Painting conservators are constantly looking for ways to increase the strength of painting canvases. One of these methods is canvas lining, where an additional canvas is adhered to the painting canvas as a support to provide additional mechanical strength. This practice has taken many forms over its history of use. A variety of application techniques, tools, and materials have been used. Some lining treatments are done with a full lining canvas covering, other have just a strip treatment to the region of high stress near the stretcher bars, and others are just covering tears in the painting canvas (1). The tools and materials used in lining have changed drastically with time. Early lining treatment in the late 19th and early 20th centuries mostly involved applying an adhesive made of animal glue and paste to an un-stretched painting canvas and then hand ironing on an additional lining canvas (2). Concerns about the longevity of the paintings caused this treatment to become a standard preventative measure of conservation. The treatment was undertaken almost as insurance against the future damage to paintings (3). However, this technique relied heavily on the steadiness of the conservator’s hand to avoid the risk of damage to the painting by means of flattening. The paste-glue mixture was quite susceptible to hygroscopic and thermal fluctuations. A wax-resin adhesive rose to prominence due to its atmospheric inertness in the early 20th century, but this adhesive lead to the impregnation of the painting’s pigments with a darker hue (4–6). New technological developments led to the improvement of the application tools; this brought the invention of the vacuum hot table that would replace hand ironing (6, 7). While this tool was invented to reduce the forces on the painting canvas, the flattening effect was still occurring due to the high heat.
Growing concerns with the immediate and long-term effects of lining on canvas reached its pinnacle with the Greenwich Conference in 1974. One of the main concerns was the so-called ‘lining cycle’ outlined in Westby Percival-Prescott’s research, who pointed out that a lined canvas would eventually need to be relined due to the degradation of the lining and adhesive, placing the painting in a cycle of invasive treatments that without initial lining would have been avoidable (8).
After the Greenwich Conference, the tools and materials developed for lining changed drastically. This included the development of a cold table with air flow that reduced the amount of heat being applied directly on the canvas (9). New adhesives were developed during this time such as Gustav Berger’s BEVA copolymer (10, 11). This synthetic glue was developed with conservation specific properties in mind. Other lining canvas materials were also considered during this period, such as sailcloth (12).
Towards the end of the 20th century, the use of lining as a treatment has decreased as shown in surveys conducted in 1975, 1984, and 2001 (13). This left the problem of treating canvases that had already been lined and were now engaged in the “lining cycle” (8).
Cotton canvas is the material for lining for paintings that this study focuses on. Canvases traditionally have been made from cotton, linen, or jute. The major component of cotton canvas is cellulose, a polymer that is able to form both crystalline and amorphous regions. The polymer chain backbone forms hydrogen bonds with the other parts of the polymer backbone giving the cellulose its strength. The degradation of cellulose is through hydrolysis where the chain is broken (14). This leads to a reduction of the tensile strength and other mechanical properties of the cellulose. This degradation is acid-catalysed and can be assessed by measuring the degree of polymerisation of the cellulose in canvas through viscometry. The weakening of canvas through chemical degradation prompts the re-treatment, often with a relining of the canvas.
The term glue in this study is used to refer to an adhesive as used in conservation practice. The glues used in this study are rabbit skin glue (RSG), isinglass (a glue made from a sturgeon bladder originating from Russia) and bovine hide and bone glue. These glues have been historically used for conservation treatments, including lining and consolidation treatments. The purpose of consolidation treatments is to reattach flakes of paint, historically achieved through lining (1). These glues are all collagen based and therefore exhibit similar degradation mechanisms. Collagen is made up of a long protein chain that has been denatured in order to become soluble in water and form gelatine. The formation of gelatine is essential for collagen to become a successful glue. Once denatured, the now free functional groups are able to form hydrogen bonds leading to the glues gel and adhesive properties. Research in the past has focused on the response of glue to environmental fluctuations but there is significantly less information about the degradation behaviour of collagen based glues (15, 16). Earlier research has shown that with thermal and ultraviolet ageing, mammalian animal glues (RSG, bovine hide and bone) will increase tensile strength and become stiffer and more brittle whereas isinglass will remain more elastic (14).
The research on canvas lining has historically focused on the immediate effects of lining. Studies are now focusing on the longevity of the treatment and how to manage collections with lined paintings. There still are many gaps in the understanding of the degradation of canvas and glue and how this contributes to the overall mechanical strength of the painting. This study aims to understand what mechanical reinforcement traditional animal glue itself provides to canvas and how this reinforcement changes during degradation. The variables in the experiment are the type of glue, degradation of canvas, degradation of glue, and thickness of glue. Tensile testing is used to determine the mechanical strength of the samples.