THE FEASIBILITY OF USING THERMOGRAPHY TO DETECT SUBSURFACE VOIDS IN PAINTED WOODEN PANELS
BRUCE F. MILLER
Before this technique is applied to works of art more information must be obtained. The maximum temperature change to which a panel painting can be safely subjected should be studied. Although much has been written about the importance of relative humidity in panel paintings, to the author's knowledge no data have been published on the effect of small and brief temperature changes on panel paintings when relative humidity is kept constant. Although the suggestion of warming or cooling panel paintings sounds somewhat ill advised at first hearing, one should remember that most panels, except those kept in scrupulously controlled environments, have probably undergone considerable and repeated temperature variations throughout their lives. Indeed, many necessary processes of conservation such as setting down cleavage and applying balsa backs often require the application of heat to panel paintings. It should also be considered whether or not a brief rise or decrease in the temperature of a panel is more or less desirable than rubbing one's fingers over the surface of suspected areas of cleavage or worm tunneling.
For the sake of the art works subjected to future thermographic analysis, however, the minimum and optimum amounts of temperature change required to produce accurate images of subsurface voids should be determined. The heating methods used for this research were to a great extent dictated by convenience. Although the technique of heating the front surface of the panels was gentle, it did not produce adequate images of the voids. However, this technique may have the potential of considerable refinement. The method of heating the reverse of the panel provided better images but was more severe in that the panel was heated throughout its structure rather than just on its painted surface, and the temperature used happened to be 90°F. Although controlled studies at lower temperatures were not carried out, it is the author's impression that the required temperature change within the panel could be as little as 5°F to 10°F. By using heating and cooling techniques simultaneously, or by using short pulses of infrared radiation on the order of tenths of a second in duration, harmless but detectible temperature variations over voids might be created. Refinements in technique and information about minimum required temperature change must be determined experimentally, however.
Unfortunately, the matter of cost must also be raised. Raster scanning radiometers are available from a number of manufacturers.8 Their costs range from roughly $27,000 to $40,000. However, for occasional thermographic studies of portable panels, a conservator may be able to use thermography equipment in hospitals as did this author. Because the major medical use of this equipment is the detection of breast cancer, possible methods of locating an instrument would be to get in touch with a local chapter of the American Cancer Society or to speak with someone working in the mammography unit in the radiology department of a large hospital.