Stretchers and Strainers: Treatment Variations

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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.
Compiler: Barbara A. Buckley
=Modifying Stretchers= 
==Addition of Beads==
===Introduction===
The development of stretcher bar marks, corner draws, and other planar deformations is normally associated with the construction of the stretcher/strainer. The increasing emphasis put on maintaining the historic stretcher/strainer as an integral unit with the painting often requires modification of original or historic stretchers/strainers to minimize deformations including stretcher bar marks, corner draws, and other planar deformations. Furthermore, tears along the edges of a painting can often be linked to a sharp edge on the stretcher/strainer at the fold-over edge of the tacking margin. These deformations can be caused by a variety of deficiencies including:
*Insufficient space between the inside front edge of the stretcher/strainer and the reverse of the canvas, leading to stretcher/strainer bar marks.
*Insufficient tension of the canvas, which leads to corner draws, buckles, and bulges.
*Poorly constructed corner joints; for example, puckers, draws, and other deformations can form when the joints are made without miters or when the faces of adjoining stretcher/strainer members do not meet in the same plane. When the painting cannot be sufficiently keyed out to take up the slack, sometimes restretching the painting is an option. This solution can often cause trauma to the paint and/or canvas as a new fold-over edge must be created.
===Treatment Variations===
One solution that can often address all of these problems is the addition of an acid-free or buffered mat board buildup to the perimeters of the stretcher or strainer.
====Insufficient Space====
To increase the space between the reverse of the canvas and the face of the stretcher/ strainer, a bead can be made by cutting a strip of mat board to the length of each stretcher member with a utility knife or mat cutter. The strips are made with a 45-degree bevel along one edge and cut at 45-degree miters at the ends. The size of the bead normally depends on the size of the painting; generally a l/8"-to 3/4"-wide strip is cut using four-ply board. After removing the painting from the stretcher/strainer, the bead can be glued along the outside edge of the face of the stretcher using any appropriate adhesive.
====Insufficient Tension====
Similarly, mat board can be glued around the tacking edges of the stretcher/strainer to increase the overall dimensions. Layers of mat board can be built up by gluing them together, or a scalpel and sandpaper can be used to further adjust the shape to the size of the stretcher. Stress on the fragile fold-over edge of the canvas can be reduced by rounding and softening the edge of the mat board built up with sandpaper.
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::::::FIGURE 85a Stretcher before modification
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::::::FIGURE 85b Stretcher with edge modification
====Poorly Constructed Corner Joints====
Often, both buildups along the tacking edge and a bead on the outer edge of the face are desired. For example, a canvas mounted on a stretcher with mortise and tenon joints lacking a miter on the face often develops creases and bulges, particularly if the stretcher has been keyed out. These deformations can be alleviated by attaching strips of mat board along the edges (on the face of the stretcher or on the tacking edge). Each strip should be attached to only one of the stretcher members and allowed to float freely where it overlaps the neighboring member. Strips on the face of the stretcher should be made with miters as above (figs. 85a, 85b).
====Rounding Sharp Edges====
In addition to adding mat board to the edges of a stretcher/strainer, sometimes rounding any sharp edges of the stretcher/strainer can be helpful. For instance, a sharp inner edge on the face of a stretcher might lead to a stretcher bar mark. This can be done with sandpaper, scalpel, etc., and, unlike adding mat board, it can be done in most cases without removing the canvas from the stretcher/strainer (it should be noted that this involves removing original material). The painting is placed face down or face in on an easel. Put something thin and rigid between the member to be modified and the reverse of the canvas for protection. To round the edges of cross-members, slip a piece of sandpaper behind the cross member and hold it on either side of the cross-member, working it back and forth over the edges (like flossing teeth). The inside edges of the outer stretcher/ strainer members can also be rounded with a piece of sandpaper taped to the end of a Popsicle stick or with a scalpel. Care must be taken to protect the reverse of the painting and to make sure the shavings or dust do not get trapped behind the stretcher/strainer (Booth 1989).
Robert Proctor
Submitted September 2004
==A Method of Reinforcement Using Aluminum Bars==
A method is described for reinforcing weak, but historically valuable stretchers or strainers.
===Introduction===
Many stretchers or strainers do not adequately support the canvas and paint, yet warrant retention because of historical or aesthetic concerns. Inadequate stretchers include those that are too narrow or undersized to provide sufficient support, those with weak joints, those with irregular or broken outside edges on the face side, or those that are warped. However, it may still be desirable to keep the painting and its stretcher together. A stretcher may carry important inscriptions or unique designs; its shape or size may allow unobstructed viewing of inscriptions on the canvas reverse. Archiving an inadequate stretcher and employing a new stretcher can overcome some of the previously mentioned problems, but often aesthetic concerns are compromised. Furthermore, the records linking the original stretcher with the painting must be carefully maintained indefinitely; otherwise, the valued historical associations will be lost. Usually the original stretcher can be reused successfully if it is repaired and modified. The following method provides an exceptional increase in diagonal and torsional rigidity, with only a small increase in thickness and a moderate increase in weight.
===Design and Construction===
This reinforcement device consists of standard aluminum flat bars (two layers, each layer 1/16" thick x 1 1/2" wide x the length of each stretcher member minus 1 1/2") joined at the corners and laminated overall with a readily available structural epoxy (PC-7 or similar). After the epoxy has cured, the device is screwed to the front of a stretcher or strainer using countersunk flat head sheet metal screws at approximately 5-inch intervals. Once the device is attached, overall rigidity and resistance to torque increases remarkably, even though the thickness of the stretcher has been increased only by 1/8". The device is easily removed (with the painting off the stretcher), since it is attached with screws only and not adhesives. A weak or undersized stretcher or strainer can be retained for its historical value, yet continue to function successfully as a support for the canvas. Of course, the stretcher joints would no longer move (as long as the device remains attached); thus, the stretcher would functionally become a fixed joint strainer. This device works best on paintings 30" x 25" or smaller, but certainly can be adapted to work on larger pictures.
Begin by placing the stretcher face down on a single piece of 3 mil (or heavier) Mylar®. Use a Sharpie® or similar pen to trace the perimeter of the stretcher on the Mylar®. Use this template to cut two sets of aluminum flat bars to length using a hacksaw in the patterns illustrated. File any rough edges down.
Prepare for assembly by placing the Mylar® template on a vacuum hot table at room temperature, ensuring that the Mylar® is taped down well. Carefully place the first set of aluminum bars over the template using spots of glue from a glue stick to ensure that the bars stay in place during the following operations. Mix the two-part PC-7 epoxy thoroughly and begin spreading on one side of each bar of the second set. At this point, the epoxy is quite viscous and difficult to handle, but a small piece of G-10 or mat board scrap helps to spread the thick paste. Alternatively, an inexpensive 30 ml plastic disposable syringe without a needle and with its end drilled larger (to approximately 1/8") will provide better control of the epoxy. If using the syringe, lay out two parallel “strings” of epoxy lengthwise on one piece of aluminum and one “string” down the center of the other piece. As each piece is coated, place it over the corresponding bar already on the template, making sure to overlap the corners to create a half-lap joint. (fig. 86, place “A” over “a”, etc.). Tape each piece down after pressing in place. Don't worry if the pieces do not appear well joined at this stage; just make sure they are well aligned over the template. Set up the vacuum system with good airflow around the aluminum device and cover everything with a piece of silicone-coated paper and a Dartek® or Mylar® membrane. Establish a strong vacuum pressure and set the heat to 140$$-150$$F for about one hour.
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::::::FIGURE 86 Template for assembly of aluminum reinforcement bars
Initially, the heat lowers the viscosity of the epoxy, allowing it to flow readily. The vacuum pressure can now effectively press the aluminum bars close together, leaving the epoxy to fill any remaining gaps. The heat accelerates the hardening process and improves the final bond strength of the epoxy.
After curing, pull the Mylar® off and remove the excess epoxy using a medium cut, 12-inch metal flat file, or similar. Mark the locations on the aluminum for the screw holes at approximately 5-inch intervals down the center of the front face all around. Check the stretcher for knots or other problems and adjust the screw locations accordingly. Select #8 or #10 flat head, stainless steel, sheet metal screws of the appropriate length. Sheet metal screws are preferred to wood screws because the threads are coarser and extend the full length of the screw, ensuring a better hold in old wood. Drill and countersink the aluminum to match the screws and leave the heads flush or slightly below. Place the aluminum over the stretcher, mark the holes with an awl, and drill as needed, if the wood is too hard for just the awl.
Many older stretchers have little or no beveling on the face, so the aluminum device sits flat on the face without further modification. Those with edge beveling should have the aluminum framework nestle inside the edge bevel. Those stretchers with full-width beveling will require custom-made shims attached before screwing the device in place. If the beveling is smooth and consistent, use laminated four-ply mat board, wood, or G-10 cut into ~1/4-inch narrow strips and epoxied to the underside of the aluminum device at the inside edge. If the beveling is irregular or inconsistent, balsawood or epoxy putty “blocks” can provide shims at selected intervals around the stretcher.
For irregular bevels shimmed with epoxy putty, make and attach the blocks without directly adhering them to the stretcher. With the stretcher lying face up on a table, protect the face by covering it with thin Mylar® (1 mil or less). Form Pliacre epoxy putty into suitably sized blocks (about one inch long), place at various locations over the Mylar® near the inside edge of the stretcher face, and clamp the aluminum device firmly on top. After the epoxy has cured and adhered itself to the underside of the aluminum device, remove the clamps and Mylar®, trim the blocks if necessary with a file, drill pilot holes, and screw the device in place as before.
If desired, a small aluminum or wooden bead can be epoxied to the face around the perimeter of the device to lift the canvas slightly and avoid encouraging a new stretcher bar crack pattern in the paint. A wooden bead can be shaped by planing or sanding more easily than an aluminum bead.
After restretching the painting, if the inside edge of the aluminum device is visible from the stretcher reverse, paint it with acrylic paint to approximately match the color of the stretcher.
===Sources===
Aluminum flat stock unpolished finish 1 1/2"x 1/16"(preferably anodized): McMaster-Carr 
Structural epoxy: PC-7
Available in small quantities in most traditional hardware stores
Syringes: ~20-30cc
Techni-Tool
Epoxy putty: Pliacre Epoxy Putty
Conservation Support Systems
James Hamm
Submitted January 2007
=Attachment of an Original Stretcher to a Replacement Support= 
==Purpose==
Retaining the original stretcher, or parts of it, after the completion of treatment when the original stretcher is no longer part of the painting's support system, is an option. This technique is seen on occasion but has never become standard practice. The primary purpose of keeping the original stretcher, or parts of it, with the painting is to preserve the information written on the stretcher or to retain the original stretcher in order to document the original assemblage of materials that comprised the painting as a whole artifact. In other cases, the original stretcher performs a limited but important function as part of a new support system.
==Materials, Methods, and Variations==
===General Description'''
The original stretcher is removed during treatment and is later attached to a replacement stretcher. The original stretcher or the pieces serve no purpose in the new support system other than to preserve and communicate information that was found on or embodied by the original stretcher. The advantage to this extra step of the treatment is that the stretcher or strainer is retained (not just a drawing or photograph) and kept with the painting for future reference. If the stretcher is saved but separated from the painting, it can easily be lost; even more easily, the significance of the stretcher is lost and the stretcher is then thrown away. The disadvantage of attaching the old stretcher to the new may be the extra bulk added to the support of the painting and/or the extra step of treatment.
The following are examples of this:
====Thinning the Entire Stretcher====
Frog Pond, Gloucester by John Sloan, American, 20th century
Delaware Art Museum
The original stretcher was thinned to 3/16" thick and 1 5/16" wide and screwed to a replacement stretcher at the time of a wax lining treatment (fig. 87a). This treatment occured before the 1996 acquisition of the painting by the Delaware Art Museum. In the John Sloan Trust collection of paintings at the Delaware Art Museum, there are several other examples of this same treatment. The original stretchers are saved for inscriptions by the artist indicating title, date, initials, color notations, and sometimes varnish information. The trimmed original stretcher acts as a collar around the backing board (fig. 87b). In most cases, the trimmed stretcher is screwed to the replacement stretcher; in one case, brass escutcheon pins are used. The trimming of the original stretcher has the advantage of eliminating the bulk associated with retaining the stretcher in this type of procedure.
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::::::FIGURE 87a Original stretcher thinned and attached to replacement stretcher.
::::::John Sloan | Frog Pond, Gloucester, Delaware Art Museum Photograph by Barbara Buckley, Courtesy of Delaware Art Museum.
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::::::FIGURE 87b Original stretcher acts as collar around backing board.
::::::John Sloan | Frog Pond, Gloucester, Delaware Art Museum Photograph by Barbara Buckley, Courtesy of Delaware Art Museum.
====Thinning Sections of the Stretcher====
Mrs. Cornelius Waldo, Joseph Badger, American, c. 1750
Worcester Art Museum
During a 1968 treatment, a corner section and two other sections of the original stretcher of this painting were reduced to 3/16" thickness to preserve the exposed surface and were screwed or nailed to the new spring stretcher. Each section contains inscriptions. An entire corner of the original stretcher was attached to a mending plate, which was then attached to the inner edge of the spring stretcher (figs 88a, 88b). The spring stretcher was a replacement from a 1968 treatment that involved lining with a wax/resin adhesive. This was a painstaking but clever way to preserve the information embodied by the stretcher without the bulk of the entire stretcher.
====Attaching the Stretcher in its Entirety to the Backing Board====
The Savage Family by Edward Savage, American, c. 1775
Worcester Art Museum
The entire original strainer from this painting was attached with screws through the foam core backing board to the replacement stretcher (figs. 89a, 89b). The painting was wax lined in 1964 by Edmond DeBeaumont. The painting is bulky but the original strainer is retained.
===Using the Old Stretcher in a New Support System===
The original stretcher/strainer is retained and attached as a collar when the painting is being lined onto a rigid or semi-rigid panel. This technique has been widely used with variations including cardboard panels, commercial hardboard such as Masonite®, aluminum sheet, and epoxy infused fiberglass sheets such as G-10™. The stretcher provides reinforcement of the panel at the edges and also a place to attach the tacking margins. Generally, the stretcher joints are rendered obsolete.
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::::::FIGURE 88a Section of stretcher attached to replacement stretcher with mending plates.
::::::Joseph Badger I Mrs. Cornelius Waldo, Worcester Art Museum. Photograph courtesy of Worcester Art Museum.
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::::::FIGURE 88b Section of stretcher attached to replacement stretcher with mending plates.
::::::Joseph Badger | Mrs. Cornelius Waldo, Worcester Art Museum. Photograph courtesy of Worcester Art Museum.
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::::::FIGURE 89a Original strainer attached with screws through foam core backing board to replacement stretcher.
::::::Edward Savage | The Savage Family, Worcester Art Museum. Photograph courtesy of Worcester Art Museum.
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::::::FIGURE 89b Original strainer attached with screws through foam core backing board to replacement stretcher.
