The authors have built a Biaxial Stress Tester for Stretched Canvas in 1980 in order to measure the changes in tension in canvas paintings in response to controlled changes in the environment. Numerous samples of old and new, painted and unpainted linen canvas have been tested under controlled environmental conditions of 0-50°C and 5-100% relative humidity (RH), and their stress changes have been recorded. The resistance of the samples to controlled dimensional changes (Modulus of Elasticity, E ) has also been measured within the limits of the above environmental conditions. Since June 1984, information has been collected with the help of automated data logging. Based on these findings, working models of the mechanics of decay of canvas paintings were developed, documented by practical examples. These show that irreversible dimensional changes are introduced by environmental cycling which lead to cracks, blisters, and other deformations; that the stiffness of any exposed top layer relative to the stiffness of its supporting substrate is the decisive factor in the preservation of surfaces exposed to environmental changes (the rule of Fat Over Lean). Since all outer surfaces of solid bodies may be considered stressed membranes, these investigations have provided new insights into the process of aging and decay of all surfaces exposed to environmental changes.