Abstract:
An apparatus for stretching a sheet of material such as a sheet of watercolor paper utilizes a flat board and a frame assembly for bending the board into a dish shape. The board has a plurality of radially extending segments separated by radially extending slots therebetween. The board is bent into a dish shape by a mechanism attached to the frame. The sheet is then affixed to the bent board with anchors while the frame is attached to the board and subsequently the frame is released from the bent board allowing the board to resume its originally flat condition thereby resulting in the sheet being stretched.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to sheet stretching devices and particularly to graphic arts paper stretching devices wherein the paper is watercolor paper. 
     2. Description of the Related Art 
     One problem encountered when painting a watercolor is when wet washes are applied to the watercolor paper, the paper expands in the wetted areas only, and as a result the paper buckles. When the paper dries the buckling does not disappear; in fact, the buckling becomes semi-permanent and can only be removed with special techniques. The buckles, in turn, result in depressions in the paper. The ridges and depressions in the paper make it difficult to control the behavior of subsequent washes. Tinted washes tend to flow away from the ridges towards the depressions causing the intensity of colors to vary in a way the artist did not intend. 
     When a buckled painting is finished and ready for framing, the paper must be flattened first by dampening the entire sheet and letting it dry under a flat surface such as a wooden board. 
     The buckling phenomenon described above occurs more noticeably with thinner papers of 140 lb and thinner. Thicker watercolor papers of 300 lb and higher do not buckle as much and are quite manageable. However, thicker papers cost more than thinner sheets thus limiting some artists in their choice of paper thickness. 
     The solution to buckling used by most watercolor painters is to stretch the paper on a suitable board prior to painting. The way this is normally done is to soak the entire sheet of watercolor paper with water for about five to ten minutes and allowing the paper to expand. Then the wet paper is attached to a flat board along its edges, either with staples or wetted gummed tape and left to dry for several hours or overnight. While drying, the paper contracts and becomes taught and flat on the board. Stretching the paper as described above makes the paper much less prone to buckling. Even when heavily wet washes are applied to the stretched paper, and some buckling does occur, the paper will re-assume its taught and flat condition when dry. 
     The process of stretching watercolor paper is time consuming and troublesome in a number of ways. For example, sometimes the gummed tape used to stretch the paper gives way to the tension applied to it by the paper and the paper has to be re-stretched. Also, removing the tape when the painting is finished may at times damage the paper in spots or leave tape residue. When using staples, some staples do not penetrate the board as intended, and must be removed and reapplied. Removing the staples after the painting is done is also time consuming. And, of course, the most time-consuming aspect of stretching watercolor paper is the time waiting for the paper to dry. Some artists overcome the problem of drying time by preparing several stretched sheets ahead of time. 
     To ease the problems of watercolor paper stretching several inventions have appeared. However, most of those inventions address only the problem of attaching a wetted sheet of watercolor paper to a board, and have successfully eliminated the need for gummed tape and staples for this purpose. Examples of these devices are shown in U.S. pat. No. 4,126,952, 4,277,901, and 4,862,610. These inventions, however, do not solve the problem relating to the time involved with waiting for the watercolor paper to dry in order to become taught. 
     Closely related to watercolor paper stretchers are canvas stretchers for oil and acrylic painting. Numerous inventions exist addressing the problem of stretching canvas on a frame. Some of these inventions could be modified to be applicable to watercolor sheets. It should be noted here that the requirements for preparing canvas for oil or acrylic painting are somewhat different from those required for preparing watercolor sheet. Oil and acrylic painters do not need a solid backing for their canvases. Most oil painters have their canvases stretched onto an open frame with no backing under the canvas. Watercolor painters, however, require a solid backing for their papers for several reasons. First, since paper will expand when wetted, paper on an open frame will tend to sag when wet washes are applied. Furthermore, watercolor painting often involves techniques such as “Lifting” and “Scratching” that require a solid backing because these operations apply significant force onto the paper surface. Therefore, frame stretchers are not quite applicable to watercolor paper stretching. Never-the-less, a discussion of frame stretchers is presented below since some of the concepts involved in frame stretchers could, perhaps, be applied to stretching watercolor paper. 
     One problem with many frame stretchers is that most of them stretch the canvas along two perpendicular axes by simply increasing the distance between opposing frame members. And, if the canvas is attached, for example, with staples, all along the frame members, then the canvas is restricted from stretching along the frame members because the frame members themselves do not expand. Examples of such canvas stretchers are disclosed in U.S. Pat. Nos. 4,301,853, 4,565,020 and 5,918,392. One patent that recognizes the above problem is U.S. Pat. No. 4,144660. In this patent the inventor has provided means for increasing the length of each frame member after the canvas has been attached to the frame. The inventor has done this by having each frame member consist of several segments, and providing means for increasing the space between each of the segments, thereby increasing the overall length of each frame member. U.S. Pat. No. 5,115,584 has also solved the problem of restricting the canvas from stretching along the frame members. In this patent the canvas is not attached to the frames at all. The canvas is stretched using resilient clips to stretch the canvas over the frames, but the canvas is free to stretch along the frame members. The latter two cited inventions, however, are not suitable for stretching watercolor paper since they do not provide a solid backing under the stretched sheet, something that the watercolor paper should have as explained above. 
