The Feasibility of using Thermography to Detect Subsurface Voids in Painted Wooden Panels

Abstract

Thermography is a technique whereby the construction or condition of objects is studied by means of precisely measuring temperature variations over the surfaces of such objects. Voids within solid structures tend to cause cold or hot spots on the surfaces of these structures as the whole object is heated or cooled slightly. Industrially, thermography is used to detect voids in cast and laminated structures. By studying areas of known voids in test panels, the described research indicated that voids as small as 1/8 in diameter beneath the gessoed surfaces of wooden panels could be resolved with a raster scanning radiometer, the principal instrument used for thermo- graphic studies. Smaller voids were discernible when they were grouped together. Only voids im- mediately beneath the gessoed surfaces of the test panels were detectible. Additional tests indi- cated that areas of simulated cleavage between the wood support and the gesso layers on the test panels could be detected thermographically. Thermography is the technique whereby the structure or condition of objects is studied by means of precisely measuring temperature variations over the surfaces of such objects. Research was undertaken to determine whether thermographic techniques could be ap- plied to the field of art conservation to detect subsurface voids within painted wooden panels. It must be stressed, however, that this research was carried out as a feasibility study with no attempt being made to establish or refine a technique that could be im- mediately applied to the examination of works of art. Blind cleavage and worm tunneling are two manifestations of the deterioration of panel paintings that can be difficult to detect. The term cleavage refers most often to a loss of bond between two paint layers, between a paint layer and the ground layer, or between the ground layer and its support. Blind cleavage is simply a form of cleavage that cannot be de- tected visually. Worm tunneling is the condition affecting panel paintings wherein insects