1 INTRODUCTION

OVER THE past 10 years there have been substantial developments in research on the mechanical behavior of paintings. The knowledge of processes leading to mechanical failure of paintings materials and their properties has been greatly enhanced through pioneering work by Mecklenburg (1982). It has been established that environmental changes induce dramatic responses in fabric and size. The layer of size is responsible for stress development during humidity decrease; high stiffness of fabric dominates in high humidity. The most brittle component of the composite—aged oil paint—fails when movements in the supporting size and fabric exceed its rupture strains.

Stress distribution on paintings and the resulting cracks in the paint layer were simulated on a computer with the Finite Element Method (FEM) (Kilpatrick 1980; Colville et al. 1982). Elastic properties of materials were assumed and based on uniaxial tests.

In this work the humidity-induced movements were observed on biaxially stretched samples of primed fabric with a commercially available brittle coating taking the place of aged paint. The coating is widely used in industrial stress analysis as a form of strain gauge; it cracks when strains in a support exceed its breaking strains. The results reveal nonelastic properties of size and are consistent with a study on dimensional response of size in changing humidities (Karpowicz 1989).