Materials and Techniques Survey: Scratch Repair on Acrylics used in Face-Mounted Photographs

Face-mounting of photographs acrylic sheet has become an established method for solving the issue of support for large format photographs. One of the main conservation issues with face-mounted photographs is the repair of a scratched or abraded acrylic surface. Scratches frequently occur on the plastic facing material as a result of shipping, vandalism, routine cleaning, and handling. To address the problem, several fill materials (e.g. epoxies and acrylics), as well as mechanical smoothing and forming techniques (e.g. heating and roll pressing) were tested. Hxtal NYL-1 epoxy was the best performing fill material that combines a low viscosity to allow wicking into fine cracks with a long working time. The most useful mechanical technique was to transfer heat to the scratch using a block of stainless steel (hot rolling), where the edges of a scratch were preferentially affected and the fragmented trough was mildly affected. While this reduced the overall appearance, the scratch was still visible. The goal of this project was to survey new options in the treatment of scratches on photographs face-mounted to acrylic sheeting. Treatments with the potential to positively affect the visual appearance in any way were considered. Experiments were conducted using materials with refractive indices matched to acrylic, and those materials were applied using common deposition methods. Starting with this information, conservators can choose a range of appropriate materials and conduct further studies to determine applicability for their particular acrylic issues. Background A wide array of plastic materials has been used to face-mount photographs. Acrylic, poly(vinyl chloride), polyester, polycarbonate, and polypropylene are some examples. Polymethyl methacrylate (PMMA) has a refractive index of about 1.49 and is the most common material used in face-mounted photographs (Sale 1993, Pénichon and Jürgens 2005). PMMA, which is often referred to as acrylic, was developed in 1936 by Rohm and Haas in Germany. It may be cast or extruded and is also known by various trade names, such as Plexiglas, Perspex, Acrylite, and Lucite. In the 1970s, a method of face-mounting photographs to hard, acrylic sheets was developed in Europe. One main goal of this process was to increase the saturation of the image (Daffner and McGlinchey 2004). In addition, face-mounting improved the photograph‘s resistance to fading when UV inhibitors were incorporated in the plastic, protected the photographic surface from pollutants and handling, and provided rigid support (Pénichon and Jürgens 2002). Large photographs tend to have unique issues of display. Glass, when used as a glazing material, can be heavy and generally requires a frame. Acrylic, when used as a glazing material for large works, has the propensity to sag. Face-mounted photographs, however, are often unframed and can be supported by a metal or wood framework or cleats on the verso. Kim and Breitung Scratch Repair on Acrylics Used in Face-Mounted Photographs Topics in Photographic Preservation, Volume Twelve (2007) 145 These features make face-mounting a visually unique, cost effective, and simple method for supporting and displaying large-format photographs. Face-mounting Materials One of the better known face-mounting processes, Diasec®, comes from Switzerland. Diasec® was patented by Heinz Sovilla-Brulhart in 1971 and is available today at licensed lamination studios. The formulas for the adhesive and primer are proprietary secrets that are maintained by Sovilla-Brulhart‘s widow and son, who took over the business after his death in 1992 (Pénichon and Jürgens 2001). The Diasec® process uses silicone rubber to adhere the acrylic sheet to the front side of the photograph. The type of acrylic sheeting is not specified by the Diasec® process, but according to Pénichon and Jürgens, most labs use PMMA with ultraviolet absorbers (Pénichon and Jürgens 2001). An electric roll press is used to press the components together, and the silicone rubber cures for forty-eight hours. Many lamination shops face-mount using their own unpatented processes and materials, where pressure sensitive adhesives as well as silicones are used. The silicone rubbers generally used for face-mounting photographs cure at room temperature when exposed to moisture, which is obtained from the water in the gelatin layer in traditional photographs. Poly(dimethyl siloxane) is frequently used as the curable component of silicone rubber. Once cured, it remains flexible and inert. There are, however, various silicone rubber formulations that contain additional materials such as cross-linking agents, fillers, and processing additives. The content and concentrations of these materials in commercially available silicones are often kept as proprietary company information, making it difficult to determine the propensity to off-gas, yellow, or age irregularly (Pénichon, Jürgens and Murray 2002). 1 The silicones used for face-mounting photographs have viscosities similar to household bathroom caulk. Because these are quite viscous, a primer is often applied to both the surface of the photograph and the acrylic sheeting. The primer is usually a 0.5-2% solution of organo-bifunctional silane in an organic solvent such as ethanol and water. The functionalities are often reactive moieties that can chemically link the substrate and adhesive. The organo-functional silane solution is much less viscous than the adhesive and formulated to wet and coat the substrate. Thus, in addition to chemically bonding the substrate to the adhesive, it can displace adsorbed air and water vapor, which helps enable intimate contact between the plastic sheet and the adhesive (Pénichon and Jürgens 2001). Treatment of Scratches on Face-Mounted Photographs Scratches that pose aesthetic problems in the presentation of face-mounted photographs range from barely visible scrapes to deep gouges. The variety of scratch types may lend to treating with a range of methodologies and materials. The best approaches will vary depending on the depth and width of the scratch, the type and source of PMMA, and the lightness or darkness of the image attached to the sheet. Hence, the methods described herein will require optimization by conservators in the field. Table 1 lists the techniques and materials studied as well as general observations. Details on specific materials tested follow. Solvent compatibility was taken into account when choosing a fill material or treatment technique. Some fill materials 1 According to Pénichon, Jürgens, and Murray, delamination has occurred along the edges of some face-mounted photographs. Kim and Breitung Scratch Repair on Acrylics Used in Face-Mounted Photographs Topics in Photographic Preservation, Volume Twelve (2007) 146 contain solvents. Crazing of PMMA can be caused by treatment with solvents such as acetone and ethanol; this limits the types of adhesives and fill materials that may be used without harming the substrate. Table 1. Overview of Techniques and Results.