8 CYCLING ENVIRONMENTAL CONDITIONS AND DISCONTINUITIES IN THE STRUCTURE OF THE PAINTING

CANVAS UNDER stress adjusts until every yarn tends to carry an even load over all of its length. If, for some reason, certain yarns receive a bigger or smaller load than the others, they expand or contract as need be until an equilibrium is regained. The plasticity and creep of textile yarns give them the capability to adjust. This capability makes it possible to stretch canvas into a perfectly smooth plane, as opposed to a stiffer metal screen or fiberglass fabric, which lacks the necessary “give.” Plasticity and creep also make it possible for fabric to regain this plane whenever it is disturbed. This is not the case with metal plates, which once deformed (dented) cannot be straightened out without loss of paint, or with wooden panels, which warp. In contrast, the softness of slack canvas makes it possible to bring deformed canvas paintings back into a nearly perfect plane surface without loss of paint. The plasticity and creep of fabrics play an important role in their performance as supports for paint. This is why canvas paintings could be kept flat for centuries.

A stretched membrane (fabric) will maintain a plane surface only as long as it has a uniform or symmetrical cross-section (fig. 12a). Priming and painting increase the cross-section on one side of the fabric. Applying paint locally (or cracking of paint) alters the uniform cross-section of a fabric (fig. 12b). Initially the paint film is soft and has little effect on the expansions or contractions of the fabric. However, as paint ages, its stiffness increases, and it begins to resist stress. If the tension in a partially painted fabric increases, the areas covered by the paint offer more resistance to expansions than do the bare parts. As a result, a one-sided pull (bending moment) is created that bends the paint film into the line of stress until an equilibrium is reached, and the stress is met again by an even resistance from all parts of the cross-section (fig. 12c). A material with plastic flow relieves stress concentrations, causing the stress to become more evenly balanced over its entire cross-section.

Fig. 12. Effects of an Uneven Load (Schematic Illustration)

This basic principle of stress propagation was first used by G. Berger to explain deformations in canvas paintings, such as cupping and bulging, as early as 1972 (Berger 1972, 1975; Skeist 1965), and later demonstrated at professional meetings (Berger 1978, 1980). Stress propagation is one of the main causes of cupping in fabric-supported paintings, though other factors common to all surfaces exposed to environmental changes also contribute to it. After all, cracking and cupping are not confined to canvas paintings. These factors are the compacting and shrinkage of the paint film by cross-linking of the exposed top surface (Berger 1980) and the cumulative compression by cycling stress changes (Berger and Russell 1986). Any of these additional distortions can be reduced by relaxation and vapor treatments. However, all the above contributing factors stem from the same defect, namely, the one-sided interruption of the line of stress, which cannot be eliminated by relaxation of the cupping alone. Discontinuity of the stretched surface by heavy pastose paint films, cracks, tears in the fabric, patches, or glue concentrations must lead to distortions in the plane of the painting. Effective lasting treatment can only be achieved by minimizing their effect through increasing and reinforcing the cross-section of the support, that is, by lining or mounting. Relaxation treatments that are not followed by lining or mounting address only the symptoms. They do not eliminate the main causes of the defect, which are the discontinuity of the paint layers and the loss of tension and stiffness in the canvas. Thus, if relaxation treatments are not followed by reinforcement of the substrate they do more harm than good, because they require frequent repetition of potentially hazardous exposures to high temperature and humidity that could be avoided by the long-lasting effects of a good lining.

The small oil on canvas by Yves Tanguy (fig. 13) was disfigured by strong local cupping, which greatly impaired its “readability.” This cupping was produced by minute breaks caused by the design the artist scratched on a commercial priming that was probably already several years old at the time the artist painted on it. Later handling and a customs stamp must have created further breaks in the priming. These breaks formed “deformation valves” that, because the painting was in a private collection and exposed to frequent cyclic changes, produced considerable cupping in an otherwise well-preserved paint film. In a young painting such as this, where the priming and paint film are still relatively soft, such deformations can be easily corrected by heat and vapor treatments. If, following this treatment, the painting is kept in a very stable environment—as, for instance, in an air-conditioned museum—the cupping might not return because the deformation valves would not be reactivated. However, were the painting to return to the same uncontrolled environment, the cycling changes would reactivate the deformation valves and cupping would reappear. The recurrence of the cupping would be more rapid than before, because the paint film has become stiffer due to aging and the discontinuity in the stress distribution is now larger.