The first step in a conservation treatment or condition assessment is examination. When examining an object, we try to observe as much as possible about how it was made, how it may have been modified, and how it has deteriorated. In order to get the maximum amount of information about the object, we often employ tools to augment our basic senses. These tools can range from simple magnification devices to sophisticated analytical instruments. One frequently used examination tool is an ultraviolet (UV) lamp, which allows us to witness a phenomenon called fluorescence. UV fluorescence can provide useful information, especially about object composition and the presence and location of old repair materials.
“Ultraviolet” refers to a portion of the electromagnetic spectrum, the portion with energy of wavelength ranging between about 100 and 400 nanometers (nm). This radiation is of shorter wavelength and greater energy than the radiation visible to the human eye, which lies between approximately 400 and 700 nm. The UV section of the electromagnetic spectrum can be divided into three subcategories: long-wave UV between 320 and 400 nm (also known as UV-A), medium-wave UV between 280 and 320 nm (UV-B) and short-wave UV between 180 and 280 nm (UV-C).
When certain materials are bombarded with UV radiation, the absorbed energy causes electrons to temporarily move to a higher energy state. The excited molecules rapidly lose the excess electronic energy as electrons return back to the ground, or unexcited, state. As the excited electrons transition back to the ground state, they emit energy, always of longer wavelength than the absorbed energy. The emitted energy may take the form of heat (vibrational energy) or photons (radiant energy). The emission of a photon is referred to as fluorescence. When the bombarded materials emit photons in the visible range (between 400 and 700 nm), the human eye can observe fluorescence.
Some pigments, dyes, natural resins, synthetic adhesives and minerals tend to fluoresce in UV. When UV energy is directed at such materials in a darkened room, they appear to glow in different colors depending on the material at hand. Thus UV examination and documentation can be a useful tool for detecting and recording the presence of certain original or post-manufacture colorants, adhesives and coatings. For example, when we examined the painted limestone lintel of Sen-Nedjem, which had been restored in the late nineteenth century before coming to the museum, we could differentiate modern restoration materials including pigments and gesso based on their UV fluorescence.
The yellow and red/pink/peach pigments on fragments of a Greco-Roman cartonnage mask fluoresce in long-wave UV, leading us to speculate about the nature of the pigments and binding media. The distinctive UV fluorescence also gave us an additional means of comparison between this mask and several similarly constructed and decorated cartonnage objects in the collection, all of which may have been produced by the same workshop.
Because exposure to UV energy can pose a health risk to the eyes, we always wear protective eyewear when using a UV lamp!