The Conservation Division of the National Gallery of Art has long relied on beta-radiography to produce one-to-one contact images of watermarks in works of art on paper. The increasing cost of purchasing and maintaining a beta plate, the lengthy exposure times, and concerns about radiation safety have stimulated the search for a reasonable replacement for beta-radiography. Joachim Siener from the Württembergische Landesbibliothek, Stuttgart, uses a method of imaging watermarks using ultraviolet light, infrared light, and a phosphorescent pigment. The described technique is an adaptation of his imaging method.
This imaging technique works on the premise that infrared waves cancel the effects of UV waves on the light-emitting properties of phosphorescent pigment imbedded in a plate. UV waves will excite the pigment plate to phosphoresce overall. IR waves will penetrate the paper both in areas of standard thickness, as well as in the thinner areas that comprise the watermark, laid lines, and chain lines causing the phosphorescent pigment to go dark. UV waves will pass more easily through the thinner areas of the paper causing the pigment to continue to phosphoresce in those corresponding areas. A bright image of the watermark appears on a dark background. When placed in contact with photographic film, the image of the watermark will expose the light-sensitive emulsion.
The images obtained from this technique are one-to-one contact negatives from which black and white printes are produced for reference. The film shows image interference from some types of media and more grain than seen in beta-radiography. The virtue of this technique lies in the ability to obtain a fast, safe, low cost one-to-one image.