::::::Edward Savage | The Savage Family, Worcester Art Museum. Photograph courtesy of Worcester Art Museum.
In the 1950s through the 1970s in the United States, it was not uncommon to line a painting onto Masonite® with wax or a wax/resin adhesive. A stretcher was often attached to the Masonite reverse to act as a collar and to prevent the Masonite® from flexing. This procedure may also have been intended to retain the original stretcher or strainer. Some examples follow:
====Cardboard and Estucheon Pins Combined with Old Stretcher====
Self Portrait by Charles Loring Elliot, American, c. 1840
Pennsylvania Academy of the Fine Arts
In this case, a stretcher provides support for the 1/8"-thick cardboard panel to which the painting on canvas was adhered. The four-membered wooden stretcher had keyed double fork mortise joints. The inner edges of the stretcher bar were chamfered. The unusual stretcher from this painting was attached to a cardboard support by nails through the front of the painting (this cannot be recommended!). The painting was marouflaged (glued) to the cardboard; the reverse of the cardboard is painted. The support is stable overall, as well as the mended 4-inch-long tear. The tacking margins have been removed. Although there are a number of faults in this support system, it has functioned to support the tear, and the original stretcher has been preserved. The painting was acquired by the Academy in 1933. There is no record on file regarding this treatment. A subsequent treatment in 1992 involved varnish removal, but no structural work was necessary at that time.
====Plywood and Nails Combined with Old Stretcher====
The Gross Clinic by Thomas Eakins, American, 1875
Pennsylvania Academy of the Fine Arts and the Philadelphia Museum of Art
During a structural treatment carried out c.1930s by Hannah Mae Horner, a new support was constructed by nailing a stretcher to a sheet of mahogany-faced plywood. The painting was then marouflaged to the plywood. In this large painting, the stretcher acts as a strainer to support the edges of the plywood. Unfortunately, the plywood was not stable, and distortions of the plywood caused the nails to work themselves out from the plywood, causing bulges in the front of the painting. A subsequent treatment in 1961 involved removing the stretcher and the plywood support. Other treatments by Hannah Mae Horner in the Philadelphia area suggest that this procedure was not uncommon and that the stretchers used were not the original stretchers. There is very little advantage to these materials and techniques.
====Sheet Aluminum with Linen Backing Combined with Old Stretcher====
Daniel Dobbins by Moses Billings, American, 1809–1884
Erie County Historical Society, Pennsylvania
The stretcher has a patent stamp dated December 21,1886 and has curved double miter lap joints (Richard Buck listed this as Type 4b: mortise with double miter, ogee variation with key). Because of the patent date, this is not the original stretcher. The most recent structural treatment, probably performed in the 1970s, combined this stretcher with a thin sheet of aluminum. The painting was adhered with wax to the sheet of aluminum, and a new piece of linen including tacking margins was adhered to the reverse of the aluminum. The laminated layers were then tacked onto the original stretcher using the new linen tacking margins. The tacking margins are attached to the stretcher, though the stretcher joints no longer serve the purpose of tensioning the canvas painting support since the aluminum support is rigid. This technique was popularized by Morton (Bob) Bradley in the Boston area in the mid-20th century and is possibly still used today by some restorers in the Boston area. No record of this structural work was available for this painting. A subsequent treatment in 1990 involved varnish removal but no structural work. One disadvantage to this treatment is that without a cushioning interlayer between the painting and the aluminum, the original canvas weave texture is pressed into the paint layer and is too pronounced after treatment.
====Masonite and Escutcheon Pins Combined with Old Stretcher====
Mr. Alfred Daniels by J. Comegys, c.1830
Mrs. Alfred Daniels by J. Comegys, c.1830 Private collection, Pennsylvania
The paintings were glued to Masonite® and the original strainers were glued to the reverse of the Masonite®. Escutcheon pins were nailed through the face of the painting through the Masonite® and strainer. The tacking margins of the paintings were glued to the side edges of the support system. In 1992, the paintings were evaluated for conservation treatment. The recent treatment involved removal of the pins, the strainer, and hard-board support system; lining of the paintings to a new fabric support; and attachment to the original strainers.
====Fiberglass Fabric and G-10™ Sheet Combined with Old Stretcher====
Martietta Ingham by Phinias Staunton, American, c. 1840
Leroy Historical Society, New York
::The stretcher has simple mortise joints with one key per joint. It appears to be the original stretcher and is intact and stable. Restoration attempts prior to 1991 involved extensive use of adhesive tapes and overpaint to mend and cover up large complex tears. The 1991 treatment included mending the tears, infusing the canvas with microcrystalline wax, and lining it with fiberglass fabric backed with a thin (.001"), flexible sheet of G-10™ (epoxy-infused fiberglass). In this case, the stretcher remained functional for fine-tuning the planar format of the support. One advantage to this combination of lining materials is that the final lining fabric, adhesive, and thin G-10™ sheet combination is transparent so that the tears (or any features on the back of the canvas) remain visible on the reverse.
The 1 mil G-10™ is flexible like 5 mil Mylar. So while it doesn't “stretch,” it can be, or become, wavy or flat. Metaphorically, the stretcher gives skeletal-type support to the muscles of the G-10™, which hold up the skin that is the painting. Adjusting the stretcher allows the conservator to somewhat reshape the painting/G-10™ combo, ideally to a flat format. Or, to use another metaphor, the stretcher is to the painting/G-10™ combo like the metal frame on an umbrella is to the fabric of the umbrella. The umbrella fabric doesn't stretch, but it can't hold itself to the desired shape—it needs the expansion of the metal framework to do so.
==Conclusion==
The procedure of attaching an original stretcher or strainer, or parts thereof, to a replacement stretcher can be a worthwhile effort. If the old or original stretcher/strainer has inscriptions or signatures or embodies unusual information, this option should be considered during structural treatment. It is left to the discretion of the conservator and the owner on a case-by-case basis to weigh the cost of the extra time, space, and resources required for this step of treatment. Although every stretcher cannot be kept with the painting or reused after structural treatment, creative ideas can be developed to retain stretchers of importance.
If the stretcher cannot be reused due to damage and deterioration but contains important information, retaining pieces of it with the painting is a way of preserving the historical data in a material way. Due to instability, the use of Masonite, plywood, and cardboard have been largely replaced by aluminum honeycomb panels and G-10™ sheets in current conservation practices. Many of the treatments are historical and are not considered acceptable for the standards of current conservation practices.
 Author's note: The examples above come from my private practice, from my work as a student at the Buffalo State College Program in Art Conservation, and from the painting conservation files of the Worcester Art Museum, the Pennsylvania Academy of the Fine Arts, and Barbara A. Buckley and Associates. Thanks to the following conservators for their assistance during the compilation and research of this topic: Rita Albertson, Mark Bockrath, Barbara A. Buckley, James Hamm, Philip Klausmeyer, and Dan Kushel.
Barbara Wojcik
Submitted January 2007
=Variations for Double-Sided Paintings= 
==Modification of an Expansion Bolt Stretcher for a Double-Sided Painting==
Tight finances, high productivity, or the simple immediate need for a surface to paint on has occasionally led artists to paint on both sides of a single canvas. Usually an existing painting is removed from its stretcher, turned over, reattached to the same stretcher, and the original canvas reverse becomes the new obverse ready for painting. The new painting on the second obverse is fully visible but the painting now on the reverse has 2–4 inches around the perimeter obscured by the stretcher members. Special stretcher design and strip lining techniques through a conservation treatment can render both paintings fully viewable.
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::::::FIGURE 90 Framed double-sided painting in cross-section
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::::::FIGURE 91a Primary painting: before framing
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::::::FIGURE 91b Secondary painting: stained stretcher obverse
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::::::FIGURE 91c Both sides of mounted double-sided painting (portrait is shown in mirrored reflection)
An initial decision involves the type of framed presentation desired for each of the two paintings. Curatorial or owner input may suggest a primary side of the double-sided piece, which is important since framing options using this method differ depending on how the stretcher is attached.
Basically, the painting is strip lined and attached to the reverse of a new stretcher with the image edge coinciding with the inner edge of the stretcher (fig. 90). Usually the primary painting will face from the reverse side of the stretcher and the secondary painting will face from the obverse side of the stretcher (the side with the bevel). The obverse side of the stretcher acts as the frame for the secondary painting so the width of the stretcher; quality of wood for staining, veneering, or even gilding; angle of bevel; and inner edge depth are all important design factors. Since the primary painting is attached to the flat stretcher reverse, any frame for that side could be used, provided there is no rabbet depth (an old frame would have rabbet depth material routed away; a new frame would be designed without rabbet depth). The stretcher will expand with turnbuckles in the usual place on a stretcher and will be hidden by the framing of the primary painting. (The photographs in figs. 91a, 91b, and 91c illustrate this technique.)
Plexiglas could easily be installed over the secondary painting as shown in the diagram. This is a good protective measure because with the primary painting being favored for viewing, the secondary painting is at greater risk in display circumstances or in storage. Harmonious trim of some sort can cover the edges of the painting. Wall hanging can be accomplished with screw eyes from the top of the stretcher, or the framed painting assembly could be displayed resting on an easel.
Scott Heffley
Submitted October 2002
==A Modular Design for Double-Sided Stretchers==
===Introduction===
These designs evolved out of the demand for mounting and display of canvases painted on both sides: double-sided canvases. The first design (option 1) is intended for canvases having a primary painting on one side and a secondary painting on the other. In this case, the primary painting is presented in the frame, while the secondary work remains visible in its entirety on the verso. The second design is intended for canvases that must be mounted so that either or both paintings can be displayed to equal effect, whether alternately in a traditional frame (option 2) or simultaneously in a specially designed double-sided frame (option 3). Alternate presentation is often dictated by the canvas, especially when compositions are not similarly oriented, as illustrated in the example by B. J. O. Nordfeldt (private collection) depicted in Figures 92 and 93. This symmetrical system of mounting is demonstrated in a mock-up also illustrated below.
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::::::FIGURE 92 B. J. O. Norfeldt, side A, horizontal orientation
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::::::FIGURE 93 B. J. O. Norfeldt, side B, vertical orientation
Both stretchers are based on a traditional design incorporating simple double mortise and tenon joinery. The only distinction is that the latter stretcher is split to provide a symmetrical support. Both can be readily fabricated in most woodworking shops.
Both designs require that the original tacking margins be flattened and extended by edge-lining as necessary to accommodate an outsized stretcher. In both cases, overall dimensions of the stretcher are determined so that the entire image can be viewed within the stretcher. In many cases, an additional margin is provided around the image to allow the frame to conceal the stretcher (fig. 94) (Alternate Frame Finish: Options 2 and 3).
====Preferential Mount and Display (Option 1)====
This stretcher is intended for use when one wishes to display a secondary image on the verso of a canvas in its entirety. The stretcher bars are composed of flat stock. As the stretcher will be behind the flattened tacking margins and their extensions, no bead or bevel is necessary. The bars are joined with simple, open double mortise and tenon joinery. Miters are also unnecessary, further simplifying fabrication. The only deviation from traditional stretcher joinery is the placement of the key slots on the outside of the bars so that they do not impinge on the image. Planning and fabrication details follow.
Based on the size of the painting, the stretcher stock is milled to appropriate dimensions. 3/4" x 2" basswood strainer stock obtained from the framer has proven to be adequate for most small to moderate paintings up to about 30". Larger stock can be milled as larger canvases require. In any case, one should determine dimensions much as one would for a traditional stretcher, bearing in mind two things: 1) the stretcher will be outsized, so each rail will be longer than its counterpart in a traditional mounting; and 2) cross bars are not an option. Some oversizing may therefore be indicated.
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::::::FIGURE 94 Three options for double-sided mounting and framing of paintings on canvas
The stretcher bars are cut to length according to the dimensions of the painting and the strainer stock. The formula for length along any dimension is: length = site dimension of image + (2 x width of stock) + 1/2". The added 1/2" should provide a 1/4" margin around the entire perimeter of the verso painting inside the stretcher. The margin can be adjusted according to the needs of a particular painting or the preferences of the conservator.
The joints can be milled on a table saw with a tenon jig. The tenons are cut to be exactly one-quarter the thickness of the stretcher stock to a depth equal to the width of the stock, leaving symmetrical mortises between. The joints are cut on opposite sides of the stock at each end, just like commercial American double-mitered mortise and tenon stretchers, so that they are symmetrical by rotation and can be interchanged without difficulty. The key slots are cut on the tenon jig by placing the bar at an angle and raising the blade to the appropriate height. A key can be used as a bevel guide for these cuts by clamping/screwing it to the tenon jig. As long as the key slots are cut on the same side of each bar, it does not matter which side they are on, because each bar can be rotated until the slots are on the outside prior to assembly.
The stretcher is assembled like any other, except that the key slots are aligned on the outside. The canvas is mounted as usual, with the secondary image on the verso visible within the stretcher. The only difference is that the tacking margins will now cover the key slots. However, the width of the bars will generally be greater than the width of the original tacking margin so modifications necessary to accommodate the keys will affect added extensions, not original canvas. Keys are inserted into the slots and hammered out as necessary to effect proper tension. The keys can be secured by driving a small nail/ screw through the join. The keys can be trimmed to minimize interference in framing.
The painting can then be mounted in a traditional frame, the rabbet having been opened up to accommodate the increased dimensions of the oversized stretcher (fig. 94). A piece of clear acrylic may be used to protect the painting on the verso without obscuring its view. As the keys are on the outside of the stretcher, they do not impinge on the view of the painting on the verso either. However, some additional modification of the frame may be required to accommodate the keys, especially if they are not trimmed.
====Symmetrical Mounting Permitting Preferential/Simultaneous Display (Options 2 and 3, respectively)==== 
Also based on traditional design, this stretcher provides symmetrical mounting for double-sided canvases, permitting simultaneous display in a specially designed double-sided frame as well as alternating display in a traditional frame. This is essentially the same design, except that each stretcher bar is split and the canvas is mounted between the two halves. Basic design and joinery are depicted in schematics I and II (figs. 95, 96). Planning and fabrication proceed as follows.
Based on the size of the painting, flat stock is milled to appropriate dimensions. The overall dimensions of the stretcher bars should be comparable to a traditional stretcher, bearing in mind factors previously noted. Stock is milled to one-half the overall desired thickness of the finished stretcher. A 1" x 2" stretcher composed of 1/2" x 2" stock has proven to be adequate for paintings as large as 36" x 40" (i.e., the Nordfeldt depicted in figs. 92, 93).
The pieces are cut to length according to the dimensions of the painting and stock and the intended manner of framing. The formula for length along any dimension is: length = site dimension of painting + (2 x width of stock) + (2 x margin). The margin is the space around the site area necessary to accommodate some concealment by the frame (see Alternative Framing: Options 2 and 3). Instead of four pieces, eight pieces are cut.
