Several complementary approaches can be useful for identifying the materials being analyzed. Perhaps the most powerful approach is to use the “search” option present in most infrared software packages and compare the unknown sample spectrum to spectra in a reference library. For this reason, the infrared spectral library of the Kimmel Center for Archaeological Science, Weizmann Institute of Science, is posted on the Web (http://www.weizmann.ac.il/kimmel-arch/islib.html). The materials in the library are listed in Table B.1. This list is also a simple “classification” of archaeologically relevant compounds. Note, too, that it contains minerals produced by organisms, many of which are less relevant to archaeology but more relevant to the field of biomineralization. Other useful libraries are available, such as that of the Infrared and Raman Users Group (http://www.irug.org/).

Perhaps the major problem with using search software for material identification is that peak positions are not invariant; rather, they depend on the local structural environment, counterions, and probably many other factors. This makes it difficult to know when a match can be accepted with confidence. A second problem relates to identifying mixtures of compounds. In this case, the library search can be focused on key peaks that are indicative of a particular compound.