Fourier transform infrared spectroscopy (FTIR) allows conservators to determine which molecular structures are present in a material. From this information they can often determine which material was used to construct an object. FTIR is most often used in the analysis and identification of organic compounds such as resins, starches and proteins, all of which are used in the construction of ethnographic objects.
The Field Museum is currently using a DATR-FTIR unit. A small sample (about 0.5 mm across) is pressed onto a diamond stage and infrared light shines up through the diamond at an angle. Some of the infrared light is absorbed by the sample and much of the rest is reflected back to the detector. The resulting absorption spectrum can be compared to reference spectra to help identify a material, or the peaks can be examined individually to try to determine the molecule that would have resulted in the pattern. Conservators at the Field Museum are fortunate to have access to the Economic Botany collection which has provided a wide range of sample materials to expand our reference library.
Ostrich shell cup, Iraq, ED III (ca 2500 BCE)
This cup from the site of Kish in Iraq is constructed around an ostrich shell. The top of the shell has been cut off and a pottery flange, coated with a brown-black resin, has been adhered to the opening to form a rim. Small triangles of nacre have been embedded in the resin as decoration.
It was asked whether the brown-black resin was a bitumen (naturally occurring geological oil product) or a plant-derived resin such as pine pitch. These materials have similar elemental compositions, but can be distinguished by FTIR. As you can see from the accompanying graph, the FTIR spectrum for natural asphaltum (green line) and is quite different from the spectrum for pine pitch (red line). A tiny fragment of the resin was removed from the pot rim and analyzed using FTIR. The resulting spectrum closely resembled the spectrum for asphaltum.