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::::::FIGURE 95 Schematic of basic design and joinery
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::::::FIGURE 96 Schematic of basic design and joinery
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::::::FIGURE 97 Eight stretcher bars milled to make half lap joint
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::::::FIGURE 98 Four bars conforming to each dimension
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::::::FIGURE 99 Assembled stretcher
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::::::FIGURE 100 Corner detail of assembled stretcher with keys
Instead of mortising, each piece is milled to make a simple lap joint: one-half the thickness of the stock to a depth equal to the width of the stock. Instead of four stretcher bars, eight pieces are fabricated: four identical pieces conforming to one dimension and four identical pieces conforming to the other. The key slots are cut as described above. Figures 97 and 98 depict a set of eight pieces, as milled and paired for assembly, respectively.
The pieces are paired and temporarily clamped together to make four stretcher bars. Holes are drilled at appropriate intervals along the length of each to permit future joining of the halves with machine screws. One hole is drilled 1" from the shoulder of the join on each end of each bar. The pairs are marked for registration and separated. Brass machine screw inserts are driven into the holes in one piece of each pair. The pairs are reassembled with machine screws and the stretcher assembled in a dry run. Figures 99 and 100 depict the assembled stretcher and a detail of one corner with keys in place, respectively.
To mount the canvas, each composite bar is disassembled and the pieces reversed, i.e., front screwed in back. Because of the symmetry of the pieces, the joinery remains functional. The inside of the back piece is now exposed on the front. The canvas is initially mounted by pinning in place (fig. 101). When it is properly situated and satisfactory tension is achieved, the canvas can be tacked or stapled (fig. 102). The entire assembly is then placed face down on a piece of foam board (fig. 103). The front pieces, which are now on top, are unscrewed and set aside (fig. 104). A second piece of foam board is used to sandwich the painting, and the entire assembly is turned over. The canvas, now mounted to one-half of the stretcher assembly, is again on top (fig. 105). Surprisingly, with the canvas secured under tension, the joints will hold, as long as the assembly is not twisted far out of plane. Holes are cut in the edge-lining material to expose the screw holes. We use a brass cork borer to do so (fig. 106). The front pieces are retrieved and reunited with their partners according to their registration markings. As the pieces are screwed back together, the two halves of the joints come together and tighten up on each other (fig. 107). The excess of the tacking margin extensions is trimmed and tacked to the outer edges. As before, the keys are inserted, the tension adjusted as necessary, the keys secured with nails/screws and trimmed as desired. Figure 108 depicts the assembled stretcher with keys partially inserted. Figure 109 depicts a detail of one corner of the finished mock-up. Figures 110 and 111 depict the A and B sides of the assembled stretcher, respectively.
Figures 112 and 113 depict two sides of a corner on an actual canvas similarly mounted. One side reveals the Hollytex edge-lining used to extend the canvas for mounting. The other illustrates the extent of margin required to permit some concealment by the frame. Note the brass pins used to secure the keys. The keys have been trimmed to minimize their interference in the frame rabbet.
Because each painted surface lies below the surface of the stretcher by a distance equal to the thickness of the stock, some special frame design is called for (fig. 94) (Alternate Frame Finish: Options 2 and 3). The simplest solution is a liner that is the same depth as the stretcher stock. This can be fixed to the frame or floated with the painting. The visible and physical width of this liner will determine the “margin” added to each dimension in the calculation above.
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::::::FIGURE 101 Canvas temporarily mounted by pinning
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::::::FIGURE 102 Canvas mounted by staples or tacks
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::::::FIGURE 103 Painting is turned face down on foam board
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::::::FIGURE 104 Front stretcher pieces are unscrewed
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::::::FIGURE 105 Painting turned face up after removal of front stretcher pieces
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::::::FIGURE 106 Brass cork borer used to expose screw holes
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::::::FIGURE 107 Front stretcher pieces rejoined by screws
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::::::FIGURE 108 Assembled stretcher with keys being inserted
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::::::FIGURE 109 Corner detail of assembled double-sided stretcher
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::::::FIGURE 110 Assembled stretcher: A side
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::::::FIGURE 111 Assembled stretcher: B side
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::::::FIGURE 112 Corner detail of assembled stretcher with painting: A side
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::::::FIGURE 113 Corner detail of assembled stretcher with painting: B side
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::::::FIGURE 114 B. J. O. Norfeldt, framed
Whether displayed alternately or simultaneously, such stretchers are mounted in their frame by usual methods. If the painting is to be mounted in a traditional frame to display one side at a time, a protective acrylic backing can be mounted to the verso as previously described. Some additional modification of the frame may be required to accommodate the keys if they are not trimmed. Figure 114 depicts the Nordfeldt in its frame. In this case, the earlier, more important painting will be displayed in the frame. Winter Landscape remains visible on the verso, protected by a piece of clear acrylic.
This is but one of several methods employed in our studio for the mounting of double-sided paintings. However, it is the most simple, straightforward, and modular of them all and has proven to be of broad utility in a wide variety of circumstances. Others often involve the incorporation of frame elements into the structure of the support, in close collaboration with a skilled framer.
Steven Prins
Submitted January 2007
=Variations on Panel Back Stretchers=
==Introduction==
This section offers several variations on the traditional panel back stretchers with adaptations to original stretchers.
==Treatment Variations==
===Adaptations of the Expandable Paneled Stretcher Concept for Canvas Paintings===
Based on several prototypes employed in the treatment of canvas paintings, a simplified paneled design concept was developed that allows painting conservators to easily construct paneled stretchers at relatively low cost. The basic design allows the incorporation of a variety of options in the choice of both the stretcher expansion system and the material and thickness of the support panel. In general, conservators would first have their desired type of expandable wooden stretcher fabricated by their regular stretcher maker (with the modifications illustrated below to which they would then attach an independent floating panel of their choice).
In short, the new stretcher would be constructed with the same mortise/tenon joints and typical wooden keys (or dowelled joints with expansion bolts or other mechanical tensioning system) but without the standard bevel on the inner face and with either an added wooden strip around the outside perimeter or with the peripheral stretcher members themselves fabricated with an L-shaped profile (fig. 115).
This design assumes the presence of two or more cross-members (one being insufficient for secure anchoring of the panel except in very small pictures), but otherwise, the original position and number of the cross-members need not be changed. The dimension A is determined by the thickness of the panel to be used, and the width of B is determined by the required strength of the wood edge-bead as based on the size, weight, and ultimate tension of the stretched painting.
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::::::FIGURE 115 Non-beveled stretcher with added strip or L-profile to accept inset panel
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::::::FIGURE 116a Stretcher with added profile to accept inset panel
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::::::FIGURE 116b Panel secured to stretcher with countersunk flat head screws
When the stretcher has been made, the panel is cut 1/16" or so smaller all around than the interior well C in which it will sit, for ease of fitting. With the panel in place, pilot holes are drilled and the panel is secured with countersunk flat head screws (filled if desired) or other fastening devices to the stretcher cross-members only. The diagrams illustrate typical points of screw attachment on a portrait-sized stretcher with a rigid 4 mm (5/32") Alucobond* panel (figs. 116a, 116b).
I have found this to be a simple and sure mode of attachment. The panel might instead be attached with an appropriately strong adhesive, although some sort of mechanical anchoring seems more secure for the long term. The fixing of the panel solely to the cross-members allows for free movement of the peripheral members and thus expansion or shape modification when and if desired, as with a normal keyable stretcher.
:Many painting conservators have at one time or another noted the protective and general preservative effects imparted to pictures mounted on original 19th-century wooden paneled stretchers. I believe that the remounting of canvas paintings, whenever the opportunity might present itself, onto paneled rather than standard open-backed stretchers, offers considerable advantages regarding physical and environmental protection from the back, as well as a certain degree of physical protection for the front. In addition, the panel acts to reduce longitudinal movement and dampen canvas vibrations during handling and shipping, especially when a cushioning interleaf (e.g., 1/16"-1/8" Pellon or similar non-woven polyester material) is stretched between panel and canvas.
Most old paneled stretchers were made with a tongue-and-groove construction similar to that in wall paneling or furniture case pieces; certainly this and numerous other more sophisticated construction methods might be suitable for individual cases. In general, paneled stretcher systems are more complicated and difficult to fabricate for the average stretcher maker and thus can incur higher costs. The present design is a relatively straightforward one that can be assembled with a minimum of time, expense, and materials, offering an effective low-tech and low-cost alternative. Additionally, a great many prefabricated panel materials can be chosen by the conservator, such as Alucobond®*, aluminum or fiberglass honeycombs, Tycore® ragboard honeycomb, or treated hardboard (Masonite®), to name a few. Special laminates can easily be made by the conservator as well. For instance, plexiglass might be appropriate for a picture with an important inscription on the reverse that must remain visible. Existing original stretchers, if the wood remains true, could conceivably have their beveled inner face routed down to the level of the existing cross-members for the inclusion of such a panel.
*Alucobond: A material I have used with success for paneled stretchers. A dimensionally stable, machinable aluminum-polyethylene-aluminum laminate. One-half the density of aluminum. 3mm, 4mm, and 6mm thicknesses, with a white anodized finish. Available in large sheets or cut to size from, for example, Museum Services Corporation, 4216 Howard Avenue (Upper Level), Kensington, MD 20895-2418; 301-564-9538.
 Editor's note: Museum Services Corporation is now located at 385 Bridgepoint Dr., South Saint Paul, Minnesota 55075; 800-672-1107; info@museumservicescorporation.com.
Robert Sawchuck
Reprinted from 1991 AIC Painting Specialty Group Postprints Studio Tips III
===A Combination Solid Support and Keyable Stretcher for Canvas Paintings===
An expansion bolt stretcher was modified to provide a rigid surface that would support a painting after restretching. Once mounted, the painting could be keyed out in the customary manner.
Treatment of a painting in the collection of the San Francisco Museum of Modern Art included relining. After treatment, the weight of the painting was considerable due to the thickness of the paint application and the added weight of the wax-resin adhesive. In remounting and restretching the painting, a continuous, rigid underlying surface was desired that would support the painting while allowing the expansion bolt stretcher to be keyed out.
A modified 1/2-inch thick 100 percent rag Tycore® honeycomb panel was placed between the stretcher members to serve as a rigid support for the painting. The Tycore® panel was cut through the lower rag board face and the honeycomb core. The upper face of the Tycore® panel was not cut, thereby leaving an exposed rag board border approximately 1 1/2 inches wide that could be wrapped around the outer stretcher members after the Tycore® was set into the center of the stretcher. The depth of the stretcher was adjusted to accommodate the thickness of the Tycore® panel by adding wooden strips to the top of the existing stretcher bead. The rag board edges of the Tycore® were folded around the edges of the stretcher by gently pressing them with a bone folder. Care was taken while bending the edges so they would not tear. The Tycore® edges should not be scored before they are folded around the stretcher.
The extended rag board of the Tycore® was not wide enough to cover the entire thickness of the stretcher members. A margin of the wooden stretcher was left exposed so that the painting could be reattached to the stretcher with tacks or staples while taking care not to secure the underlying rag board.
Since the rag board edge of the Tycore® remained in contact with, but not attached to, the edges of the stretcher, the remounted painting could be keyed out. The stretcher members could be expanded without impediment while the interstice created between the Tycore® and the expanded stretcher members was covered by the Tycore's® extended and unsecured ragboard edge.
The Tycore® support can be used for unlined and lined paintings. Tycore® is manufactured by Archivart Products, Process Materials Corporation, 301 Veterans Blvd., Rutherford, New Jersey 07070; 201-935-2900.
 Editor's note: The Tycore® mounting panel is currently manufactured by Archivart® Products for Conservation and Restoration, 40 Eisenhower Dr., Paramus, NJ 07652; 800-804-8428; askarchivart@nbframing.com
Neil Cockerline
Reprinted from 1991 AIC Painting Specialty Group Postprints Studio Tips III
===A Gatorboard and Expansion Bolt Stretcher Adaptation===
This system describes a panel backed stretcher constructed with Gatorboard and an expansion bolt stretcher. This system was used on a large oversized painting.
====Methods/Materials==== 
Three-sixteenth-inch Gatorboard was cut to the dimensions of the painting and adhered, end to end, with a water-based, contact cement, Panelmaster 40-8007 (see suppliers list below). The Gatorboard is adhered to the inner (painting side) of the stretcher. A 1/4"-wide wood strip is fabricated to notch into the inner side of the stretcher. This piece is separate and removable or changeable if ever needed. The Gatorboard is attached only to this strip with a polymer emulsion, Vy-Lok (see suppliers list below). The painting is stretched over this system and the tacking edges are attached to the stretcher.
====Advantages/Disadvantages====
This system was used to replace an auxiliary system of plywood over which the painting had been stretched. This system replicated the rigid backing but added the versatility of expandable corners as well as greatly reducing the weight of the whole. The new stretcher plus the Gatorboard provided a stable auxiliary support but reduced the overall weight. Ultimately this allowed for easier handling and hanging.
Suppliers: Panelmaster 40-8007
Bondmaster
National Starch & Chemical Co.
New Jersey
VY-LOK 40-0160
Bondmaster
National Starch & Chemical Co.
New Jersey
Randy Ash
Submitted January 2000
===Panel Stretcher Using G-10™===
This method describes a system in which the plastic sheet G-10™ is attached to the original stretcher.
====Materials/Methods====
After removing the painting from the stretcher, make sure that the stretcher shape is not changed. Clean the stretcher and seal with Paraloid™ B-72 (optional). Trim the G-10™ to size.
Tip on cutting G-10™ to size: Place a sheet of G-10™ as large as the stretcher atop the stretcher as it lies on a level table. Build up the cavity of the stretcher if it is necessary so that it does not sag as it rests across the bars. Allow one side of the G-10™ to rest just inside the stretching edge by no more than 1/8", mark the G-10™ 1/8" inside the other three sides, and trim with scissors.
Affix the semi-rigid support to the face of the stretcher with either double-sided tape or silicone adhesive to prevent it from shifting around beneath the painting. Once the G-10™ has been secured to the stretcher, run double-sided tape around the perimeter of the stretcher and affix a loose lining of thin polyester fabric to the stretcher atop the G-10™. This layer will prevent the edges of the G-10™ from abrading the bend of the stretching edges. Replace the painting in the original position.
====Advantages/Disadvantages====
This method has been used for paintings that do not require a lining but are displayed in an unsecured location or are otherwise at risk for vandalism. The objective in using this method is long-term preservation with minimal intervention. It has often been found that the quality of the G-10™ is not consistently flat. If possible, it is best to hand-select the sheets from the supplier.
Margaret Sutton
Submitted April 1997
2012 Ed. note: Custom keyable panel stretchers with honeycomb fiberglass or plywood inserts are available from Simon Liu, Inc.
=Coating Stretchers=
==Introduction==
This section discusses the treatment of both old and new stretchers with resin or wax coatings by conservators. In addition, it discusses the staining of replacement stretchers and keys.
