In addition to UVF, we did Visible-Induced Luminescence (VIL) imaging on all the sculptures and, a couple of times, we did Infrared Reflectography (IRR) as well. In both cases, imaging is done in the near infrared range of the light spectrum. I will focus on VIL, but IRR is essentially the same (or so Mark tells me), but we used a red LED spotlight to illuminate the objects rather than the flashes. This makes for much more interesting photos and that’s why I’m briefly mentioning it here. (IRR is also regularly used in the study of paintings because it allows you to see preliminary sketches (underdrawings) underneath the layers of paint—that’s actually pretty cool.)
The setup for VIL is the same as UVF, but you mustn’t forget to change the camera and the filters on the flashes! Basically, we’d do UVF and VIL one after the other so the camera angles and distances remained the same for both series of photographs. Of course, before you do all that, you also have to take ambient light photographs of the sculpture. And to each series, you take one photo with the grey card… and another with the colour card, if it’s not already in the shot. It was a lot to think about, but we came up with a routine and all went smoothly. Although there were a couple of times when we forgot to change the camera… (Ah, hum, well, you know, we were working late in the dark galleries of the museum.)
VIL is used to detect traces of Egyptian blue—the first synthetic pigment ever made, used for thousands of years in ancient Egypt and known throughout the ancient Mediterranean. Naturally occurring blue minerals were either too precious or difficult to transform into large quantities of blue powder to create blue paint, so the Egyptians invented an artificial pigment.
Egyptian blue has the peculiar property of emitting infrared radiation when excited with a red light. Traces of Egyptian blue can easily be detected in the infrared spectrum—the pigment will glow a bright white. None of this is actually visible to the naked eye; shine as much red light as you want onto a work of art, you will not see the luminescence (bright white glow) because human vision does not extend to that part of the light spectrum. However, if you have an infrared (night vision) camera (usually a modified digital SLR camera with infrared filters), you can take an image of that luminescence. That’s what we did. We took photographs of all ancient marbles with the infrared camera when we used the flashes to excite the pigments. We found traces of Egyptian blue pigment on a few objects—some deliberately painted (like the sarcophagus fragment), others having been in contact with particles of the pigment but not actually decorated with it. (You can figure this out normally by the pattern or distribution of the luminescence.) The luminescence is so strong you can actually see single grains of Egyptian blue!
There is a great video about Egyptian blue on the British Museum website, click here to view it. One of the photos below features Jimmy in the process of posting an image to Instagram. To see what he posted, click here!