Taking marble samples

Along with Mark Abbe’s art historical and provenance research as well as his pigment analyses mentioned in great detail in earlier posts, the research on the marble sculptures includes research on the marble itself, to be conducted by Scott Pike. In order to find out from which quarry the marble was obtained for each of the statues, we have to take samples and submit them to scientific analyses.  As you can imagine, sampling is a destructive process: holes have to be drilled into the sculptures to obtain either a core or a powder sample. We needed both.

It was object conservator Amy Jones’ job to sample the museum’s sculptures. During four days, we worked together in the galleries and in the lab with a drill, little vials, small baggies, labels and a water bottle.  Amy’s main tool was the drill, mine was the squirt bottle.  In order to prevent the diamond drill bit from overheating during the drilling of a core sample, water is squirted on it (you do not need to do this when taking a power sample).  Lubricating  the drill bit with water also makes it last longer and, more important, it prevents the heating of the sample itself.  (My other job was to document this whole process and that’s why you have photos to look at below!)

We drilled into the statues in places that are hidden or unobtrusive, although this was not always possible. Each sampling location was carefully documented for our files. In order to minimize trauma to the sculpture, the powdered samples were drilled inside the hole created by the drilling of the large core sample.  In all cases where the holes are actually visible, these will be filled to make them less conspicuous.





XRF: X-Ray Fluorescence

After having identified the sculptures that still bore traces of pigments, the next step was to do x-ray fluorescence. This is a non-invasive technique used for the identification of non-organic pigments. It is not enough to note the presence of pigments on sculpture and take photographs of their location; we need to know of what these mineral pigments are made. This helps with the dating and authentication of the work of art because certain pigments are used in specific countries (or cultures) or during specific historical periods.

The analysis was conducted by Scott Pike, associate professor of geology and earth sciences at Willamette University, who had in his luggage his very handy x-ray gun. While we worked together, Scott explained how this Star Trek phaser looking thing works. This instrument send x-rays on the non-organic matter we wish to analyze. This results in electrons being bumped from their atomic orbital positions, releasing a burst of energy (fluorescence) that is characteristic of specific elements (elements of the periodic table, that is). The fluorescence is identified and recorded by the device. Each pigment will have its own combination of elements. Thanks to this technique, we can tell if the red pigment is actually red ochre (an iron oxide used in ancient Egypt) or cinnabar (mercury sulphide used by the Romans).

The x-ray gun is controlled with a computer. Although I did have my photo taken with very cool looking instrument (I just had to!), I assisted Scott by manning the computer while he held the portable XRF device to do the analysis. Elizabeth joined us in the galleries for a while and that allowed me to take a few pictures. Luckily, x-ray fluorescence does not require to be done in the dark. So we didn’t have to work late nights at the museum. All we had to do was stanchion off a work area around the sculpture under examination so that people did not disturb us… but we did have some people intrigued enough to stop by and observe what we were doing.