The macroscopic record (what you see with the naked eye) and the microscopic record (what you “see” with the aid of technology) together constitute the archaeological record. All the information that we can learn about past human behavior is embedded in this combined record. The first requirement for exploiting the microscopic record is to have an overview of the types of information that can be obtained from the record, and to understand the basic principles on which this information is based; the subject of this chapter.

This chapter provides brief overviews of many, but not all, of the different categories of information that are embedded in the microscopic record. The chapters following this chapter provide information on various common materials that are present in archaeological sites, underlying mechanistic processes that influence the preservation of these materials, and at the end of each chapter or subsection, more information is given on the embedded information that can be extracted from these materials. Emphasis is placed on the concepts rather than the methods. For a review of the most common methods used to extract information from the microscopic record and the principles on which these methods are based, see chapter 2 of the work by Pollard and Heron (2008).

It is noteworthy that many of the methods used to extract information from the microscopic record were not developed by archaeologists. They were developed mainly by chemists, geochemists, botanists, and others for their own purposes and were later adapted and applied to archaeological problems.

The archaeological record is mainly composed of materials related to past human behavior. Some of these are visible to the naked eye – the so-called macroscopic record – and some require instruments, such as microscopes or spectrometers, to be seen and characterized. This is the so-called microscopic record, and the study of this record is referred to here as microarchaeology.

The macroscopic record is composed of strata, buildings, graves, floors, and so on, as well as artifacts such as pottery, bones, stone, and metal tools. The microscopic record is composed of the materials of which the macroscopic artifacts are made, as well as the sedimentary matrix in which the artifacts are buried. Thus the investigation of the archaeological record as a whole involves the integration of both the macroscopic and microscopic records. It incorporates activities that span the humanities, social sciences, and natural sciences, with the former two disciplines being focused mainly on the macroscopic record and the latter discipline being focused mainly on the microscopic record. Herein lies a problem: the different parts of the archaeological record are studied by investigators with diverse backgrounds and approaches. Often lacking is an integration of these different worlds.

In this book, the focus is on the archaeological information that can be extracted from the microscopic record and, in particular, from the materials commonly found in most archaeological sites such as ceramics, bones, rocks, ash, and sediments.

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.