Changes in tension in canvas paintings caused by fluctuations of the environment have been recorded by automatic data logging. Samples of fabrics in their raw state, after wetting, sizing, priming, and painting were stretched and tested bi-axially. These samples were exposed to fluctuations in temperature between 0°C and 50°C and relative humidity of 5%-100%. The resulting changes in tension of over 1500% were recorded. The divergent reactions caused by the different treatments were evaluated. The conclusion was that expansions and contractions of the components of the painting cause plastic deformations of all of its layers. Accumulations of small dimensional changes produce one-sided stress concentrations within the material. Graphic models are given of the mechanisms causing both deformations and stress increases ( Stress Valves or Deformation Traps ). These models show the relative stiffness of the different layers to be one of the decisive factors of the deterioration of outer surfaces exposed to environmental changes. In other words, for surfaces to be preserved, their outer layer must be softer than their supporting layer (<i>Fat Over Lean</i>). Therefore, the measurements performed by the Art Conservation Research Foundation, Ltd. on stressed un-isotropic membranes have wider applications: since the skin of every object can be considered a stressed membrane, these findings are instrumental for the understanding of the decay of surfaces exposed to environmental changes.