==Treatment Variations==
===Old and/or Original Stretchers===
The most commonly cited varnish used for coating original and/or old stretchers and strainers by conservators is Paraloid™ B-72 (manufactured by Rohm and Haas Company) (Vo 1999). Paraloid™ B-72 may be commonly chosen because it is readily available in most painting conservation studios, and it is generally considered to be harder and more moisture-resistant than most painting varnishes. Rather than applying a resin coating, one conservator cited using a naphtha-based, custom-made wax polish that is applied to the stretcher with a cloth and rubbed along the grain of the wood.
The stretcher is first cleaned by an appropriate method (generally dry-cleaning) prior to the application of a surface coating. B-72 is diluted in xylenes or acetone and is applied by either spraying or brushing. One conservator prefers to use foam brushes when applying the B-72 to a stretcher.
===New Stretchers===
Conservators seemed to differentiate between applying a coating to a new stretcher as opposed to an old stretcher. In addition to B-72, dilute shellac was noted as a choice for applying a surface coating to new stretchers. One practitioner generally brushes two coats of dilute shellac onto a replacement stretcher. Paraloid™ B-72 is also commonly sprayed on a new stretcher. ICA stretchers (see section 2.3 in Stretchers and Strainers: Materials and Equipment), when manufactured by Glenn Hobbs in Oberlin, Ohio, were coated with a commercial varnish, Sand 'n Seal. A surface coating on a new stretcher would be useful in preventing sap migration, especially on stretchers made from hard pine or redwood rather than basswood.
It has also been noted that new stretchers are sometimes stained with a brown wood stain to age their appearance, as is often seen on dealer or trade restorations. Several restored John Sloan paintings (it is estimated that they were restored in New York City in the 1980s) have replacement stretchers that have been stained with a brown wood stain and then finished with a wax coating (Buckley 1999). Wax coatings may be applied as a separate step in a treatment but are also often part of the process of finishing a wax lining by attaching the tacking edges of the painting to the stretcher with molten wax. The excess wax is then melted onto the surface of the stretcher, thus coating only the verso of the stretcher.
When new keys are made to replace lost keys on an old or original stretcher, some conservators stain the keys so that their appearance is more in keeping with the original keys and stretcher.
===Advantages===
Some conservators believe that a surface coating helps protect the stretcher surface. One conservator felt that any grime that was not removed by surface-cleaning the stretcher was “locked in” by varnishing. A surface coating may protect an inscription on a stretcher depending on the material used for the inscription (a chalk inscription would disappear).
Some conservators also cited that applying a surface coating is an attempt to make the stretcher more inert in an already acidic system; the surface coating acts as a barrier to keep the acidic by-products of the wood from the canvas. Although there hasn't been any research to substantiate this view, this may be most relevant when treating an unprimed modern canvas.
Conservators also mentioned that varnishing the wood improved its appearance; they thought of it as a nice finishing touch to a treatment.
===Disadvantages===
Some conservators believe that coating an old stretcher is an unnecessary step to add to a conservation treatment. In informal discussions, it is often mentioned that if a stretcher was uncoated originally, then it should remain so. If a strip lining or lining has been part of the treatment, it is thought that this will act as a barrier or buffer for acidic migration from the wood to the original canvas support, thereby making the surface coating an unnecessary step.
Many conservators consider staining a replacement stretcher as deceptive, and they retain the pale color of raw or lightly coated new wood as an indication of the use of replacement materials.
Barbara A. Buckley
Submitted August 2000
=Restretching a Lined or Unlined Painting=
==Introduction==
A crucial but little discussed step among conservators is in regard to the stretching of paintings, whether lined or unlined. There are many considerations for preparation, positioning, stretching, and attaching a canvas to a support. However, most of the guidelines for stretching paintings are based on those written for artists. For example, Ralph Mayer, in his book The Painter's Craft, describes the “proper procedure” for stretching a canvas (Mayer 1991, 65-71), as does Max Doerner (1949, 7-8). Although this may be practical for raw canvas, can it be applied to a painted canvas, especially one that has considerable age and damage? Certainly, the paint layer and potential damage to that paint layer must be considered before retensioning the painting.
There is little historical record of the procedures used by artists prior to the 20th century. Stretching pliers were supposedly not in common use until 1889 (Gettens and Stout 1966, 284), although it is highly possible that pliers used by other trades were employed by the artist before the late 19th century (Hodkinson and Child 1995, 40). Even so, the common working method of stitching a fabric to a working stretcher, the so-called Dutch Method, would have required a subsequent mounting onto a strainer or stretcher, but there is no documentation on how the artist would approach it.
There has been previous research studying the mechanics of the stress and strain of a stretched painting and the amount of tension that would be appropriate to achieve (Berger and Russell, 187–204). However, the practical application of such information would be difficult to implement during a stretching procedure. An empirical deduction must be taken during stretching, while factoring in all of the variables: size, fabric, paint application, age, degree of brittleness, etc. In order to determine the stretching methods used today in the conservation field, several conservators were interviewed for the procedures followed and considerations taken when stretching a painting. These discussions clearly illustrated the great variation of approaches employed by conservators.
==Preparation==
In order to prepare the canvas and the stretcher, several steps may be incorporated prior to stretching, depending on the needs of the painting. Strip linings are often added when the tacking margin is not substantial enough to facilitate stretching. It is usually recommended that the strip lining be of a similar weight and material to the original canvas. Conservators use a variety of materials for strip linings, including Hollytex, linen, Sunbrella®, and PeCap.
Tears of the canvas at the foldover edge can be patched with strips of Japanese tissue and an appropriate adhesive. The tacking margins can be sized for added strength as well, possibly using BEVA® 371 or Paraloid™ B-72. If the foldover edge was flattened prior to treatment, or is in danger of cracking or breaking, the area can be warmed with a Leister hot air tool, hot air gun, or tacking iron to make the canvas more flexible for restretching. Although common treatment procedures often require removal from the stretcher, this does not always necessitate the flattening of the tacking margin. A strip lining is often added while maintaining the foldover edge, reducing the possibility of damage.
Working in a warm room can facilitate stretching. A rise in temperature has shown that the stress within the paint film and the canvas decreases, even though there often is a subsequent reduction of relative humidity (Berger and Russell 1988, 200–201). This can assist in stretching because the brittleness and stiffness of the structure become less of a factor.
Some conservators seal the stretcher with a spray of varnish on the face and sides, especially if it is new. Usually, the stretcher is adjusted to match as closely as possible to the current configuration of the tacking margins. For example, if the painting is not square, the stretcher can be adjusted to match the out-of-square contours, thereby preventing stress to the existing foldover edge; this is easier when the old stretcher has been reused. If the strip lining or lining fabric will add too much bulk to the foldover edge, the sides of the stretcher can be planed down to accommodate the thickness of the fabric, thereby alleviating further stress.
==Positioning==
A conservator can opt for three different orientations for the painting during the positioning and stretching procedures: face up, face down, or vertical. The initial positioning is easiest when working horizontally to roughly estimate the centering of the painting and the alignment of the corners. Temporary attachment is commonly achieved by using pushpins. Afterward, the painting can be placed face up or vertical for further tensioning. Although a painting can be stretched face down, this is not often recommended since the conservator is working blind, and the reaction of the canvas to the stretching cannot be gauged. However, the advantage of face-down stretching is that canvas pliers can be used more easily in this position and there is no worry about the canvas sagging inward on the stretcher. By placing the edge of the stretcher over a table, however, canvas pliers could still be used with the painting face up. Conservators may place the painting vertically when the canvas is large, although some prefer to work vertically for most of the stretching. For example, Dean Yoder has constructed a special stretching easel that holds the painting vertically.
Small pieces of foam board, mat board, or other appropriate material are often inserted between the stretcher bar and canvas. An appropriately thick support can be placed under the canvas between the stretcher members as well. This imitates the desired plane of tension, and the stretching can occur without working against gravity by eliminating sag.
==Stretching==
The topic that has raised most of the discussion among conservators has concerned where the actual stretching process should begin—starting from the middle or starting from the corners. Most guidelines for artists suggest starting in the middle and working outwards, and this is how most conservators were initially trained. However, several conservators have suggested that it is better to work from the corners inward; only one source was found suggesting this for artists as well (Wehlte 1975, 346). An upcoming publication will explore this topic by using computer models to extrapolate the stress fields while stretching paintings. The result of these studies has shown that it is better to start stretching from the middle and work out to the corners as is commonly practiced (Mecklenberg 2000). For this article, the common steps of stretching a painting, as gathered from conversations with practicing conservators, will be discussed.
Following the initial positioning, the painting is held in place by pushpins in the middle of each side and at the corners with only a minimal amount of tension on the canvas. Most commonly, then, the conservator works from the middle out. A few pushpins are used to set the middle of the long side, with very little tension placed on the canvas. Then the canvas is rotated to the other long side and a few pushpins are placed opposite those placed previously. At this point, the canvas is stretched using either the fingers or canvas pliers. In this way, the pushpin on the opposite side can act as a point of attachment for pulling. The next pushpins are then placed on the short side, with no tension, after which pushpins are fastened to the opposite short side with tension. Then the stretching goes back and forth, from long side to long and short side to short. Usually, a few pins are placed to the left and right of the middle, slowly working out to the corners. Finally, the pushpins in the corners can be removed and the tension adjusted.
Conservators use either fingers or canvas pliers during stretching. Usually this depends on the stability of the canvas and whether the painting is lined or unlined. Canvas pliers can prevent fatigue in the fingers and give more overall even tension since the conservator's strength does not diminish throughout the stretching procedure. Also, canvas pliers spread the stress over a greater area whereas finger stretching concentrates the stress at one point. However, too much tension can be applied, endangering a weak, unlined canvas.
Once the entire canvas has been stretched, it is often recommended to let it sit for a day or two to reach equilibrium with the surrounding environment. The tensioning procedure can be repeated as often as necessary to approach the desired final tension. This helps prevent excess keying out at the end of the treatment.
Another method mentioned above is starting the stretching from the corners. The procedure follows the “middle” method, but the pushpins are placed at the corners first, and then the canvas is stretched working in towards the middle. Several conservators use this method solely for the stretching of lining or loose-lining canvases. One system is described below. Others have found that this can also be used for paintings.
While formulating an approach to stretching, two different points of view may be considered. One is that the conservator should preserve the previously introduced stress patterns. This would be achieved by maintaining the present shape of the stretcher and picture plane, as well as trying to closely imitate what appears to be the original stretching method. Another approach would be to correct any perceived deficiency.
For traditional paintings, it would seem prudent to maintain the original configuration as much as possible. A painting can compensate for the stress and strain by cracking and buckling in order to reach a state of equilibrium between all the layers of the painting and the surrounding environment. By introducing a new stress pattern, the canvas would have to undergo the same equilibration, thereby causing new patterns of cracking or buckling.
In the case of contemporary paintings, one possible hypothesis could be drawn. Often the paint layer has not yet manifested any damages from stress and strain forces inherent in a stretched painting. If the painting is properly tensioned before such damage occurs, one could conceivably prevent deterioration of the paint layer. However, research has shown that the damage to a sizing layer, such as rabbit skin glue, usually occurs immediately upon drying. Contemporary paintings are not usually traditionally sized, however. Rather, artists often used acrylic gessoes. Because the reaction to stress and strain differs in this regard, it is possible that the forces involved in eventual paint deterioration have not been set into place. By correcting this through proper stretching, it may be possible to prevent this result.
==Attachment==
The two most common methods of attachment use staples and tacks. However, the residual adhesive from a lining can also be used, such as wax or BEVA® 371, or BEVA® film strips can be adhered to the sides of the stretcher for the same purpose. To cut down on bulk, the strip lining material can be adhered to the stretcher with the lining adhesive, and then the tacking margin can be attached with tacks or staples. It gives overall even tension, but logistically can be both difficult to adhere while maintaining the appropriate tension on an unlined canvas and difficult to remove during a future treatment.
Staples and tacks are used interchangeably. Staples offer two points of attachment, and the use of an electric staple gun minimizes the force required to apply the staple. Tacks, however, offer a more aesthetic choice, and the old tack holes can be reused to minimize the addition of new holes. To alleviate the force required to insert the tack, new holes can be predrilled with a hand or electric drill. Some prefer not to use the old tack holes, as these areas can be the weakest in the structure. Traditional paintings are usually attached to the sides of the stretcher while modern pictures can be stapled to the reverse, usually mimicking the original attachment method employed by the artist.
Tacks can be staggered and staples can be put in at an angle. In order to spread out the tension, the tacks or staples are usually placed closely together, about 1/2 inches apart. If the painting is unlined, this becomes more of a consideration, since a lining material usually helps distribute the stress within the painting.
The original canvas can be protected from the tack or staple in various ways. Some use blotting paper washers or linen lozenges. Others use Sunbrella strips, nylon tape, or fabric binding tape to edge the tacking margin, and then the staple or tack is applied through that. Because modern paintings must often be removed from stretchers for transport, these strips can be used as a “ripcord” to facilitate staple removal.
==Finishing==
Following the attachment of the canvas, the corners and the excess fabric must be finished. Usually, the corners are tucked in to safeguard during handling and framing. For unframed modern pictures, the corners can be finished off on the top and bottom so they are not visible during exhibition. The excess fabric is usually brought around to the reverse of the stretcher bars, ironed, and attached with staples, tacks, or escutcheon pins. The holes for the tacks or pins can be predrilled with a bookbinder's awl. Residual wax or other lining adhesive can be adhered to the reverse. BEVA® film can also be used, as this will help provide more even tension than just the tacks or staples used alone.
At times, the excess fabric is trimmed flush with the edge of the tacking margin so that the material is not visible. This would be an ideal aesthetic choice, but further restretching would require the addition of yet another strip lining fabric.
==Variations==
An Italian technique employed by Nancy Pollak for stretching lining canvases ensures that the weave pattern is perfectly square (see fig. 117, Variation 1).
Threads are marked on the warp and weft to act as a guideline for parallel attachment. The canvas is attached on both sides of the upper left corner. Then that parallel thread is followed along the top edge and attached to both sides of the upper right corner.
:
::::::FIGURE 117 Variations 1 and 2
Next, go to the bottom left corner and attach both sides, making sure the thread is parallel along the left side. Then attach the left side. Then attach the top edge. Then go to the bottom right corner and attach the corner on both sides, making sure that the threads at the upper right edge and lower left edge are followed along their respective edges. This will usually be quite difficult. For the right and bottom edges, start in the middle and work up toward the top right and out toward the bottom left. Then work toward the bottom right corner. As the bottom right corner is approached, it may be easier to adjust the corner by removing the staples there.
Laurent Sozzani uses a similar technique for stretching lining canvases, loose linings, and lined and unlined paintings (see fig. 117, Variation 2). It differs from the above technique in that the four corners are attached first, working clockwise from the top left. After all the corners are stretched, the sides are stretched, working from the corners in toward the middle.
==Recommendations==
Based on both discussions with other conservators as well as personal experience, several recommendations can be made. However, each conservator must factor in all of the variables before proceeding.
