If you have ever looked at a white shirt under an ultraviolet (UV) "black light" you will have noticed that the shirt appears to glow purple. UV light can have a similar effect on many materials. Glue, epoxy and some paints will fluoresce brilliantly. Like a white shirt, substances may not catch the eye until viewed under UV light.
Ultraviolet (UV) scanning of art objects has been used for many years, and the technique is practiced on site at TK. Under certain wavelengths of ultraviolet light, erasures can be detected on paper documents. Modern alteration of jade and marble can cause the stone to fluoresce brilliantly, and the same is true of ivory. Many touch-ups and alterations on oil paintings become readily apparent. Unfortunately, the same is not true of the powdered pigments generally used on painted Chinese pottery. However, many of the adhesives used to bind those pigments will fluoresce, giving the altered area a distinctive glow. Certain epoxies, used to join fragments of pottery or metal, appear a brilliant purple or blue under UV light.
At TK, UV scanning may be used to help in the identification of certain minerals in corrosion layers, but the main use is to detect touch-ups and repairs. It should be stressed that repairs are not the same as alterations. Many antiquities are expected to have sustained some damage, as they have been recovered from collapsed tombs. Under such conditions it is highly unusual to find large pottery objects that are unbroken. Scanning is performed in order to ascertain the extent of the repairs, and to locate modern repair materials so that they may be avoided during sampling for TL and other tests.
The image above left is of a glazed pot, from the Spring and Autumn period of the Eastern Zhou Dynasty, viewed under normal fluorescent lights. Close visual examination will reveal suspicious areas, but the quality of the touch-up is quite high. The image on the right is of the same pot as viewed under a combination of long and short-wave UV light. The extent of the damage and repair is immediately apparent. The epoxy used to join the fragments fluoresces a brilliant purple. Despite the fact that this pot is constructed from numerous pieces, the nature of the edges and the fit between pieces, including alignment of the low-relief pattern, makes it clear that the fragments are all from the same pot. For this reason, despite being broken and repaired, the pot is still fairly valuable. It is not a fake, nor even an intolerably restored piece, utilizing parts from different vessels. Chinese pottery artifacts of this age are seldom found undamaged. While the pot would be significantly more valuable if unbroken, the fact that the pieces are all original to each other makes it a more valuable and desirable piece than if portions were replaced.
Not all adhesives fluoresce under UV light, and different materials fluoresce different colors. There is no single color that always indicates a modern substance. Most adhesives generally fluoresce green, blue or purple, if they fluoresce at all. Certain waxes also appear purple under UV light, and minerals fluoresce in almost every color. It is important to examine any fluorescent substance carefully in order to avoid mistaking wax, paper fibers, authentic corrosion or a stray smear of glue as an indication of repair. It should also be stressed that the UV light used for these examinations is not the same as the common black light available for recreational home use. The common black light uses long-wave UV light only. The instruments for use in examinations are filtered to produce either long- or short-wave UV light, as some substances respond differently to each. Attempts to scan objects with a common black light will not yield the same results. In addition, it should be stressed that certain wavelengths of UV light are quite harmful to the skin and eyes, as well as to some materials.