9 PROTECTION OF PAINTINGS UNDER ADVERSE ENVIRONMENTAL CONDITIONS

IT IS rarely possible or economically feasible to protect paintings from all possible changes in the environment. Indeed, it could be said that many paintings have kept quite well without any environmental controls. Most paintings are quite tough, and paint has been used to protect other surfaces from decay since ancient times. Moreover, some paintings crack or blister profusely over part of their surfaces, while other parts of the same paintings that are exposed to the identical environment remain in perfect condition. It follows that certain conditions innate to the painting have protected it from deterioration. As long as an oil or an acrylic painting is “young” and flexible, it keeps quite well under the most adverse environmental conditions. Our tests have shown that this is the case because a soft film moves like a liquid to absorb its own expansions and contractions and, therefore, does not create sufficient stress to cause its own rupture. Only when a paint film becomes harder and stiffer than its substrate can it begin to deform on its own. Therefore, if the substrate stays harder than the aging film, as is the case in a painting on metal, it ages without developing cracks (fig. 15)(Berger and Russell 1987). It develops them only after the top surface of the paint becomes harder than its bottom layer (or when the adhesion or cohesion of the paint fails under repeated stress (figs. 5, 6, 7).