*Proper preparation: addition of strip lining, sizing of tacking margin, repair of the foldover edge.
*Initial positioning: position the painting on the stretcher face up or face down, depending on the painting surface. Place pushpins in both the middle and the corners, with very little tension. When the desired position is reached, place the painting in the most convenient orientation for stretching.
*Tensioning: using pushpins, work around the painting to achieve good initial tension. Either finger stretching or canvas pliers can be used. Even if the tension desired is low, canvas pliers can help reduce finger fatigue, enabling more consistent tension from start to finish.
*Attachment: either tacks (copper or other) or staples can be used. Pieces of mat board or other appropriate material can be added under the tack heads to help prevent further deterioration and damage to the tacking margin. Both methods of attachment dictate a certain amount of force applied to the sides or the reverse. Predrilling with an awl or hand drill can prevent some of this force. The use of BEVA® film as an attachment method could reduce this, keeping in mind that reversal may be difficult and there remains the possibility of adhesive failure.
*Corners: the corner should be tucked in for protection during handling. The frame may dictate whether the corners should be finished on the top and bottom or on the sides. Unless originally in a certain orientation, unframed modern pictures should have the corners tucked in on the top and bottom.
*Keying out: care should be taken in keying out. If too much tension is achieved while the canvas is slack due to ambient temperature and RH, the canvas can split during a change in the environment. It is advised to let the painting equilibrate for a while before keying out.
==Conclusion==
Conversations with many different conservators revealed the myriad of choices for each step in restretching a painting. Research has shown that restretching affects the paint layer more than the canvas layer, and by aggravating the stress field, a conservator can introduce more problems than are solved. Through practice, experience, experimentation, and scientific research, the merging of the actual and the theoretical can occur.
 Author's note: Several conservators were consulted for this article: Jim Bernstein, Ruth Cox, John Gayer, Gay Myers, The Philadelphia Museum of Art, Nancy Pollak, Laurent Sozzani, and Dean Yoder.
Noelle Ocon
Submitted November 2000
=Fixing Paintings in Work Stretchers without Flattening the Tacking Margin=
The following tips are different methods to suspend a painting in a work stretcher while maintaining the fold in the tacking margins. It is often helpful and possibly better for the paint layers to avoid flattening the tacking margin during a treatment.
Hollytex, Kraft paper, or Mylar® (or other suitable material) can be used. A right angle fold is made in the Hollytex to mirror the folded edge of the tacking margin. The Hollytex is then affixed to the outer edge of the tacking margin with BEVA® film (fig. 118). You may want to support the “bridge” where the folded right angles meet with wet strength tissue to avoid putting too much stress on the fold-over edge of the painting (fig. 119).
:
::::::FIGURE 118 Hollytex adhered to tacking edge with BEVA film
:
::::::FIGURE 119 Bridge the tacking edge and Hollytex with wet strength tissue
:
::::::FIGURE 120 Hollytex ironed to half of Fome-Cor with half of BEVA film left exposed
:
::::::FIGURE 121 Folded Fome-Cor is placed over the tacking margin and clamped with bull dog clips
:
::::::FIGURE 122 Hollytex cut perpendicular to edge of stretcher to allow for tension adjustment
Another method has been developed to make the attachment mechanically using Bulldog clips, Fome-Cor® (or mat board), and BEVA® film as follows. In this method, no BEVA® is attached to the painting. Four pieces of Fome-Cor® are cut to a length slightly shorter than each tacking margin and twice as wide as the tacking margins. Iron BEVA® film to one side, then score the Fome-Cor® down the middle lengthwise on the non-BEVA® side (so it can function like a hinge). You will also need folded Hollytex, as in the method described above, to suspend the painting in the work stretcher. Remove the silicone Mylar® carrier from the BEVA® film and iron the Hollytex to one side of the Fome-Cor®, covering only half (fig. 120). You will be placing the two right angle mirror folds of canvas and Hollytex together as you did above. The scored Fome-Cor® sits over these folded edges to support them. One side is ironed to the Hollytex and the other side contacts the canvas but is not attached. The folded Fome-Cor® (mat board) is then placed over the tacking margin (fig. 121). The uncovered BEVA® keeps the tacking margins from slipping out when under tension, and the bull dog clips act like a constant tension spring stretcher. This allows for a little give if too much tension is applied (for example, in the unfortunate occasion that the canvas shrinks rather than expands when placed in a humidity chamber).
After using one of the above methods, you are ready to attach the painting to the work stretcher. Velcro® is used to attach the Hollytex to the work stretcher in the following manner. Using the self-adhesive type Velcro®, adhere the back of the hook side (plastic side) of the Velcro® to the tacking edge of the work stretcher (the outer edge) and attach the loop side (fuzzy side) to the hook side. Center the painting in the work stretcher face down. Expose the adhesive backing of the loop side along one edge of the work stretcher and fold the Hollytex over onto the adhesive (fig. 121). Next, do this on the opposite side, making sure there is enough tension to properly support the painting when suspended. Repeat this on the last two sides. It usually helps to separate the hook and loop and staple the loop part of the Velcro® onto the Hollytex to secure it in place. The Hollytex can then be cut perpendicular to the edge of the stretcher every few inches to allow for proper adjustment of the tension when the painting is restretched onto the work strainer (fig. 122).
Robert Proctor
Reprinted from 1999 AIC Painting Specialty Group Postprints Studio Tips
=Using a Barrier Between Tack and Canvas= 
==Introduction/Purpose==
A barrier between metal fasteners and the canvas can be introduced to provide additional protection to the canvas by isolating it from the potential effects of corrosion. Much of the damage to the canvas around fasteners such as nails, tacks, or staples is caused by corrosion of the metal, which in turn degrades the canvas in the area of contact. Introduction of a barrier minimizes the area of contact between metal and canvas, thus limiting the area of damage. This can be especially important if original fasteners, quite often corrosive, are reused. Another advantage of a barrier is that it makes removal of nails, tacks, or staples less intrusive. Because the fasteners are raised, they may be easily removed without damage to the canvas.
The use of a barrier between fasteners and the canvas generally has been done within the context of conservation. The barrier typically is introduced as a preventive measure if a painting is to be remounted or restretched. Cases in which such a barrier has been included in the original structure by the artist are not unknown, but are relatively rare. Even in cases where the barrier appears original, it remains a possibility that the barrier was introduced by a restorer if the painting has been treated previously (McGinn 1997).
==Materials and Methods==
===Piece Goods/Tab Barriers===
Various materials can be cut into small pieces to serve as a barrier between fastener and canvas. These materials are generally cut into small squares or tabs slightly larger than the tack head or into small circles using a stamp or hole punch. The small pieces of material are first pressed onto the point of the nail or tack, and then the tack and barrier are attached to the painting as a single unit. The barrier may also be held in position while the fastener is applied. This is typically done with staples and can be awkward without practice (Chevalier 1997, Walker 1997).
Materials cut into small pieces to serve as barriers can be divided into two categories: thick and thin materials. Thick materials include blotter paper, acid-free mat board, and Nalgene® foam. Leather has also been used in the past, particularly in Europe (Chevalier 1997, Walker 1997). The thicker materials provide good barrier protection, more cushioning, and the tacks tend to be easy to remove as they are slightly raised. Thin materials used by conservators include Mylar®, neutral pH or acid-free paper, and filter paper (Seifen 2000a). These thinner materials also provide good barrier protection and may be more appropriate in situations where any additional thickness could be problematic for framing (Seifen 2000a).
===Twill Tape/Continuous barriers===
Another method involves the use of cotton twill tape. The tape is held in place along the tacking margin while the nail, tack, or staple is applied. In this case, the twill tape forms a continuous barrier around the tacking margin of the painting, thus offering edge protection as well as barrier protection. Twill tape is widely available in a variety of widths and colors, and may be chosen to suit the painting (McKay 2000, Phillips 1998, and Webster-Cook 1998). A variation of this method involves the use of single-ply mat board strips in place of the twill tape (Webster-Cook 1998).
==Variations==
===Tape===
In combination with staples, conservators have also used paper tape, linen tape, and Tyvek® tape to serve as barriers (Seifen 2000b). The tapes also provide continuous edge protection.
:b) Mat Board
Strips of single-ply mat board can be used in place of the twill tape to form a continuous barrier around the tacking margin (Webster-Cook 1998).
===String===
String is also occasionally used as a barrier, although the main objective of its use is to facilitate remounting. The string is placed along the tacking margin and staples are applied over the string as the painting is restretched. If the staples need to be removed to adjust the canvas placement or tension, one need only pull on the string to remove the staples quickly and easily. The string can be left out in the final stages of restretching and staples can be applied directly to the canvas, or the string can be left in place to serve as a barrier (Lawrence 1998).
===Fabric===
Strips of fabric, such as scraps of lining fabric, can be similarly used. The scraps of fabric are stapled in place as the painting is being aligned on the stretcher, and then they are pulled out for final attachment of the canvas to the stretcher (Pollak 2000).
Maria Sullivan
Submitted February 2000
=Removing Tacks=
A method is described for safely removing tacks from tacking margins.
==Introduction==
Iron tacks are often quite difficult to remove safely without damaging the tacking margins. Rusted tacks are even harder to remove because the corroded iron expands slightly, making the shank fit tighter in the surrounding wood. Ordinary double bevel wire cutters cannot get close enough to the underside of the tack head to be very useful. Sharpened screwdrivers, or other thin-bladed tools, rely on prying from one side at a time, inevitably resulting in damage to the tacking margins and possibly breaking off the tack head.
The electronics assembly industry routinely uses small flush-ground wire cutters with bevels on the topside to trim copper wire smooth with the surrounding surface. In paintings conservation, this same tool works exceedingly well for the removal of tacks whose heads are flush, or even below, the surrounding surface. The flush wire cutter is not used to cut the tack heads off, but just to facilitate removal of the entire tack.
Jaws in cross section (fig. 123):
:
::::::FIGURE 123 Jaws of wire cutters in cross section
==Use==
In preparing to remove the tacks, place the painting face up on a table at a convenient height and near the edge. Slide the painting off the edge of the table just enough to hold onto the stretcher bar firmly from below. Using the flush cutters with the handles down, grab hold of the tack head, twist slightly side to side to loosen the shank, then press the jaws toward the tacking margin and squeeze the handle grips slightly. The jaws should suddenly slip under the tack head, causing the tack to pop out of its hole. The condition of the tack usually allows reuse, if desired. The tacking margin should remain intact.
On certain paintings, the tack heads have been hammered below flush with the tacking margins. If the above technique fails to work, a slightly invasive procedure usually makes removal possible. With the handles roughly parallel to the picture plane, use the pointed tips of the flush cutters to puncture the tacking margin on either side of the tack head. Now grab the tack head firmly, twist slightly, and pull the tack outward. Even if the tack does not come out, this action loosens the shank's bond to the wood and should now allow a successful removal using the first technique. Usually, even the most tenacious tack can be removed with only a small amount of damage to the tacking margin. A limited and controlled damage is preferable in this case to the alternative of slowly wearing away the tacking margin around the tack head in an attempt to “gently” remove the stubborn tack.
Since the painting is face up while removing the tacks, leave about every other tack in place, but loosened, before turning the painting face down for removal of the stretcher. While face down, the loosened tacks can be quickly and easily removed. Many times, one tack is overlooked. Check for additional tacks by sliding a microspatula all around the perimeter between the inside of the tacking margin and the stretcher bar. You will not only find a tack or two occasionally, but also release the canvas fibers caught in the tack holes in the stretcher.
Flush wire cutters are available from many sources, including suppliers for jewelers and electronics assemblers. Most companies sell several different types, any of which are suitable for this use. The best designs have pointed tips, soft grips, spring-loaded handles, and full flush cutting jaws, not shear flush cutting jaws. Realize that these small single bevel wire cutters are not as durable as double bevel cutters; consequently, they wear out faster. Occasionally, filing the flush side of the jaws smooth will keep them working well for years.
==Distributor==
Techni-Tool: typical flush wire cutters
*Xuron Micro-Shear Cutter: #892PL1026 (manufacturer's #170IIA)
*Techni-Tool Shear, Standard Flush Cutter: #758PL172
James Hamm
Submitted January 2007
=Reusing Tacks= 
==Introduction==
A method is described whereby tacks carefully removed from a stretcher can be reused if they are in sound condition and are of historical importance.
==Materials and Method==
Tacks are carefully removed from the painting's tacking or stretching edge and inserted into the edges of a foam core board scrap. The tacks are placed in order as they are removed. The scraps are labeled to identify from which edge of the painting the tacks were removed. When the painting is ready to be remounted onto the original stretcher/strainer, it is positioned so that the tacking edge holes realign with the holes in the stretcher. The tacks are simply twisted and when they are properly “seated,” they are pushed into place. If care has been taken to keep the painting and stretcher system in the same shape as originally found, the tacks should fall back into place without requiring hammering.
Margaret Sutton
Submitted September 2000
=Securing Stretcher Keys=
==Introduction==
Stretcher keys are secured to prevent their falling out of the slots and being lost or being lodged between the stretcher and the back of the canvas where they may cause distortions and paint damage. References were found in the conservation literature, in artists' handbooks and manuals. Information was also derived through personal experience and conversations with colleagues. The securing methods fall into three categories: tying the keys in place, adhering the keys in place, and blocking the keys with some mechanical system.
==Materials and Methods==
===Tying Keys in Place===
====Description====
This system involves drilling holes into the keys through which thread, string, monofilament (fishing line), or wire is fed, which in turn is tied to a nail, screw, or screw eye on the adjacent stretcher bar verso.
====Precautions====
The element attached to the stretcher bar should be screwed into place as opposed to being nailed, to avoid unnecessary vibration and shock and risk to the canvas verso.
====Advantages====
Provided line is well tied, this holds keys securely.
====Disadvantages====
Keys must be removed for drilling of the holes and may split during the process. Threading the line can be awkward. Requires drilling or screwing into the stretcher verso.
===Adhering Keys in Place===
====Description====
This system involves applying a bead of adhesive along the edge of the key lying parallel to the stretcher bar. The tips of keys can also be dipped in molten wax. Adhesives have included wax, white glue, and silicon caulking or sealant. A variation uses adhesive tapes such as masking tape, duct tape, or gummed paper tape applied over the key. Surfaces should be dust-free to ensure good adhesion.
====Precautions====
Adhesive bead should be minimal. All applications should be left to dry overnight before application of backing boards to allow adhesive fumes time to dissipate. Silicon caulking should not emit excess acidic fumes (CCI 1993).
===Advantages===
Generally easy to do and the materials are readily available. The appropriate silicon sealant is readily reversible, with minimal damage to the keys or stretcher bars.
====Disadvantages====
The keys are not able to expand the stretcher without removing the adhesive. Some adhesives dry out or lose flexibility and adhesive power with time (e.g., wax, masking tape, and duct tape). White glue dries harder than the wood, and it becomes impossible to remove the keys without splintering the wood of either the key or the stretcher.
