From cataloging to previewing, the black light is an essential tool for the American & European Works of Art department. There are two main types of black light devices we use in the department, one being affectionately termed “the beast,” as it produces quite a strong ultraviolet light that can cover a wide surface area; the second has both black light and incandescent light, of which the black light produces a softer glow than “the beast” and can pick up more subtleties, such as older retouch. While black lights come in all shapes and sizes, the most important thing when using a black light is to know what you are looking for!
With paintings, black light can be used to help determine if a work has been retouched or “inpainted” by shining it across the surface to see if anything fluoresces. What you are looking for are any areas of a vibrant dark purple; they can come in any shape and size, from small pinpoint dots to large swatches. The shades of fluorescence can also vary depending on the age of the retouch. Paintings with newer conservation will be a deep purple while works with older retouch will have a much more subtle, even faint, fluorescence in a lighter shade of purple.
However, it is also important to note that some pigments do fluoresce under UV light, such as some chromium pigments and rose madder. Assess the color, the shape of the area, as well as the brushstrokes (if applicable) to see if it’s more likely the artist’s hand rather than that of the conservator. Today’s conservators don’t try to fool you; their inpainting is applied in a way that makes it apparent what is their work and what is that of the artist. In incandescent light and visible to the naked eye, conservators’ colors will generally be a shade or two lighter or darker than the field of color that they’re repairing, so make sure you look at the work under both types of lighting. Ask yourself questions like “Is the fluorescence relegated to only one color in the work?”; “Do the fluorescing brushstrokes match the rest of the work?”; and “Is the shape of the fluorescence irregular?”; all help narrow the field of retouch versus original.
There can be other factors in why a work fluoresces, such as varnish, mold, stains, and accretions. Typically, varnish will give the work a flat appearance under UV with a yellow to green cast. Something to keep in mind about varnish is that it’s not always applied as a single layer; in fact, some varnish jobs can look downright patchy and even drippy under UV. There are also masking varnishes that help slow down the aging process by using a UV-filtering material. This kind of varnish is a forgery favorite, as it can be used to hide retouch and fake signatures and will present as a greenish fluorescence. Meanwhile, certain molds can appear yellow or orange with black light. Stains and surface-level accretions can also fluoresce, but these can often be easily spotted by alternating UV light with incandescent light. Operating under UV can be quite handy, but a comparison with incandescent can alert you to discolorations that you can discern with the naked eye. Once you’ve determined retouch is likely, consider the shape and size of the retouch as a key is to a map; thin, narrow lines are likely fill in lines of craquelure, small squares could correspond to flaking (when the paint literally starts flaking off the canvas), arcing diagonals can illustrate the path of a previous abrasion or tear, and large irregular-shaped portions could indicate areas that have been stained or water damaged.
The fluorescence of a signature is often a red flag for specialists and dealers alike, and conservators take special pains to protect the quality and character of a signature. For instance, think of a work done by your favorite painter (who happened to sign their work) and under UV the only fluorescence to be found was the signature. Rather strange, no? Forgers will often take older works with no signature and sign them in the manner of a known artist in order to increase the work’s value. Though do keep in mind signatures can also have a layer of protective varnish covering it that may fluoresce, but the whole area should light up, not the characters themselves. If the signature looks odd under UV light proceed with caution and seek out an expert.
UV light is also a helpful tool when trying to identify signatures. There have been countless times when a signature is just too faint or too jumbled in visible light to discern the artist’s name. Enter the black light. Often the UV light will bring a faded signature out or the different cast of the light can allow you to make out letters you couldn’t quite pick out before. Again, the instance of flipping back and forth between UV to incandescent can help you tease out more details.
Black light isn’t just for paintings though, it can also tell you things about works on paper. Mold and mildew still fluoresce a yellow-orange, but bleach and optical brightening agents will shine a cool bright blue – think how a white shirt looks under UV. For prints, if a work has been bleached to remove stains or water damage by a conservator the paper will fluoresce this way. In photography, beginning in the early 1950s, manufacturers began producing paper with optical brightening agents to create whiter and brighter whites. By the mid-1960s a large portion photographic paper was being produced with these brighteners and by the 1980s the majority of photographic paper produced had them. Why does this matter? While dating photographs isn’t down to an exact (and accessible) science, knowing the addition of optical brighteners came after the 1950s can help you identify some vintage or “of the period” photographs from later printings. For example, if you have an image taken in 1932 by Henri Cartier-Bresson, a photographer known to not have dated or assigned edition numbers, and it fluoresces a cool blue, then you have a later printing of the work and not a vintage 1932 photograph.
While the help of a black light might end up leaving you with more questions than answers, it can reveal a work’s hidden history. As always at Skinner, UV photos of artworks can be requested with a condition report and during our live previews work can be shown to you under black light, just ask!
 Paul Messier, “Notes on Dating Photographic Paper,” Topics in Photographic Preservation, Volume 11 (2005): 123-124.