     It is evident from the above that it would be highly beneficial for watercolor artists if they had a device that could stretch watercolor paper in its dry state within a matter of minutes, never having to wet it prior to stretching, nor wait for hours for the paper to dry. Furthermore, what is needed by watercolor artists is a paper stretching device that provides a solid backing under the watercolor paper. Another improvement that would benefit watercolor artists is the reduction in the time consumed by attaching and detaching watercolor paper from a board. 
     SUMMARY OF THE INVENTION 
     To overcome the shortcomings associated with paper and fabric stretchers as enumerated above, the present invention provides an apparatus and method to stretch watercolor paper in its dry state, without having to pre-wet the paper as is currently the practice. 
     In the present invention a paper mounting board is first bent on a special frame and assumes a dish-like shape. While the mounting board is still on the frame in its bent state, the paper is mounted onto the board and attached to its edges with a plurality of special anchoring devices. Then the board is separated from the bending frame and allowed to assume its original, flat shape. However, since the board is prevented from completely returning to its flat shape by the paper attached to it, the board consequently exerts a stretching force all around the edges of the paper that pulls the paper taught and flat. In this stretched state the paper and board are ready for the artist to paint on. 
     Utilizing the apparatus and method of the present invention, artists can stretch their watercolor paper within a matter of minutes in contrast to the hours they must currently wait for their pre-wetted paper to dry. 
     Another improvement provided by the present invention are the means whereby the paper is attached to the mounting board. The common practice among watercolor artists is to attach their paper to a mounting board using either a large amount of staples or lengths of gummed paper tape, both methods requiring lengthy times for attaching and detaching the paper to and from the board. The present invention provides special anchors for attaching paper to a board, each anchor taking the place of five or more staples so that the times involved attaching and detaching the paper are greatly reduced. 
     It is therefore an object of the present invention to provide a watercolor paper stretching board that is capable of stretching watercolor paper in its dry state. 
     It is a further object of this invention to provide a watercolor stretching board that will stretch the watercolor paper substantially radially, so that it is stretched in all directions simultaneously. 
     Yet another object of the present invention is to provide time-efficient means for attaching and detaching watercolor paper to and from a board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of the watercolor paper board employed in the preferred embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of the frame and crossbar used for bending the board. 
     FIG. 3 is the same view as FIG. 2 with the board shown in its bent state. 
     FIGS. 4 and 5 are perspective views corresponding to FIGS. 2 and 3. 
     FIG. 6 is a partial perspective view showing the top of the board with paper attached to it with anchors. 
     FIGS. 7 and 8 are perspective views of the anchors used to secure watercolor paper to a board. 
     FIG. 9 is a perspective view of the frame used to bend the watercolor board. 
     FIG. 10 is a perspective view of the crossbar and screw used to bend the watercolor board. 
     FIG. 11 is a cross-sectional view of an anchor just prior to engaging the watercolor paper. 
     FIG. 12 is a cross-sectional view of an anchor after it has pierced the paper. 
     FIG. 13 shows the watercolor board being bent by a lever system. 
     FIG. 14 shows the watercolor board being bent by a pulley and crank system. 
     FIG. 15 shows how the frame is modified to accept a half sheet of watercolor paper. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to facilitate an understanding of the various elements of the present invention, the principle of its operation is described first. 
     Let us take a flexible object shaped like a shallow dish and cut several thin slots in it starting from its rim and up to its base. If we then flatten this object, say by placing a transparent sheet of glass on top of it, we would see right away that the outer periphery of the flattened dish-like object would be significantly longer that the periphery of the original rim. 
     Now let us say that prior to flattening the dish-shaped object we covered its opening by gluing a thin, round sheet of rubber along its rim. If we then place our glass plate over the assembly and flatten it, we would of course notice that the rubber sheet has stretched evenly and radially in all directions. 
     This is the geometric phenomenon employed in the present invention for stretching a sheet of material such as watercolor paper. However, the logic followed here is somewhat reversed: We start out with a flat board in which slots are cut, then force the board into a dish-like shape, then attach the sheet to be stretched onto the periphery of the board, and then just let the board seek its original, flat state while stretching the attached sheet along with it. The slots in the board essentially create peripheral space so that the periphery of the board is allowed to contract freely while the board is forced into a dish-like shape. 
