Example: Egyptian Scarab and Its Hieroglyph Pigments
Okay, okay... The Egyptian Museum in Cairo did not actually lend me a priceless gold scarab artifact. No, this is a reproduction loosely based on the few known gold scarabs. I suspect, though, that its look derives more from The Mummy movies than actual artifacts. Several years ago, underwater archaeologists excavating a late Bronze Age shipwreck found a real golden scarab with hieroglyphs indicating that it was owned at one time by Egyptian Queen Nefertiti. Below is a photo of this authentic scarab (Credit: Institute of Nautical Archaeology):
I wanted to use this scarab reproduction as an example here for three reasons. The first is a very technical reason: scarabs are cool, and people recognize them. The Mummy films had waves of flesh-eating scarab beetles, and the movie Bubba Ho-Tep had a flying killer scarab. Scarabs are prominently featured in any museum with an Egyptian collection, and their gift shop will have scarab necklaces, pendants, and other jewelry. In reality, scarabs represent common African dung beetles, but popular culture has seized on them.
The second reason is to show off the superior depth-of-field of electron microscopy. Visible-light microscopes have shallow depths-of-field, meaning only a thin slice of an object is in-focus at a time -- only those features at a certain height appear sharp and focused. Usually with visible-light microscopes, the depth-of-field is measured in microns. With scanning electron microscopy, though, the full height of the scarab falls within the plane of focus, meaning the depth of field can be measured in centimeters (dozens or hundreds of times better).
Below: Secondary-electron image of the scarab; the depth-of-field allows the entire scarab to be in-focus.
A third reason is to demonstrate how paints with different colors also have different compositions, and the electron microprobe can highlight these chemical differences. The combination of scanning electron microscopy and X-ray microanalysis has been used to find traces of pigments that are no longer visible.
Below: Clockwise from upper left: (1) the bottom of the scarab has hieroglyphs painted in different colors; the red square represents the area shown in the other images; (2) a secondary-electron (SE) image of the hieroglyphs; the different paints do not show up here because SE images mainly show topographic, not compositional, compositional; (3) a backscattered-electron (BSE) image of the same area; here the different colors of paint on the hieroglyphs exhibit different contrast; and (4) a combination image which also shows compositional contrast.
Below: These red-green-blue combination element maps (one element is red, another is green, and a third is blue) show the compositional variations in paints with different colors. Notice that the hieroglyph in the upper left and the one in the lower left are painted the same color and have the same color in these combination maps.
Below: Here are red-green-blue combination element maps for a smaller area, showing compositional variations within the paint on the hieroglyphs. Note that the hieroglyphs all have different colors in the combination maps.
Clearly we didn't need electron microscopy or X-ray microanalyses to find the different paints on this replica scarab; however, these same techniques can be applied to artifacts where pigments have mostly worn away and are barely visible or invisible to the naked eye. Analyzing pigments was among the first applications of an electron microprobe housed in Oxford's Research Laboratory for Archaeology and the History of Art in the 1960s.
12/5/07
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
