AN IMPROVED STRESSED-SKIN SUPPORT FOR ARTWORKS ON CANVAS OR PAPER

Providing acceptable reinforcement for fragile paper or painting supports, such as those made of cardboard, metal sheet or compressed wood fibers (Masonite or Presdwood brands) has left few options. Choices have been largely limited to aluminum honeycomb/aluminum skin panels, paper honeycomb/paper skin panels, or plywood. Stressed-skin panels made from aluminum skins and aluminum honeycomb are relatively thick, costly and prone to denting. Those made of paper skins and paper honeycomb are thick, susceptible to warping in changing environments, and prone to dimpling and denting of the skins. The best grades of marine plywood are heavy and expensive. The present design is an improvement over existing secondary support panels. Besides being lighter, cheaper and not prone to denting or warping, a panel can be made in various thicknesses and strengths to meet a variety of structural and aesthetic demands. Most of the materials used to make the support may already exist in the paper and/or painting conservation studio. A basic stressed-skin panel is comprised of three elements: thin, rigid skins on both flat faces, a lightweight spacer core between the skins, and a suitable adhesive to hold them together. The mechanical principle is essentially the same as for an I-beam: the parallel top and bottom flanges separated by a spacer web work in concert to prevent the I-beam from bending under load. Without either top or bottom flange, the beam would be significantly weaker. The skins on the stressed-skin panel function like the flanges of the I-beam, while the core is the spacer web. The major difference is that the stressed-skin panel resembles a multidirectional I-beam, providing a planar load resistance. The skin material must be capable of resisting tensile and compressive forces beyond those normally endured by artwork. This material must also be inert to changes in relative humidity or temperature, and be sufficiently hard to resist dents. The core material comprises the greatest bulk of the panel and therefore should be lightweight. The core should ideally provide uniform continuous support for the skins, although a small-cell honeycomb design works well, too. The adhesive should be strong, resistant to changes in relative humidity and temperature, and relatively non-toxic. Fiberglass-reinforced epoxy resin sheets, commercially available under the name 0-10, are an ideal choice for the skins. 0-10 is strong, stiff, inert to wide environmental fluctuations and virtually impossible to dent. Thicknesses suitable for the present use range from 0.25mm to 0.75mm, depending on the overall dimensions of the finished panel. Although available from many companies in the USA, strength, surface finish and overall quality can vary significantly from manufacturer to manufacturer. The core material for a small panel (less than 40 x 40cm) is made from 6or 8-ply acid-free, 100% rag matboard. A larger panel requires a thicker core in order to maintain overall rigidity. To reduce weight, rigid end-grain balsa wood is preferred. Endgrain balsa is lightweight and rot resistant, especially when the end-grain faces are sealed by the 0-10 skins. Moisture cannot enter the wood from the panel edges, either, because the wood cells are naturally closed. The balsa is available in thicknesses ranging from 6.35mm up, in 6.35mm increments. The core can be made from pieces of material firmly butted together, so there is very little waste. There is no loss of strength, either, since the skins carry any imposed stress, not the core. The preferred adhesive, for structural reasons, is an epoxy paste requiring a heat cure. The handling and application of the epoxy is awkward and messy at best, and the shelf life is usually only one year. Oiven the structural needs of most artwork, Beva film provides sufficient bond strength to exceed all ordinary demands on a secondary support, so it is used instead of the epoxy paste. Assembly is fairly simple, if one has access to a vacuum hot table. The G-I0 skins are cut to size, using scissors or a board shear for the cleanest edges. Do not bother to use a mat knife. Any irregularities can be cleaned up when the panel is finished, by using an ordinary hand-held block plane. Cut Beva film slightly larger than needed and bond to the 0-10 sheets on the vacuum hot table. Meanwhile, cut the balsa or matboard core, using a mat knife, slightly larger (3mm) than the 0-10, to allow for alignment adjustments. After the G-I0/Beva film laminate has cooled, trim the excess Beva with scissors and remove the Mylar release film. Assemble the panel components like a sandwich: G-I0/Beva on either side of the core. Bond these components together on the vacuum hot table at approximately 71 °C and 130mm of Hg vacuum pressure. Cool to room temperature. Afterwards, use the block plane to true up the edges, as needed. The panels can be used in a number of ways. Small paintings on metal, Masonite or cardboard can be supported in a 'slip case' by attaching a rabbet made of brass or aluminum channel to the sides of a panel (matboard core with 0.25mm G-I0). The inside surface of the channel in direct contact with the face of the artwork can be padded using a Goretex/polyester material adhered with Beva film. The channel can then be adhered to the perimeter on three sides with epoxy. The top channel is attached on the reverse with a sailcloth 'hinge' and Beva film. The artwork is slipped into the case from the top, and the case is fitted into a normal frame in the usual way. Larger panels (6.35mm end-grain balsa core with 0.38mm G-I0) can be attached to the front of a stretcher to support a fragile canvas painting without lining. Sometimes, a single sheet of 0-10 (0.75mm) is suitable for this use. In either case, the original stretcher can be retained. In preparation for use with paper artwork, the surface of the panel can be sheathed with 2-ply rag matboard using Beva film on the vacuum hot table. The artwork may then be either hinged in the usual way or even mounted overall with starch and/or methylcellulose, if appropriate. There are many other uses for this type of support: inclined surfaces for exhibition of small paper artworks or objects, protection of artwork from puncture during shipping, removable lightweight shelves for flat storage of fragile artwork, and more. This design fulfils the need for a relatively thin, lightweight, rigid, environmentally stable, secondary support panel which could reinforce or replace deteriorating supports. Canvas paintings experiencing the effects of stress imposed by transport, handling or exhibition would also benefit from the support provided by such a panel, without sacrificing the original strainer. Author's address: Art Conservation Department, RH230, Buffalo State College, 1300 Elmwood A venue, Buffalo, NY 14222, USA.