     Although the following detailed description depicts the stretching of watercolor paper, it should be understood that a sheet of any material, including fabrics, plastic and canvas, may be stretched in a similar manner. 
     FIG. 1 shows a top view of the preferred embodiment of a board  10 . The board is designed to stretch watercolor paper as well as to hold it stretched until the painting is finished. The board  10  shown is for a full watercolor paper sheet which is 22×30 inches. The board  10  may be constructed from any number of flexible materials such as plywood, plastic, aluminum, or foam board. Board  10  is 1 inch longer and 1 inch wider than a full watercolor paper sheet. 
     Board  10  is provided with a number of slots  11 . The outer ends of slots  11  are equally spaced around the periphery of the board, and the direction of the slots are substantially radial, pointing towards the center of board  10 . The number of slots  11  and their length may be varied depending on requirements such as paper tension, paper thickness, board material and so on. Those skilled in the art will realize that the longer the slots, the less force will be required to bend each segment of the board  10  between the slots  11 . Experiments with ¼ inch thick plywood boards have shown that for a 22×30 inches, 140 lb (a measure of sheet thickness) sheet, twenty slots are sufficient. The width of the slots  11  is kept to a minimum so that the slots  11  are not noticeable to the artist while painting. The maximum width of the slots  11  should be about one tenth of an inch, and the minimum about one sixteenth of an inch to allow for the peripheral contraction of the board  10 . Each segment  12  of the board  10  between two slots  11  acts more or less like a cantilevered beam. When the board  10  is forced into a dish shape, each segment  12  bends along the line  13  drawn between the inner ends of the slots  11  bordering the segment  12 . The length of bending lines  13  determines the bending strength of each segment  12 . The length of lines  13  may be varied from segment to segment in order to optimize the bending strengths of the various segments  12 . The center of board  10  is provided with a threaded hole  14 . A metal threaded insert with a ¼″-20 thread is recommended. Further details of the board will be discussed later. 
     How the board  10  is forced into a dish shape will now be explained with reference to FIGS. 2-6. 
     Frame  16  (shown by itself in FIG. 9) serves as a fixture on which board  10  is bent. Frame  16  has four sides with bottom edges  17  and  18  curved in order to assist in the shaping of board  10  into a dish shape. The top edges of the four sides are straight and have a notch  22  cut into each of its longer sides. These notches  22  serve as guides for positioning a crossbar  19 . 
     Referring now to FIGS. 2 and 4, board  10  is placed upon a flat surface  15 . Frame  16  is placed centered atop board  10  with curved edges  17  and  18  facing down. Crossbar  19  along with screw  20  and nut  21  (shown as an assembly in FIG. 10) is placed into slots  22  in frame  16 . Screw  20  is engaged into threaded hole  14  until ring  23  prevents further engagement. Nut  21  is then turned clockwise, causing screw  20  to pull the center of board  10  upwardly towards crossbar  19 . As screw  20  is turned further, since frame  16  prevents the edges of board  10  from moving upwardly, board  10  is forced into assuming a dish-like shape as shown in FIG.  5 . The amount of curvature of board  10  is easily controlled by how many turns are imparted to screw  20 . FIGS. 4 and 5 show perspective views corresponding to the sectional views of FIGS. 2 and 3. After board  10  assumes the desired curvature, the whole assembly is turned upside down and is now ready to receive the sheet of watercolor paper. 
     Referring to FIG. 6, a sheet of watercolor paper  29  is placed onto the board  10  and is centered on it. Anchors  25  are attached to the board and paper simultaneously, one anchor to each segment  12  of the board  10 . First the anchor guiding holes  26 , shown in FIG. 7, are registered in precise position by barbs  27  in the board. The front section  40  of anchors  25  are registered against the outer edges of board  10  . This precise registration assures that the anchor barbs  31  will enter holes  30  in the board  10  which holes  30  cannot be seen when covered by paper  29 . Then the top sections  28  of anchors  25  are pressed downwardly causing barbs  31  to penetrate paper  29  and to enter holes  30  in board  10 . 
     Barbs  27  serve an additional function which is to prevent the anchors  25  from disengaging from the board  10  when the stretched paper  29  pulls on the anchors  25 . FIG. 11 is a cross-sectional view of an anchor  25  just prior to being pushed through the paper  29  and FIG. 12 shows the anchor  25  after it has been pushed through. 
     In the preferred embodiment of the present invention the board  10  accommodates a full, 22×30 inch watercolor paper sheet  29 . Twenty anchors  25  are used to secure the paper sheet  29  to the board  10 , each anchor  25  having five barbs  31 . This means that a total one hundred barbs  31  are used to secure the paper sheet  29 . Board  10  has one hundred holes  30  that accept the anchors  25 . The diameter of each hole  30  in board  10  is slightly larger that the diameter of a barb  31  in an anchor  25 . 
