The identification of materials and their characterization is, in many respects, the key to elucidating the microscopic archaeological record. Among all the methods used for this purpose in archaeology, infrared spectroscopy is one of the most useful. Infrared spectra are easy to obtain but can be difficult to interpret. The purpose of this chapter is to provide information on interpreting infrared spectra of archaeological materials as well as to review some of the more common applications. The latter are referred to as overviews in this chapter and throughout the book.

Infrared spectroscopy is a sensitive method for obtaining information on the molecular structures of crystalline and amorphous/ disordered materials as well as organic materials. Infrared spectroscopy can thus be used both to identify materials and to characterize their states of atomic order and disorder. In these respects, infrared spectroscopy is similar to powder X-ray diffraction, although the latter cannot be used to identify and characterize amorphous and highly disordered materials. Amorphous and highly disordered materials are common in archaeology.

Infrared spectroscopy is based on the manner in which radiation interacts with material in the infrared range (4,000 to 250 centimeter−1 [cm–1; wavenumbers, or the inverse of wavelength]). Some of this radiation is absorbed by the sample because it causes the chemical bonds of the sample to vibrate. The result is that less radiation reaches the detector at specific wavenumbers, and this is recorded as a series of peaks in the infrared absorbance spectrum.