6 STRESS-VALVES OR DEFORMATION-TRAPS

THE TYPE of mechanism that leads to deformations and dimensional changes in materials exposed to environmental changes was first observed by Richard Buck (1978). He noticed that a panel painted on one side only is inexorably deformed by repeated cycles of humidity changes. In fact, it is deformed to such an extent that no single physical force could bring it back to its original condition. This happens through the one-sided accumulation of small dimensional deformations created by every rise in humidity.

In Buck's example only compression is accumulated, while elongation is eliminated. In stretched canvas the opposite is the case. Our research has found many such one-sided layouts in the geometry of materials, which lead to the accumulation of deformations during aging. We have called these configurations stress-valves or deformation-traps because they permit only one type of stress, either compression or tension (Berger and Russell 1986).

Figures 5 and 6 show schematically the most common stress valves that occur in paintings. The top diagram, for cracking, shows the paint film at rest (fig. 5a). There are two heavy layers of paint at both ends and a weak layer between them. Here the weak layer is shown as being thinner, but it could also be weaker for a number of reasons. The paint layer expands when the temperature rises or when exposed to solvents. This is a well-documented fact supported by ample evidence, some of which will be shown later. If the paint is not held firmly, the strong, heavy layers expand and compress the weak layer, which, as a result, becomes shortened (fig. 5b). In the subsequent cooling or drying cycle, the contraction of the paint shortens all parts of the paint film (fig. 5c). However, the weak portion of the paint film cannot resist the strong tension of the adjacent heavy layers. It becomes elongated against its own tendency to contract and is overstretched in the process. As a result, the overstretched part is further weakened and eventually ruptures. After the rupture, the paint film can only be compressed in the expansion cycle (fig. 5d). From now on, contraction will only open the crack, but no longer cause the film to elongate (fig. 5e).

Fig. 5. Cracks, Figures 5 and 6 give a schematic representation of a stiff paint film held by a soft support (slack canvas) that is restrained at both edges either by the stretcher or by adjacent paint films.

Fig. 6. Blisters, The central section is detached from the support.

Figure 6 (6a–6c) explains the circumstances that lead to blisters. Here, a failure of adhesion between the paint film and the substrate is shown by the open space between them. In figure 6b, a contraction leads to an overextension of the weak, unattached film. A subsequent expansion cycle (fig. 6c), does not compress the loose film, which evades compression by buckling. Thus every contraction elongates the unattached section, but subsequent expansion no longer shortens it.

Cracks are a sign that the paint film has contracted and has become smaller because cracks take space. Blisters are a sign that the paint film has expanded since a bent paint film must be larger than a flat one. Both cracks and blisters can often be found side by side in the same painting. It would be hard to explain this seeming paradox in any other way than by the stress valves described above.

Cupping of the paint film and warping of the canvas can be explained by similar mechanisms. We could safely generalize and say that such stress-valves can be found whenever aging causes physical or dimensional changes. They work like a pump: every environmental change is another stroke of this “pump,” which fills up the reservoir of stress or deformation. The elements that lead to the deterioration of surfaces exposed to environmental changes are as follows:

• Changes in environment cause surfaces to expand and contract. They generate the forces that drive the mechanism of decay.
• Certain spatial arrangements between the layers of the materials cause these forces to be restrained in only one direction. This leads to the accumulation of stresses and deformations.
• With every change in temperature and humidity, some minute dimensional changes in the stressed material remain nonrecoverable and cause its permanent deformation (Berger and Russell 1984).

Repeated variations in temperature and RH (e.g., during day and night or due to thermostatically controlled devices, such as cycling air-conditioners and heaters, turning on or off electric lights, using radiating heat sources such as incandescent lights and the light coming through the windows), lead to the accumulation of the nonrecoverable dimensional changes. As a result, the materials fail, just like a wire when repeatedly bent back and forth. For example, in the case of paintings, the paint film cracks to avoid extension or blisters to avoid compression, depending on the location of the initial failure.