A STUDY ON DEVELOPMENT OF CRACKS ON PAINTINGS
DESPITE ITS limitations, the brittle coating method proved useful in studying humidity-induced movements on paintings. The film of size was found not only to expand in high humidity, but also, following the brief expansion, to contract when its moisture content reached sufficient levels. The magnitude of the contraction will vary depending on humidity range and the length of time a painting is kept in high humidity. Short exposures may not be sufficient to bring about cracking of paint. Subsequent periods of lower humidity, however, will induce further contraction of the size layer and progressive increase of stress and strain (Karpowicz 1989). At lower tension the direction of the strains followed that of the most common pattern of craquelure on paintings. This direction will be the same in high humidity and during later desiccation. At higher tension the stress distribution will be different. A recent paper, however, reported that sized fabric stretched at high tension appeared to relax rapidly and stabilize at lower levels of tension (Berger and Russell 1988).
During the process of viscoelastic recovery in high RH, the layer of size could have acquired anisotropic properties through molecular orientation in the direction of shrinkage (Karpowicz 1989)(figs. 5, fig. 6). It is possible that the stiffness of size became greater in the direction of orientation (Turner 1948). This may explain why strains during drying that followed were small, despite the development of high stress, and caused only limited cracking in the same typical direction. It is important to realize that the size, and presumably the fabric, are both nonelastic materials. The effects of time on the development of stress and strain are of primary importance.
Fabric, which also contracts in high humidity, may contribute to these movements and requires another study. The ultimate mechanical properties of aged paint, especially breaking strains at lower rates of loading, must also be investigated to take into account the viscoelastic, time-dependent behavior of size.
THE AUTHOR wishes to thank Stefan Michalski and Debra Daly, Canadian Conservation Institute, and the late Gerry Hedley, Courtauld Institute of Art, for their helpful remarks at the beginning stages of this project.