THE FEASIBILITY OF USING THERMOGRAPHY TO DETECT SUBSURFACE VOIDS IN PAINTED WOODEN PANELS
BRUCE F. MILLER
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 applied 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 immediately 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 detected visually. Worm tunneling is the condition affecting panel paintings wherein insects have consumed either the gesso ground or the wood support beneath the gesso, thus leaving the paint layers unsupported. Even if the appearance of the reverse of a panel indicates that it had been attacked by insects, it is often difficult to determine whether insect damage is present immediately beneath the paint surface that is the location where the presence of any void would have the most serious effect on the security of the paint film.
Because blind cleavage and worm tunneling have the potential of resulting in extensive damage to the design layers on panels, there is a need for a technique of detecting these defects. Presently the technique of “sounding” the paint surface is the chosen method for detecting blind cleavage and worm tunneling. Unfortunately, this technique is capable of detecting only large voids and the required contact with the paint surface can be extremely dangerous for an unsupported paint film.
X-radiography has some potential for the detection of worm tunneling. However, the presence of x-ray absorbing pigments in the paint or ground layers would prevent the detection of small worm tunnels within a wood support. In addition, only sophisticated x-ray techniques have the ability to establish the exact location of worm tunnels within panels. X-ray techniques are not at present capable of detecting cleavage.
An ideal method for detecting cleavage and worm tunneling should have the following characteristics, listed in order of importance:
- Harmlessness. (There should be no possibility of damage to the object, and ideally there should be no contact with the paint surface.)
- Sensitivity. (The technique should be capable of detecting small voids, and it should be capable of distinguishing between voids just beneath the paint surface and those voids deep within the panel.)
- Low cost.
X-radiography and the technique of sounding do not satisfactorily meet the first three requirements. This being the case, the author feels justified in proposing another technique that has the potential to meet at least the first three requirements, if not all six.