The Mechanics of the Deterioration of Surfaces
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THE MECHANICS OF THE DETERIORATION OF SURFACES GUSTAV A. BERGER* AND WILLIAM H. RUSSELL" "Art Conservation Research Foundation, Ltd., 115 West 73rd Street, New York, NY 10023 "*Georgia Institute of Technology, College of Architecture, Atlanta, GA 30332 INTRODUCTION Paint layers on a stretched canvas or solid support present complicated behavior and failure patterns. The cracks, cups, blisters, and sagging commonly found in paintings give ample evidence of the detrimental effects of environmental changes on a composite of mechanically dissimilar materials. Under variations in stress caused by environmental conditions, the materials are inelastic and non-linear individually and collectively are anisotropic. The authors have conducted extensive biaxial tests on dozens of painting samples under variable stress, strain, relative humidity, and temperature 11,2,3,4]. IN-PLANE STRESSES: BIAXIAL TESTS The overall stress variations caused by strain, temperature, and humidity can be determined on a painting sample mounted in a biaxial stress tester developed by the authors and shown in Figure 1.
Figure 1. Testing Arrangement
Mat. Res. Soc. Symp. Proc. Vol. 185. "c)1991 Materials Research Society
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A 25 cm square canvas sample (A) is mounted in a rigid steel frame (D). The stress in the warp and weft directions is measured separately by the load cells (E). The sample can be stretched or relaxed in each direction by the screws (H). The edges of the canvas sample are linked to the frame with fiberglass threads which are glued at each end with plasticized epoxy. These threads provide continuous support for the sample while permitting biaxial movements. The load cells have a sensitivity of 0.25 Newtons and the screws can adjust strains to 0.02%. The testing frame is enclosed in an environmental chamber with temperature ranges from 0 to 50 °C and relative humidity ranges from 5% to 100%. Due to the size and thin cross section, the sample quickly responds to temperature changes of 0.1 °C and humidity changes of 0.1%. Automated data logging equipment permits short term sampling every 15 seconds as well as long term continuous testing over thousands of hours. One sample recently tested consisted of a medium strength linen canvas weighing 370 g/m 2 with a heavy paint layer weighing 2100 g/m 2 . The use of thick paint layers weighing 5 or 6 times the canvas weight is not unusual for many modern artists such as Balthus, Appel, and Cucchi. Test results for this sample of canvas-paint laminate subjected to moderate temperature and humidity changes are shown in Figure 2. The changes in temperature and relative humidity are shown on the top graph, and the resulting changes in stress on the bottom graph. These graphs show clearly that both temperature and humidity have a strong influence on the stress in the sample. For example, the rise in humidity from 17% to 55% at hour 30 causes a drop in tension from 196 N/m to 183 N/m. This gives a Humidity Stress Index (HSI), defined as the ratio of the stress change to the humidity change, of -0
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