===Blocking Keys in Place===
====Description====
Nails, brads, or screws are driven into the stretcher bar just behind the short side of the key in order to prevent the keys from moving. Plastic “wedge retainers” available in the UK are a variation of this method, whereby the retainer is nailed into place on the stretcher bar, with its “fingers” around the ends of the wedge.
====Precaution====
Canvas must be protected from the hammer used to insert brads or nails.
====Advantages====
Generally easy to do and the materials are readily available.
====Disadvantages====
Vibration and shock resulting from hammer blows and accidental contact between the hammer and canvas verso may damage the painting. Brads and nails may eventually work loose, so that they and/or the keys become lost or wedged between the stretcher and canvas verso.
  Author's note: In addition to the resources consulted below, I also drew upon personal experience and discussions with colleagues.
Sandra Lawrence
Submitted May 2001
=Tips for Making Keys=
==Materials and Methods==
===Wood Type===
Keys are made in a variety of sizes and thicknesses ranging from 1/32" to 1/4" thick. A wood of medium hardness, such as a kiln-dried poplar, is generally a good choice. If the wood used to make the key is too hard, it can be difficult to insert into the stretcher. If the key is thin (such as 1/32" thick), a harder wood such as maple or cherry would be chosen. The most common key angle is 16° (although this can vary from 12° to 25°).
Barbara A. Buckley
Telephone conversation with Simon Liu, March 1999
===Replacement Keys===
Wood paint stirrers are useful for making replacement keys for stretchers.
Michael Swicklik
Submitted February 1999
===Cutting Stretcher Keys===
Similarly, a lath strip can be used to cut stretcher keys using the following pattern:
::::::This can be done easily with a jigsaw or small handsaw.
Mark Bockrath
Submitted 1999
===Plexiglass Stretcher Keys===
Replacement stretcher keys were needed for a 19th-century American canvas painting as the worn original keys no longer served their proper function. Problems arose when trying to replace the original thin wooden keys with new wooden keys of the same thickness (less than the standard 1/8" or 3/32"), as the wood tended to split when being cut or might have split later when being tapped into the stretcher. Plexiglass keys were found to be a viable alternative.
Using 1/16" thick plexiglass, the desired shape and size of the key was scored into the surface of the plexiglass with the aid of a template. The keys were then cut out of the plexiglass with a band saw. The edges of the plexiglass keys were roughened with coarse sandpaper (a file could also be used) so that they had greater “tooth” and would not slip out after being installed into the stretcher. The keys can be secured by dipping them into Lascaux Acrylic Adhesive 360HV before they are tapped into the stretcher, or they can be tied on with nylon monofilament cord anchored to the corner of the stretcher.
Neil Cockerline
Reprinted from 1991 AIC Painting Specialty Group Postprints Studio Tips III
===A Table Saw Jig for Replicating Stretcher Keys===
The following is a schematic drawing of a jig used to cut stretcher keys on a table saw (fig. 124). It adds 45° to the angle of the miter-gauge, allowing one to adjust for cuts to duplicate any angle of wedge key needed. To make keys using the jig, mill stock to the desired thickness. Cut strips to the desired width, taking into account the direction of grain desired for the key. Stacking the strips four high, eight keys are produced with two cuts: first the angle cut and then the square cut, which can be done on a chop box if one is available.
:
::::::FIGURE 124 A table saw jig for replicating stretcher keys
Steven Prins
Reprinted from 1991 AIC Painting Specialty Group Postprints Studio Tips III
===Patterns for Common Sizes and Shapes of Keys===
Because the wrong size key can damage an old stretcher, we save old stretcher keys and also have a stockpile of new keys of the most common sizes and shapes, as well as special shapes (for folk art stretchers that have tiny little slots, for example).
Patterns for A and B are, perhaps, the most commonly used key sizes. Pattern D is useful for 19th century stretchers with tiny slots.
Gay Myers
Submitted January 2001
:
::::::FIGURE 125a Key pattern A: 3/32" or 1/8" thickness
:
::::::FIGURE 125b Key pattern B: 5/32" thickness
:
::::::FIGURE 125c Key pattern C: 5/32" thickness
:
::::::FIGURE 125d Key pattern D: 3/16" thickness
:
::::::FIGURE 125e Key pattern E: 3/16" thickness
:
::::::FIGURE 125F Key pattern for single key slot
===Pattern for Common Single Key===
A pattern for a single key used for a half-mitered bridle joint with diagonal key slot (fig. 125f). The pattern was traced from the stretcher key of a Jacob Eichholtz painting from 1814.
Barbara A. Buckley
Submitted 2001
=Cutting Out Holes for Expansion Bolt Stretchers=
Two methods are described for removing excess canvas/fabric over the ring openings of expansion bolt stretchers.
==Use of a Template==
Excess fabric covering the hardware in the metal collars of expansion bolt stretchers can be cut away using a coin as a template, with tidy results:
*Find the location of the covered expansion bolt hardware.
*Size the fabric with Rhoplex or diluted PVAc or PVAc-EVAc emulsion adhesive, extending 1/2" beyond the collar circumference.
*Place a nickel or a quarter over the metal sleeve, making sure to center the coin.
*With a pencil, mark a circle around the coin and cut out the fabric with a sharp blade, such as a #11 blade surgical scalpel, or simply cut out a circle in the fabric using the coin as a template.
Dee Ardrey
Reprinted from 1991 AIC Painting Specialty Group Postprints Studio Tips III
==Use of a Cattle Dehorner or Arch Punch==
For all painting conservators who labor over cutting away the fabric over the ring openings of expansion bolt stretchers, an option for easier cutting is available. Because of our “western” influences, we began using a cattle dehorner, purchased at a veterinary supply. It is inexpensive and is the correct diameter. Its drawback is that the steel is not hardened and needs to be sharpened
often. Of course, this specific tool may not be accessible to everybody. After investigation, we found a second tool, an “arch punch,” which works even better. It can be found through any company that sells industrial tools. It is hardened cast steel and holds a better edge. We put a more comfortable handle on the top for easier cutting of the fabric in a single turn.
Randy Ash and Hays Shoop
Reprinted from 1991 AIC Painting Specialty Group Postprints Studio Tips III
=Backing Boards=
==Introduction==
===Purpose===
Backing boards are attached to the reverse of a painting's stretcher to protect the canvas from dust, debris, framing wires, and accidental damage. By protecting the canvas reverse from knocks and scrapes, the chance of mechanical cracking caused by these blows is reduced. Backing boards have even at times prevented a blow to the face of the painting from becoming more severe by reducing the potential damage (i.e., a blow to the face of the painting by an object may cause a dent in the canvas rather than a tear). Backing boards have become a good place to put shipping, exhibition, and ownership labels (see section 15). Paper dust covers are a framers' variation on backing boards. As their name implies, they prevent dust and debris from accumulating on the reverse of the support but offer little protection from accidental damage.
===Environmental Buffer===
Until about the 1980s, the corners of a backing board were often canted to create an air space, or holes were cut into the center of the board with the reasoning that the canvas should “breathe.” It was felt that otherwise the backing board would create a microclimate. Current understanding is that a backing board should not have air holes and that it creates a buffer from the surrounding environment and can be helpful in slowing down the reaction time of the painting to shifts in climatic conditions (DiPietro and Ligterink 1999). The exception to this would be for paintings in uncontrolled tropical climates; conservators often opt not to put a backing board on paintings that will return to uncontrolled environments (Levenson 2006).
===Travel/Vibration Control===
A backing board dampens vibration during travel by making a plenum of dead air space between the board and the fabric support. Sealing the backing board will maximize this effect (Green 1991). If a backing insert is not used for travel, to add stiffness to the system it is recommended that a backing board be attached to each opening on a large painting with crossbars. It is the stiffness of the backing board that resists the pushing effect of the resonating canvas, thereby reducing vibration.
==Materials==
=== Foam Board===
:Foam board is available from many manufacturers. Foam board is composed of an extruded polystyrene foam inner core with paper facings that can be either acid-free or acidic. Many conservators believe that a painting is an acidic system and therefore that it is unnecessary to use an acid-free foam board. An advantage of using an acid-free foam board is the longevity of the board itself.
===Corrugated Paper Board===
Corrugated paper board is available both in acid-free and more commonly available acidic boards. Unless other materials are unavailable to the conservator, an acidic corrugated backing board is not recommended because it will not have the long-term durability of a better quality material. A double-wall, acid-free corrugated board is generally recommended for use, not only because of its durability, but also for its strength, thus offering better protection for the painting.
===Coroplast®===
Coroplast® is a rigid corrugated plastic that is a copolymer of polypropylene and polyethylene. It is available as white, brown, or semitransparent in 3 millimeter-thick sheets. The ability to see through the semitransparent type has its advantages.
===Kraft Paper===
Kraft paper is most commonly used for a dust cover. Typically, the Kraft paper is attached to the frame reverse (rather than the stretcher) with double-sided tape or, more traditionally, with gummed tape or an aqueous glue or paste. The disadvantage to paper dust covers is that they can be damaged in handling and do not provide protection from mechanical damage. In addition, they do not provide the advantages that a backing board can have of buffering climatic changes or absorption of vibration during travel. Another disadvantage is that a paper dust cover has to be removed when unframing a painting.
==Materials for Attachment==
===Washers and Screws===
Washers and screws are the most commonly used materials for the attachment of backing boards. They provide a secure attachment to the stretcher that can readily be removed. The disadvantage to using washers and screws is that they put holes into the stretcher.
===Velcro®===
Velcro®, a hook and loop tape, was a method of attachment used in the 1970s. The advantage to this system is that no screw holes are put in the stretcher. Glue, however, often didn't hold the tape to the stretcher, so the tape was stapled to the stretcher. The disadvantage to the method is that the backing board can be hard to pull off and reattachment can be awkward.
===Mending Plates===
Mending plates can be used to avoid the use of screws in the stretcher. In this method of attachment, the backing board and painting are both held in place within the frame with mending plates. A disadvantage of the system is that when the painting is unframed, the backing board is removed thus exposing the painting to possible mechanical damage during handling. Some also feel that the painting is not held in the frame as securely.
==Method==
a) Backing boards are generally cut with a utility or mat knife although some technicians use a table saw. The edges of the board can be finished by sanding.
b) When replacing a backing board, line the new backing board up with the old one and, using an awl, puncture a hole into the new board at the same points as the old board so that new screw holes will not have to be made in the stretcher.
==Variations==
a) A window in the backing board can be made if there is an inscription on the canvas reverse that could be easily viewed through the window. The window can be covered with a heavyweight piece of Mylar® (5 mil) adhered with double-sided tape secured to the inside of the backing board. A window in the backing board may reduce the vibration dampening effect during travel. Other variations, for reasons of transparency, might include the use of Mylar® or plexiglass.
b) In addition to paintings on canvas, backing boards are often used for paintings on thin wood panels, hardboard, or canvas panel but are secured only with bent mending plates (no hardware through the panel support). They are used to protect the reverse of the panel from scratches and also provide a place to put labels. Backing boards can act as a cushion for framing hardware, such as mending plates, from pressing into the surface of the panel. An acid-free mat board can also be used for protection, especially in the instance of a canvas panel.
c) Polyester mesh or screening fabric can be attached to the stretcher verso. The mesh or screening will allow for the circulation of air, which may be required in an uncontrolled tropical climate, and also offer some protection from debris or accidental knocking of the canvas verso (Levenson 2006).
Barbara A. Buckley
Submitted January 2007
=Labels on Stretchers=
==Label Preservation==
Many label types may be found on stretchers, and the variety of information gleaned from labels may prove useful for analysis, authentication, or study. Stretcher labels may be classified into one of the following two categories: those relating to the manufacture of support(s) and those relating to the history and use of the painting (i.e., any label placed on the stretcher during or after the artist used the support for painting). The former category includes those labels that convey information about manufacturer or supplier of stretchers and/or pre-primed canvases that were tensioned onto stretchers. Also included in this group are labels that identify a patent date for a new type of stretcher, panel, or compressed board (Katlan 1992). The latter category contains a greater array of label types, mentioned below. For example, artists have been known to attach labels to their painting's stretchers, which may give information about a painting's date, title, or subject. An artist's label may even go so far as to offer extensive information related to the artist's philosophy or to the meaning of the painting (Hamm 1999). A label may contain the name and address of the gallery that once owned the painting. An accession or inventory number related to current or past ownership may often be found alone on a label, without benefit of related information about the owner. Labels often convey information regarding a prior auctioning of the painting. Private owners may also place a label on a stretcher containing information about themselves or about the identity of the artist, subject, title, or date, among other potential information. Exhibition labels are commonplace today, stating information about an exhibition in which the painting was displayed. Labels are important to preserve primarily for the information that they contain and the support that they can lend to a painting's provenance. Furthermore, the preservation of labels may be significant in studying various histories, such as that of artist's materials (Katlan 1987).
The study of stretcher labels can profoundly influence the process of unraveling a painting's history. Alexander Katlan describes such a process (Katlan 1998). Katlan explains how a pair of stretcher labels aided in the reattribution of a 19th-century painting. One of the labels came from the firm that owned the painting in the early 20th century. The second label contained an inventory number, which, it was discovered, related to the company listed in the other label. Because the company exists now as M. Knoedler & Co., of New York, Katlan was able to trace the inventory number to a 1907 catalog. In the catalog, the painting was described as being signed by Jakob Simon Hendrik Kever. Through infrared reflectography and ultraviolet-induced visible fluorescence, inconsistencies related to the current attribution to the artist Joseph Israels were noted. Thus, the use of analytical examination in conjunction with research into the history of the painting's stretcher label supported the reattribution of the painting to Kever.
The history of stretcher label preservation has not been well recorded. Several references to the topic, however, indicate that documentation and preservation of the labels have been of concern to conservators since at least 1965. Caroline Keck discussed the importance of transcribing information found on stretchers during the examination of paintings (Keck 1965, 8, 52). Eleanor Quandt also mentioned the routine recording of such information by anyone who examines paintings (Quandt 1971, 345). At the Philadelphia Museum of Art in 1971, labels were routinely photographed and removed from their stretchers and stored in curatorial files, according to Theodore Siegl. Where removal was not possible, the stretcher was preserved (Quimby 1971, 373).
==Label Age==
Paintings from as early as the 14th century may be found today with labels attached to their supports. According to the results of a recent questionnaire, the age of these labels seems most often to be no earlier than the late 18th century. Earlier labels do exist, however, and these labels most frequently contain collector's marks, information about shipping or excise fees, and sometimes auction data or inscriptions (Curtis 1999). The majority of labels found today date to the 19th or 20th century. Reasons for this paucity of earlier labels vary. Public collections of paintings have only existed since the 18th century, as have public exhibitions of paintings. There may have been less of a need to label paintings prior to that time. Furthermore, labels may simply not have survived due to their vulnerability. Stretchers dating earlier than the 18th century are rarely still found with the paintings they originally supported, as newer stretchers have often replaced them. Such labels, if they existed, were not commonly preserved.
