KRESS

EXAMINATION METHODS AND SCIENTIFIC TERMS

ULTRAVIOLET ILLUMINATION

What is Ultraviolet Fluorescence?

Ultraviolet light that falls with the 320-400 nm range (long-wave) of the electromagnetic spectrum is generally used to examine artworks; although short/mid-wave UV light (200-280 nm and 280-320 nm, respectively) is also useful in identifying certain materials. This technique is non-destructive and can reveal important information relating to surface coatings and other organic materials (e.g. varnishes, adhesives) as well as an object’s restoration history (e.g. retouching). Occasionally, UV light can also be used to tentatively identify certain pigments depending on their auto-fluorescent properties (e.g. red lakes, indian yellow, etc.).

History of Ultraviolet Fluorescence

Examination of paintings using UV light first began in the 1920s, although the phenomenon of ultraviolet fluorescence was discovered as early as 1801. James Rorimer, director of the Metropolitan Museum of Art from 1955 to 1966 (and former Monuments Man) published a book focusing on the use of ultraviolet light in the examination of artworks in 1931.

Commonly Used UV Light Sources

Sources that generate ultraviolet radiation (“black lights”) tend to emit long wave UVA light, with an emission spectrum that peaks at around 365 nm. A special type of blue-purple glass is used to block the small amount of visible light that it is also emitted, which is why these “black light” sources tend to give off a blue-purple color. As light emitted in the UVA range is damaging to the eye, protective glasses must be worn when examining artworks under UV light.

How does Ultraviolet Fluorescence work?

Light within the ultraviolet radiation or UVA range (200-400 nm) has the ability to cause many organic materials (e.g. varnishes, oils, adhesives, organic pigments, etc.) to auto-fluoresce as well as a handful of inorganic materials (e.g. inorganic pigments. When UV light is absorbed by these materials, the light that is reflected back towards our eye is called fluorescence. Different materials will have varying degrees of oxidation or may possess other chemical properties that will directly influence both the color and intensity of the observed fluorescence. Materials that have not had time to age or oxidize will not produce a strong auto-fluorescence. UV light is therefore useful for revealing areas that have been recently restored (retouching will often appear dark under UV). Conversely, some modern materials will auto-fluoresce brightly (e.g. zinc-white containing paints), so caution should be exercised when drawing conclusions based solely on examination with UV light. Often additional testing is required to characterize the nature of surface coatings, retouching, and other materials.

References

Kirsh, Andrea, and Rustin S. Levenson. Seeing Through Paintings. New Haven: Yale University Press, 2000.

MacBeth, Rhona. “The Technical Examination and Documentation of Easel Paintings.” In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 294-6. Routledge: London and New York, 2012.

Rie, Rene de la. “Ultraviolet Radiation Fluorescence of Paint and Varnish layers” in PACT: Journal of the European Study Group on Physical, Chemical, and Mathematical Techniques Applied to Archaeology 13 (1986): 91–108.

Rorimer, James J. Ultra-violet Rays and their use in the Examination of Works of Art. New York: Metropolitan Museum of Art, 1931.

Taft, W. Stanley Jr., and James Mayer. The Science of Paintings. New York: Springer, 2001.