1 INTRODUCTION

PRIOR TO these investigations, it was impossible even to estimate the forces created by shrinkage or expansion of the different layers of a painting. The plasticity of the materials involved causes them to be continuously modified and transformed by their own responses to mechanical stress and/or environmental changes. Therefore, in order to repeat or compare stress measurements of polymers with any degree of accuracy, one must know the previous history of exposure of the measured materials (Dwan 1987; Guilly, Sotton, and Chevalier 1981; Kuzmitch 1978). For this reason, some of the initial results of this research were hard to interpret, as long as the measurements were recorded manually and had to be interrupted at night. With the introduction of continuous, automated data logging, the investigations took a giant step forward. Feedback of the electronic equipment made it possible to repeat the responses of the tested materials to environmental changes and to demonstrate the variations in reactions that occur with every cycle. Thus, some of the forces that lead to the physical decay of paintings, and possibly to the decay of all surfaces exposed to changes of environmental conditions, can now be measured reliably.