==Paper Type==
Very little has been published regarding the history of stretcher labels. It is reasonable to assume that the history of stretcher labels mirrors the history of paper. That is, early labels, or those dating to the late 18th and early 19th century were most likely made from rag paper. Labels from the latter half of the 19th and early 20th century were likely to have been made of wood pulp paper. Some labels made of wood or wood pulp paper may have been coated with a pigment or clay adhesive mixture to improve the finish. Coated papers present special problems for treatment.
==Adhesive==
Household manuals published throughout the late 19th and early 20th centuries contain recipes for labeling adhesives that include all of the major classes of water-based adhesives (vegetable and protein), often in combination (for examples, see Campbell 1867, Hopkins 1932, Moore 1879). These types of adhesives include animal glue (including fish glue), casein, vegetable gum/mucilage, and starch. Many adhesives were modified with odorants and preservatives. Adhesive mixtures might contain such ingredients as glue, dextrin (a modified starch), casein, gum arabic, gum tragacanth, and/or acacia gum (Hiscox 1944, 42–43). One historic recipe contains gum arabic, gelatin, glycerin, and camphor, while another contains gelatin and rock candy. Furthermore, aluminum sulphate was recommended as an addition to ordinary mucilage to improve the adhesion of paper to wood and other surfaces (Hopkins 1908, 342–343). In the United States, in 1862 Josiah Mitchell began making gummed labels in a small-scale production. This company grew into a company known as Salem Label in 1907 (Klein et al. 1994, 3). Modern gummed tapes generally consist of adhesives applied to a Kraft paper carrier. The adhesives are primarily composed of animal glues (bone and hide), with a small amount of dextrin added, and frequently other additives (Skeist 1962, 578). A similar type of gummed paper may have occasionally been used for stretcher labels during the 20th century.
Rubber-based adhesives for tapes and labels have been used since 1928, when the first masking tape was introduced by 3M Company. In addition to rubber, these adhesives often contain several types of additives, including tackifiers, plasticizers, pigment fillers, and antioxidants (Book and Paper Group 1992, Section 15:14–15). Rubber-based adhesives do not age well overall, especially those manufactured before the late 1950s, which contained wood rosin or modified resin tackifiers (Feller and Encke 1982; O'Laughlin and Stiber 1992, 282). As the adhesive oxidizes and deteriorates, the paper carrier often releases. For this reason, it is unlikely that many of the early labels containing rubber-based adhesives would have survived intact on a stretcher or frame.
Polyacrylates have been used widely for pressure-sensitive adhesives since the late 1950s. These adhesives may contain tackifiers, plasticizers, and fillers (Book and Paper Group 1992, Section 15:19–20). Pressure-sensitive tapes and labels tend to lose their adhesive qualities over time. When they are still tacky, however, they can be quite difficult to remove. Removal may require the use of solvents or the application of cold or heat. Aside from pressure-sensitive labels, pressure sensitive tapes, with either rubber or acrylic adhesives, may be found attaching the edges of the labels to stretchers. Although adhesives were typically used to attach labels to stretchers, one might also find a label attached to its stretcher with staples or tacks.
==Media==
Commonly, labels carry printed information, while some may also be typed or handwritten. In general, printing inks are insoluble in water and most organic solvents (Book and Paper Group 1990, Section 3:26). Writing inks are frequently quite sensitive to moisture, depending on the type of ink used. Iron gall ink is relatively water-sensitive, and contact with water should be limited or even avoided. Fountain pen, ballpoint pen, and felt tip pen inks can be extremely sensitive to water and/or organic solvents. Such inks should be tested for sensitivity before any contact with moisture. Other media applied to labels are graphite and wax crayon. Graphite is generally stable in contact with moisture but can be smudged. There is always the possibility, however, that copy pencil, which contains a water-soluble purple dye, was used rather than graphite. To be safe, it is a good idea to test media that appears to be graphite with a water droplet. Wax crayon will often soften or dissolve in contact with organic solvents (Book and Paper Group 1990, Section 3:8). The use of solvents in the removal of a label that contains wax crayon should therefore be avoided.
==Treatment Rationale==
===Paper Preservation===
Stretcher labels may be found in a poor state of preservation. Such degradation may stem from poor quality paper. Additionally, the acidity of the stretcher may have accelerated or created paper degradation. If the stretcher is relatively old, especially over one hundred years, it is unlikely that the wood will still off-gas and cause further damage to the paper. However, if the paper is tearing off on its own and is terribly brittle and friable, the paper may be best preserved through its removal from the stretcher and subsequent treatment to stabilize the paper.
If the label is no longer well-adhered to the stretcher, it is vulnerable to any type of handling. The label, in this case, is subject to being lost, partially or entirely. The conservator needs to decide whether it is best for the label and the painting to leave the label in place and attempt to reattach any loose edges or to remove the label and possibility treat it.
A stretcher label may also simply lie in a vulnerable location. For example, if the label is attached to the stretcher near the hanging hardware, it might easily be damaged during the hanging of the picture (Hamm 1999). It might be wise in this case to devise protection for the label and/or move the label to a better location in contact with the stretcher or backing.
===Treatment Interference===
Sometimes a label requires removal in order to treat the painting. A clear example of this situation is when a label bridges the stretcher and the frame. It would not be possible to unframe the painting without removing the label from at least one of the supports. A label may also be covered during treatment, if it is not removed. If a lining fabric would otherwise cover a label, the conservator might wish to remove the label (Curtis 1999).
===Stretcher Disuse===
During the treatment of paintings, it is sometimes desirable to create a new stretcher rather than reuse the old one. In this case, the removal of the label may help ensure that it is kept with the painting, especially if it is reattached to the new stretcher or backing board. In some cases, the stretcher along with its label(s) may be preserved. In order to better ensure the preservation of the information contained on a label, however, it is recommended that the label be returned to the new stretcher in addition to placing the information into the conservation and/or collection object file in the form of a transcription, photograph, and/or a photocopy of the label. If the label is particularly difficult to remove, another option is to cut off the section of the stretcher bearing the label and attach this whole section to the new stretcher (Curtis 1999).
===Label Preservation without Removal===
====Loose Edge Reattachment====
If the edges of a label are not well adhered to the stretcher, one solution is to reattach the loose edges to the stretcher. This may be the best option if it seems that the label is too fragile to remove, if there is not enough time to remove it, or if it is in good condition and is simply in danger of being damaged.
Wheat starch paste or a combination of wheat starch paste and methylcellulose (1:1) are appropriate adhesives for reattachment. The adhesive may be applied with a small brush to the detached portion of the label or to the wood. The label should then be reattached to the wooden support with gentle pressure. One method of applying pressure is to place a small piece of Mylar® over the label and then press a stiff brush across the surface of the Mylar®-covered label, brushing from the center toward the edges. This method helps ensure that even attachment occurs. The Mylar® should then be slowly peeled back at a low angle from the label. Any excess adhesive should be removed with small damp swabs. Avoid excessive moisture and/or paste down the label in increments to avoid cockling.
====Protective Cover====
A Mylar® cover may be applied over the stretcher label. For this purpose, a thick Mylar®, such as 3 or 5 mil, offers the most protection and is least likely to become creased during application. The Mylar® should be cut larger than the label to allow for sufficient room for the points of attachment to the stretcher. Double-sided tape, 3M #415, may be used around the perimeter of the cover. Although the tape creates a new problem for future removal, this technique is quick and easy and gives a good seal between the cover and stretcher. Another possibility is to use copper tacks or screws to attach the cover to the stretcher. It may be difficult to apply the tacks or screws while keeping the Mylar® perfectly smooth, as the tacks have a tendency to cause the Mylar® to bulge unevenly. The use of Monel or stainless steel staples is another option. If it is desirable for air exchange to take place between the Mylar® and the stretcher to avoid off-gassing acids being trapped, one of the latter two methods may be best. If the stretcher is fairly old, though, such damage would be unlikely. A small piece of Plexiglas® may also function as a protective cover (Curtis 1999). While Plexiglas® is a strong material offering good protection, it is much more difficult to cut and manipulate than Mylar®.
Whether the label is left alone or reattached to the stretcher, a cover serves as protection against future damage to the label. A cover allows for easier handling of the painting and guards the label from abrasion or damage from sharp instruments. The cover also protects the label from pollutants.
====Backing Board====
A backing board offers protection for a label, but it also can hide the label. Current practice, as noted in a 1999 questionnaire (Curtis 1999), may include various techniques that make stretcher labels more accessible. In order to alert the viewer, one option for the conservator is to note the stretcher label's location on the backing board. This notation alerts the viewer of the label's existence and location. Further steps may be taken to make the label or its contents more visible. A window may be cut in the backing board so that the label itself may be seen. Another option is to attach a photograph or photocopy of the label to the backing board.
==Removal Technique==
===Dry Removal===
Dry removal incorporates the use of purely mechanical means to detach a label from its support. For dry removal, one can use a variety of tools, including a microspatula, a Teflon®-coated microspatula, a Teflon® spatula, a palette knife, a bamboo spatula, or silicon release paper. The microspatula gives the conservator a great amount of control in its manipulation. The use of freshly cut, double-sided, silicon release paper can perform well as a tool that releases the label from the stretcher and offers support for the label (Book and Paper Group 1992, Section 15:31). The use of a scalpel can be risky for the label and/or the stretcher.
In a situation where label removal from a discarded stretcher is particularly difficult, the conservator may choose to cut into the stretcher to begin removal. The wood may then be pared down from the reverse, partially or completely (Curtis 1999).
In order to remove a label that is already partly detached, the microspatula or palette knife may be placed under the loose edge(s) of the label. The tool may then gently slide farther between the label and the stretcher. It may work best, if there is more than one area of detachment, to attempt gradual removal. Each of the areas of detachment can be tested, and the label may be released in one area at a time. It may also work best to move across the whole label beginning from one edge, depending on how easily the label releases from the support. One might also try dry removal on a label that is not partly detached and quickly judge whether or not this technique would be useful. This method is particularly useful for the removal of new labels that were recently applied and are not very well attached to their stretchers. Its primary advantage is that the label and stretcher are not subjected to moisture. Also, if the painting is attached to the stretcher during removal, there is no chance of humidity disturbing the nearby canvas. If a label contains a medium that is moisture sensitive, it would be advantageous to attempt the dry technique. On the other hand, dry removal might cause damage to a label if the label does not readily release from the support. In this case, the label might easily be skinned or torn. Caution should be used when attempting this technique.
===Moisture===
====Special Consideration====
=====Sensitive media: testing and fixing=====
If any medium on a label is suspected of being water sensitive, it should be tested before beginning an aqueous treatment. To test for sensitive media, a blotter moistened at the corner with water may be lightly touched to the edge of the least conspicuous area possible of the medium and quickly released. If no offsetting occurs onto the blotter and no bleeding occurs in the area tested, further testing may be undertaken, gradually increasing the amount of water and increasing contact between the blotter and the medium. This procedure should be repeated on different colors and/or types of media found on the label until the conservator is satisfied that the medium is safe in contact with moisture. The same type of procedure should be carried out with any organic solvent that is being considered for use.
If the medium is determined to be water sensitive, the conservator might first wish to consider dry removal or only limited use of moisture. If dry removal is not possible, then the medium might be temporarily fixed before proceeding with the use of moisture. Options for the medium fixing include the use of either cyclododecane or Paraloid B-72. Cyclododecane, which is a volatile cyclic alkane, has the distinct advantage of its sublimation after a period of fifty or more hours (Brückle, Nichols, Strickler, and Thornton 1999, 162). Thus, working time is allowed and solvents are not required to remove the cyclododecane from the label. B-72 may also be applied to water sensitive media. It requires the use of a solvent such as xylenes to remove it, once the aqueous label removal and possible subsequent label treatment have been completed. The advantage of B-72 over cyclododecane for the painting conservator is that B-72 is much more readily available and may be easier to use. Furthermore, the cyclododecane must be melted before application as a fixative. Some practice using this material is advantageous. If available, cyclododecane seems the better choice for some applications.
=====Coated paper=====
Coated paper consists of paper that has been covered on one or both sides with a mixture of an adhesive and a clay or pigment. Such coatings provide the paper with certain qualities, such as a very smooth surface for printing. Coated papers present special problems for treatment. Moisture can easily damage such papers. In addition, coated papers may cause flaking media, because inks often do not adhere well to a coated paper surface (Book and Paper Group 1990, Section 4:36). This problem may result in a loss of media and, consequently, the information conveyed by a label.
:::It is important to be able to identify a coated paper as such before attempting to remove it. This type of label seems best left in place on the stretcher. If the medium is flaking, consolidation may be attempted using methyl cellulose, Klucel, or B-72. Darkening of the coating may occur.
====Humidity Tent====
If it seems important to limit the contact of the label with moisture, a humidity tent using an indirect application of moisture seems to be the safest method. For this method, a moisture carrier is used in conjunction with a moisture barrier. One simple application of this technique is to cover the label with a barrier, such as a lightweight Hollytex. A moistened blotter is then placed atop the Hollytex, and a piece of Mylar® or polyester sheeting should cover the blotter to prevent quick evaporation of the moisture. The package is allowed to sit for a short period, approximately ten to fifteen minutes. The label should be checked periodically in order to determine whether the label is beginning to release from the stretcher. When the label seems to be releasing easily, the label should be uncovered. If the label is very weak and/or seems too damp, it may be allowed to dry slightly on the surface before beginning its removal. The paper strength is increased in this somewhat drier state. The conservator may then apply gentle mechanical pressure to completely remove the label. A microspatula may be glided between the label and stretcher to release the label. Another option is to cover the label with a small piece of Mylar®, slightly larger than the label. The label might then be slowly peeled back from the stretcher (Curtis 1999). If this method works correctly, it can be safer than using a microspatula, which could potentially skin the paper. The conservator may also try the use of a microspatula in tandem with the Mylar®. The Mylar® would then serve as a support as the label is released from the stretcher.
An advantage to this system of indirect application of moisture is that, ideally, only enough moisture is used to soften the adhesive and release the label. The stretcher does not become very wet, nor does the label.
====Direct Moisture Application====
=====Poultice=====
If the paper appears very sturdy or the above method was attempted and there did not seem to be sufficient moisture for the label removal, a direct application of moisture may be preferable. Although some conservators may prefer to use a brush or cotton wool to apply water directly onto the label, it is easy to cause tide lines using such means and may be safer for the label to use a poultice. For this technique, moistened blotters, cotton wool, paper towels, or Gore-tex® may be used. A Mylar® cover over the poultice will keep the moisture contained near the label. After applying the moisture, the label should be checked over a short time, approximately five minutes. Once the label seems to be releasing easily, the label should be uncovered. The conservator may then apply gentle mechanical pressure to completely remove the label. As described previously under the “Humidity tent” section, Mylar® might prove helpful in the label removal, as might a microspatula or a palette knife.
