A black light is an ultraviolet light source used in the examination of artworks to determine what, if any, alterations have been made that may not be visible in natural or raking light. Long wave electromagnetic radiation, invisible to the naked eye, is emitted from the lamp and produces an ultraviolet fluorescence that can reveal recent restoration on older paintings. A black light is used in a dark area, to examine a painting and determine its condition. Reading the fluorescence of a painting may be complicated, and should be done with the assistance of a trained eye. Areas of a painting examined under ultraviolet light that fluoresce as dark purple can indicate inpainting or overpaint applied onto the original paint or varnish layers, sometimes covering old structural damages such as tears. Newer restoration will appear more pronounced. As chemical changes occur in paint over time, restoration may lose it’s degree of fluorescence. More subdued discoloration or a lighter muted purple may indicate older restoration buried below a natural resin varnish that appears as a green film over that area. Aged layers of dirt and/or oil added to a natural resin varnish will block the ultraviolet light from penetrating below the surface, obscuring hidden information. Under examination “masking” varnishes, which absorb the UV frequency, can also fluoresce as a dense green color. Although these coatings can appear similar, a discerning eye should be able to distinguish subtle differences in tone and application. A few pigments also have fluorescing characteristics.
An example of this can be seen in the bright yellow/ green fluorescence of zinc white or the day-glow orange of some red pigments. Restorations are usually made well after the artists’ completion of a painting, however, it is important to recognize later artists’ changes and certain original glazes that may also fluoresce and appear to be restorations. A trained professional can provide further analyses and determine condition. There are numerous methods in the examination of a work of art to be considered. The black light is a useful tool that is only as good as the person who makes use of it, with knowledge and discernment.