4 THE ROLE OF THE GLUE SIZE LAYER

In the Sawyer painting, large areas of the ground and paint film were delaminated from the canvas support. Examination of the canvas revealed that it had been sized with an animal glue before the application of the ground layer. This technique is typical of commercially prepared artist's canvas of the 19th century. In paintings susceptible to water damage the canvas support usually begins to shrink 2–3 minutes after wetting. Because the forces would also be transferred to the ground and paint layers, one might expect that compression damage to these layers would occur immediately at the onset of canvas shrinkage. However, there is usually a delay of some 5–10 minutes before the paint layers undergo compression damage.1 This interval between the canvas shrinkage and the beginning of tented cleavage is significant. The reason for the delay is that the glue size layer is slower to absorb water and soften. Once the glue size has softened and adhesion between the canvas and ground is lost, compression damage to the more rigid paint and ground layer occurs. Moreover, the manner in which the glue size was applied to the canvas may play an important role in the delamination process. In the Sawyer painting, microscopic examination revealed that the size layer coated the canvas support but did not penetrate the fabric structure. This indicates that the size was applied as a cold gel by the manufacturer. Delamination is more likely to occur in this instance because the glue size is a descrete layer not intimately bonded to the fabric. On the other hand, a glue size applied hot would penetrate the fabric structure, thereby locking the yarns and reducing the canvas shrinkage and paint cleavage that could occur.

Several factors accounted for such severe and extensive damage to the Sawyer painting. First and foremost was the phenomenon of canvas shrinkage producing high compressive forces. Second, the damage was exacerbated by the presence of a discrete glue size layer that lost its adhesive strength as it gelled and allowed the ground layer to release from the canvas. Third, the thin, brittle paint and ground layers, too weak to resist the compressive forces generated by the shrinking canvas support, were fractured and tented out of plane. This phenomenon of rapid canvas shrinkage followed by delamination of the paint layer was also reported by Hedley (1988).