A damp poultice may prove more effective than a humidity tent in releasing a water-based adhesive, due to the increased moisture in contact with the label. This technique might still be performed on a stretcher with its painting attached because the amount of moisture introduced is kept to a minimum and may be effectively contained in a local area.
=====Facing=====
A facing may be applied to the label, if necessary. Materials that may be used for the facing include wet-strength tissue or thin Japanese paper, slightly larger than the label. A good adhesive for this purpose is methylcellulose (Methocel A4M, 2.5% solution). A small, synthetic fiber brush works well for the application of the facing. The brush should be somewhat stiff. A non-aqueous alternative for facing is the use of heat seal tissue.
To apply the facing, place the tissue over the label. Begin applying the adhesive through the tissue at the center of the label, working toward the edges of the label. Take care not to allow the adhesive to get into the stretcher bar. It might be necessary to relax the label with moisture before the facing application. A variation is to apply the adhesive directly to the label, then place the tissue onto the label and brush it out with a stiff brush. However, the first method allows less adhesive to lie directly on the label and thus provides for less adhesive removal in the end.
There are a couple of options for label removal after facing. Depending on the condition of the label, whether the painting is still tacked to the stretcher, and the ease of the label's removal, a specific technique for removal may be most desirable after facing. One option is to allow the moisture from the facing to work into the label and soften the adhesive underneath. After a short period of time (five to fifteen minutes), the label may be tested with a microspatula or palette knife and gently removed, if possible. If this does not work, steam might facilitate the treatment.
=====Steam=====
Steam might work when other possible techniques fail. While a steamer intended for paper conservation works best, a small, household steamer will serve nicely. Household steamers have a tendency to drip water, especially when held at an angle. It is a good idea to wrap a small towel around the joint holding the two parts together (the nozzle and the reservoir/heating section) to avoid dripping water onto the label or stretcher. The steam should be directed at one edge of the label and gradually moved down the length of the label until it is fully removed. A microspatula should be used as an aid in releasing the label from the stretcher. Thus, the microspatula is held in one hand, directing the release of the label while the other hand directs the steam at the label.
The use of steam would not be appropriate for a stretcher that still carries its painting, due to the amount of heat and moisture. Another disadvantage of steam is that the heat and moisture will cause the stretcher to become fairly damp in the area treated. The advantage of steam is that it can frequently release a water-based adhesive very quickly. The heat helps to soften the adhesive, especially if it contains animal glue.
=====Combination of facing and steam=====
For this method, a facing should be applied to the label as in part (b) with methylcellulose. Methylcellulose has the unique characteristic of gelling when steam is applied to it. Thus, it still performs well as an adhesive during this treatment. Immediately after the facing is applied, steam may be used to release the label as described in (c).
This technique is especially useful if the label to be removed is badly torn, very fragile, and/or very desiccated. That is, this method may be appropriate when normal techniques would be likely to damage the label further. Once again, the stretcher must be removed from the painting in order to perform this method. If there were no need to remove the painting from its stretcher, then this technique would not be applicable. The application of the facing serves to protect the label during its removal, while it also serves to pre-dampen the label and thus the adhesive beneath it.
If heat seal tissue is used to face the label prior to steaming, consideration should be made for the expansion of the paper during steaming. The heat-set tissue will hinder the natural movement of the paper during and after the application of moisture. Thus, if the label is made of paper that tends to expand excessively when dampened, then only a very thin Japanese tissue should be attached lightly or this combination of techniques should be avoided.
=====Immersion=====
If the stretcher will not be reused and its label is extremely difficult to remove, one might consider placing the entire stretcher in a water bath to help release the label (Curtis 1999). The condition of the paper should be carefully evaluated before considering such a technique.
===Cold===
For pressure-sensitive labels, reducing the temperature has been known to reduce the adherence of the label to the support. Pressure-sensitive adhesives are not as effective at low temperatures. One application to label removal is to apply a cold compress on the label. For this purpose, an instant cold pack would work well. Care should be taken to avoid condensation from forming next to the label. An additional container, such as a small plastic bag, would provide extra protection against such an occurrence. The cold pack should be allowed to sit until the label becomes sufficiently cold that it begins to release from the stretcher. It should become easily removable at this point. The advantage of using a cold pack rather than heat or solvents is that it is safer to the label and the wood. Also, it may prove to be the easiest solution in removal of a pressure-sensitive label. One concern in freezing a label, however, is the possibility of causing embrittlement of the paper (Book and Paper Group 1992, Section 15:43).
===Heat===
The use of heat is frequently effective at softening or releasing pressure-sensitive adhesives if the adhesive is still tacky (Book and Paper Group 1992, Section 15:40). Sources of heat include a heated spatula, a heat lamp, a radiant heat tool, or a hot air gun. A heated spatula would be the best to try on a stretcher that is attached to a painting because the heat is contained locally. A heat lamp or a hot air tool might be appropriate if the stretcher has been removed from its canvas.
A thin-tipped heated spatula is the easiest type to manipulate between a label and its support. The temperature should be carefully controlled. Use of a warmed microspatula (heated by contact with a tacking iron or by hot air application) can also be effective (Book and Paper Group 1992, Section 15:40). The spatula should be placed next to, or slightly under if possible, an edge of the label. As the adhesive releases, the spatula is moved across the edge and then systematically used to evenly release the rest of the label. Tweezers may be used to hold the released edge of the label away from the exposed stretcher during removal. Another option is to place silicone-coated Mylar® between the released label edge and stretcher to avoid readherence of the label.
If a heat lamp is used, it should be placed at a safe distance from the label and stretcher. Check the label periodically to determine whether the adhesive is softening and/or releasing; if and when it does, a microspatula or similar tool may be used to release the label.
A hot air gun may be directed at the label until the label gradually releases. A hot air tool, with controlled temperature and degree of air output, is preferable to a hot air gun. Again, an implement such as a microspatula or similar tool may be used to release the label.
Of these methods, the use of a heated spatula seems to have the most applications, since the painting may still be in place on the stretcher. The heat lamp might also be a good method to try. The advantage of using heat is that solvent use may be avoided. This technique may be effective and advisable, for example, if solvent-sensitive media are present. However, heat may prove damaging to the label or the wood, especially if too much heat is used. Heat may also cause changes in the adhesive. For example, the adhesive may become more resistant to solvents, especially with use of the heated spatula (Book and Paper Group 1992, Section 15:41). Also, the application of heat may cause the adhesive to migrate into the wood.
===Solvent===
Sometimes it may be necessary to use organic solvents to remove pressure-sensitive labels. A variety of solvents may be tested at the edge of the label to determine which solvent(s) will be effective. The solvent may then be applied at the edge of the label by feeding the solvent under the label using a swab or small brush. Another method that might be the most effective at releasing the whole label evenly involves the use of a solvent vapor mini-chamber. The cotton wool or a blotter is suspended in a jar or Petri dish larger than the label. A solvent or solvent mixture is then applied to the solvent carrier. The dish is then placed upside down over the label and allowed to sit until the adhesive releases. The label might then peel away easily. A microspatula or similar tool may be needed to help release the label. Warming the microspatula often works well following the label's exposure to the solvent mini-chamber (Book and Paper Group 1992, Section 15:70–71). Tweezers might also be used to hold the label away from the stretcher as it is released.
A disadvantage of solvent use is that solvents can affect certain media. Solvents may also drive the adhesive into the support. In some cases, however, the use of solvents may be the only method to remove a label effectively.
===Splitting===
When a label cannot be removed by any other method without serious damage, this technique might be considered. In some cases, for example, any use of moisture may cause an almost complete lack of structural integrity of the label. The label must first be faced with Mylar® and a non-aqueous adhesive, such as BEVA-371 film. A heated spatula or tacking iron may be used for this attachment. The label may then be separated from the stretcher by splitting the label with a microspatula and smoothly pulling the Mylar® and attached label away from the support. Ideally, the label should split in half evenly. Once the label is removed, it should be lined as described in 8.c below (“Lining”) to give the label greater strength. The Mylar® may then be released using heat and any excessive adhesive may be removed with solvents or a crepe eraser.
:Disadvantages of this method include a reduction in paper strength of the label, as well as the possibility that the label will not split evenly. Furthermore, paper and adhesive are left behind on the stretcher. However, there may be a case where the desirability of preserving a label through its removal from the stretcher will necessitate the use of any means possible. The advantages and disadvantages of using this technique must be weighed very carefully.
==Label Treatment==
===Adhesive Reduction===
Once a label is removed, it will almost invariably have adhesive residues on its reverse. If the label will not be immersion washed, it is best to immediately turn the label face down on a clean surface. A water-based adhesive may be reduced using moistened cotton swabs or small wads of cotton. If the label is to be bathed, the adhesive is generally most efficiently removed during immersion. After the label has been immersed 10 to 20 minutes, a soft brush may very gently be brushed across the reverse to release residual adhesive. If the label was faced, the facing can be released by first dampening the facing with dampened cotton wool and then gently peeling it back from the label. Residual methylcellulose may also be removed during immersion as described above. Adhesive residues may also be present on the stretcher. If the adhesive is very degraded and brittle, it may be removed mechanically with a scalpel or spatula. If the residue is minimal and the adhesive is water-based, moistened cotton swabs rolled across the surface might be effective at reducing the adhesive. If the adhesive residue is substantial, damp poultices may be useful, followed by gentle, mechanical action. Crepe or vinyl erasers are often effective at reducing pressure-sensitive label adhesives.
===Bath===
If the label is safe to immerse in water after media testing and examination, washing can be an effective method to remove soluble degradation products in the paper, reduce the acidity of the paper, and reduce the labeling adhesive (for water-based adhesives).
If possible, deionized or distilled water conditioned with calcium hydroxide to pH8-9 should be placed in a small tray. The label may next be pre-dampened by spraying on both sides with deionized water. The label may then be placed in the water bath. If the label is very fragile, it should be placed on a piece of Hollytex larger than the label. Then, it may be handled via the Hollytex. After placing the label and Hollytex on the surface of the water, the Hollytex is pressed to the bottom of the tray from one end to the other. The label may then also be gently submerged beginning at one edge and moving to the opposite edge, in order to immerse it and remove any air bubbles trapped beneath it. An alternative method is to simply dip one edge of the label under the water and quickly immerse the label. For related treatment variations, Chapter 16 of the Paper Conservation Catalogue may be consulted (Book and Paper Group 1990, Section 16, 26–27).
The label should be washed for 15 to 20 minutes. If a significant amount of impurities is being removed and the water is turning yellow very quickly, the label should be transferred to a fresh bath after approximately 15 minutes. One to three baths should be sufficient. Adhesives may be reduced during washing as described in 8.a.
To remove the label from the bath, the Hollytex may be pulled straight up, beginning at one end and slowly continuing until it is removed. The label and Hollytex may then be placed on a small screen to air-dry.
===Lining===
If the label is fragmentary, contains tears, or is very fragile, a lining might be advisable. This additional support will allow for easier handling and will help to prevent further damage to the label.
If the label was previously washed, it should be dried at least partially before lining. Depending on the quality and thickness of the label paper, an appropriate lining paper should be chosen. In general, the strength, weight, flexibility, color, and opacity of the lining paper should be compatible with the label paper (Book and Paper Group 1988, Section 29:5). Many thin Japanese papers, especially those that are kozo-fibered, make good lining papers. Once the paper is chosen, it should be cut 1/2 inch or more larger than the label on all four sides. The rough side of the paper should be determined. It is this side that should be attached to the label. The label and the lining paper should be misted with water and the grain directions noted before cutting. The grain directions should be parallel during lining. Each paper should be placed onto a piece of Mylar® that lays flat on the table. The label should be face down, and any tears in the label should be aligned at this point. The adhesive should then be applied evenly to the lining paper. A dilute mixture of wheat starch paste and methylcellulose (1:1) makes a good-quality lining adhesive that is reversible. The Mylar® and lining paper should then be taken, flipped over, and placed down onto the label. A stiff brush may be used to brush across the Mylar®, lining, and label. The whole package should then be flipped over onto the table and the Mylar® removed from the back in the same fashion. At this point, the lined label may be partially air-dried and flattened at the same time by placing the label between polyester web and blotters. Felts, glass, and weights may then be applied. The other option is to air-dry the label completely and to rehumidify and flatten the label later. The latter method has the advantage that excessive contraction is avoided and the separation of tears is less likely (Book and Paper Group 1988, Section 29:25).
===Deacidification===
:If the label has been washed, deacidification is really not necessary. If it is not possible to wash the label, but the paper seems very brittle and acidic, a paper or board with an alkaline reserve might be placed behind the label before encapsulation.
===Encapsulation===
Label encapsulation in Mylar® may be the best way to protect a label from abrasion and other damage. The easiest method of encapsulation utilizes an ultrasonic or heat welder. The advantages of sealing the Mylar® using this method include the avoidance of adhesives and simplicity of technique. However, most painting conservators do not have such a device readily available. Thus, the use of 3M #415 double-sided tape seems to be the next best alternative (Ogden 1994, 1-2). Two sheets of Mylar® should be cut larger than the label, allowing plenty of room around the edges for the tape. The tape should then be applied around the perimeter of one of the Mylar® sheets. The label may then be placed within the area circumscribed by the tape, and the second sheet of Mylar® may be carefully adhered into place.
===Storage===
Storage or placement of the label may be dictated in part by the owners or museum curators. Many conservators believe that the label should remain with the painting, on its stretcher or backing board, so that the information will remain easily accessible (Curtis 1999).
====Stretcher====
To preserve the original placement of the label, it would be ideal to return the label to its original location subsequent to removal and/or treatment. If a label would not be visible due to the application of a backing board, one might consider cutting a window in the backing board.
If the label has been encapsulated in Mylar®, 3M #415 double-sided tape can be placed around the perimeter of the Mylar® and adhered into place on the stretcher.
====Backing Board====
Placement of an encapsulated label on the backing board allows for easy visibility. This option is perhaps the most simple because no further accommodations for the label are necessary.
The use of 3M #415 double-sided tape seems the best way to apply an encapsulated label to a backing board.
====Report or File====
The placement of the label and/or report is dependent on the filing system within an institution. If the painting is privately owned and the report is filed, there is a greater chance of the report becoming separated from the painting as the latter gets passed down through the generations. Keeping the report with the painting, somehow attached to the backing board, for example, may be the best way to keep the information and label with the painting. In the case of ownership, however, the owner may be more willing to follow the advice of a conservator in returning the label to the stretcher or attaching it to the backing board.
If the label must be stored with the report or in some type of file, it is advisable to protect the label through encapsulation or in a small, labeled storage envelope (such as negative envelopes), such as Mylar®, glassine, or alkaline-buffered, archival storage paper.
Based on the 1999 stretcher label questionnaire, some institutions tend to file labels with the curator's file. It is advisable to make a copy (i.e., photograph and/or photocopy) and/ or transcription to place with the conservation report and in all appropriate files related to the painting (Curtis 1999).
Marla Curtis
Submitted September 2006
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