5 SUMMARY

To assess the risks to cultural objects such as photographs and paintings brought about by changes in temperature or RH, an accurate determination of the stresses in the different layers of the object is necessary. In turn, those stresses must be compared to the strengths of the artists' materials at the specific environments in question and the rates of loading encountered by the object. Computer modeling in the form of finite element analysis readily lends itself to performing the needed analysis. However, the ability to numerically model multiple layered structures such as paintings and photographs is extremely dependent on an accurate determination of the mechanical properties of the constitutive materials. Reaching this determination includes assessing alterations to the mechanical properties brought about by time.

This study has examined some of the materials typically found in a large proportion of canvas paintings and photographs. It has been shown that the same systematic numerical approach applies to a wide variety of multilayer materials, ranging from precision-coated photographic materials to handcrafted oil paintings on canvas. The effects of temperature and relative humidity brought about over moderately long periods of time can be modeled successfully if the material properties are characterized in an equilibrium or quasi-equilibrium state rather than using rapid-loading test data commonly used to report the mechanical properties of polymer and nonpolymer materials.

A remarkably accurate representation of the deflection and image layer stresses was calculated by the computer and verified by the actual measurement of the deflection of Ilfochrome print specimens. This result was due to the development of the proper protocols necessary to establish the equilibrium mechanical properties of the polymeric materials. It was also no doubt achieved because of the precise manufacturing uniformity necessary in each coating layer of a photographic material to ensure the required product consistency.

Computer modeling of a typical canvas painting with glue size and lead oil paint layers showed that deep drops in temperature at low RH (from 23� to −3�C at 5% RH) resulted in both uniform and high stresses in the paint layer. These stresses equaled or exceeded the measured breaking strengths of 13-year-old paint. Paint and glue tested at −3�C and 5% RH showed extremely brittle behavior, shattering into multiple pieces when broken. It could be inferred that an actual painting, constructed with the materials modeled, would exhibit severe and extensive cracking over the entire surface of the painting. The same computer analysis also showed that the glue layer reaches stresses slightly more than twice those of the paint layer. Computer modeling of the painting with respect to desiccation (from 70% to 10% RH) showed that crack patterns similar to those found in cooling could develop, but they would not be nearly as extensive.

Finally, a comparison of the results for paintings and photographs reveals that the temperature parameter is of greater consequence to a typical oil painting on canvas, whereas RH turned out to be the more significant stress-inducing parameter for Ilfochrome photographic print media. This result confirms that optimum storage of different types of cultural objects must consider both the physical and chemical well-being of the objects in any chosen environment. Computer modeling using the method of finite element analysis provides the only suitable way to quantify internal stresses so that a comprehensive view of “ideal” storage and handling of valuable cultural artifacts, one that considers both chemical as well as structural integrity, can be realized.