     Other embodiments of the present invention may not require anchors or predrilled holes in board  10  as described above. In these other embodiments paper  10  may be attached with staples, adhesive tape, or wetted gummed tape as mentioned previously. It should be noted, however, that if adhesive tape or wetted gummed tape is employed, they should not span the slots  11  in board  10  so that each board segment  12  remains free to seek its original, flat state. The tapes could be applied one piece per board edge, and then cut between board segments  12 . 
     After all the anchors  25  have been applied to the board  10 , screw  20  is turned counter-clockwise until board  10  is free from frame  16 . In its free state board  10  will tend to resume its original, flat condition, and this tendency provides the radial forces that stretch the paper sheet as previously described. 
     Board  10  may be used to stretch and hold watercolor paper sheet  29  without using the technique described above, but by simply pre-wetting the paper sheet  29  as is normally done to this day. Anchors  25  may be used to secure the wetted paper sheet  29  to the board  10  and indeed would save the time and trouble of stapling or taping the edges of the paper sheet  29  as described above. 
     It should be understood that using a screw and nut system to force the board  10  to assume a dish shape is only one way of many that could be employed to achieve the same result. This point is emphasized here that the method of bending board  10  is secondary to the novelty of the present invention. It is the construction of the board  10  itself that affords this invention a unique method of stretching watercolor paper, canvas, and the like. 
     The following are brief descriptions of other methods and systems that could be employed for bending board  10 . 
     Vacuum System 
     With slight modifications to the preferred embodiment of the present invention, a vacuum activated board-bending device could be constructed. Using frame  16  of FIG.  9  and attaching a bottom to its straight edges and a continuous rubber seal to its curved edges would create a vacuum box. A hole in the side of the frame  16  is drilled to receive a vacuum hose. The threaded hole  14  is eliminated. 
     Board  10  of FIG. 1 is modified by taping all the slots  11  so that the board is vacuum tight, but the slots  11  would be free to contract and expand. Alternatively, the entire board  10  could be covered with a flexible membrane making it vacuum tight. 
     The vacuum box described above is placed on a flat surface with its curved edges and vacuum seal facing upwards. The modified board is then placed on top of the box, and a normal house vacuum cleaner is used to draw a vacuum through the hole in the modified frame. The user must push downwardly on the center of the board to help establish a seal between the board and the frame. Once the seal becomes effective, the board is drawn down by the vacuum with sufficient force to bend the board. 
     Lever System 
     FIG. 13 shows how a lever system could be adapted to pull the center of board  10  upwardly thus forcing the board  10  to assume a dish shape. Details of how the board  10  is held in its bent state are not given here since anyone skilled in the art of mechanical design could solve the problem. 
     Pulley System 
     FIG. 14 shows how a pulley and crank system could be used to perform the same function as screw  20  and nut  21  does in the preferred embodiment of the present invention. 
     As mentioned above, the size of the board  10  used in the preferred embodiment of this invention is for a full watercolor paper sheet, namely 22×30 inches. Often, however, a half sheet is also used by watercolor painters. FIG. 15 shows how frame  16  can be modified to  16 ′ to accommodate a half sheet as well. 
     The modifications required are: 
     1. Plate  36  is added to support the long edge of board  35 . The bottom edge  37  of plate  36  is curved to accept board  35  when it is bent. Alternatively, plate  39  of frame  16 , if made removable, could be used instead of adding plate  36 . 
     2. Frame  16 ′ has curved side edges  37 ′ and  38  to accept board  35  when it is bent. 
     3. Paper holding board  35  is added. It is inch longer and inch wider than a half watercolor paper sheet and its design features are the same as those of board  10 . Because of its smaller size, board  35  has only fourteen segments  12  instead of the twenty on board  10 . 
     Crossbar  19 , screw  20 , nut  21 , and anchors  25  (not shown in FIG. 15) remain the same and the bending procedure as described above remains the same as well. 
     Frames  16  and  16 ′ are shown in the drawings as being made from one piece of material. However, anyone skilled in the art could design frames  16  and  16 ′ in a way that would make them more conducive to efficient packaging. For example, the frames could be hinged at the corners, or they could be attached at their corners with suitable hardware. 
     While an illustrative description has been given for a preferred embodiment of the present invention as well as several approaches to applying a bending force necessary for the practicability of the invention, it should be understood that different embodiments may be made using the principle of operation of the present invention. Hence, all matter disclosed heretofore is merely illustrative and the scope of the present invention is defined by the following claims.