UV fluorescence analysis

In addition to the Athenian white-ground lekythos I talked about in my last post, I’m studying a Japanese sue jar from the 12th century CE. After a solid week of looking at my two ceramic pieces under the microscope and doing thorough condition reports, I was pretty sure I knew them well – that is, until I looked at them under ultraviolet light.

Ultraviolet (or UV) light falls in the wavelength range of 100 to 400nm. If you’ve ever seen someone wearing a white shirt under a black light, you know that UV can cause seemingly dull materials to fluoresce brightly, sometimes with unexpected colors in the visible range. This occurs when the UV light excites electrons in the material and the electrons fall back to a lower energy state, emitting longer-wavelength, lower-energy light that our eyes can detect. See the Techniques page for more information about UV fluorescence.

It turns out that many materials used in art production and conservation, especially lacquers, glues, and pigments, fluoresce strongly under UV light. For example, when I looked at my lekythos under UV, several brownish stains that I hadn’t even noticed before fluoresced bright orange, indicating that shellac had been used to repair the vase. There were several orange spots near the handle and one on the white ground near the base – it looked like someone had accidentally dripped a little too much shellac on, then tried to wipe away the excess. The brown spots blend in well enough with the off-white surface, but when you turn off the lights and shine a UV lamp on it, the handle looks like someone sprinkled it with a glow stick.

lekythos_uvCompareHandle2

Shellac used to repair the handle of the lekythos, under white light (left) and UV light (right).

The fluorescence on the sue jar is much more varied. It also has a few brownish stains that fluoresce dull orange, both on the outside and the inside surfaces. More interestingly, it has some cotton-like fibers, barely visible under normal lighting, that fluoresce white, yellow, yellow-orange, and even green in some areas. In addition, the occasional blue fleck is visible in the clay matrix. I’m studying this jar to see if it is coated in urushi, a typical Japanese lacquer that was traditionally used on a wood substrate to create beautiful lacquerware.  It’s possible that the fluorescence on the jar comes from shellac or some other lacquer, as urushi does not fluoresce very strongly.

Comparison of sue jar under white light and UV light

The sue jar under white light (left) and UV light (right), with fluorescent patches visible on the inside of the mouth.

Analyzing these ceramics under UV did more than just create some pretty colors – it revealed new clues about the history of the objects and opened up new questions about what they were used for (how did those weird fibers get on the sue jar?) and how they were conserved (why were two different adhesives used to repair the lekythos?). In the coming weeks, I’ll be carrying out some more tests using different spectroscopy techniques to try to answer these questions.

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