Survival of Residues

The full chemical process behind the preservation of organic residues is incompletely understood. It is known, however, that some organic compounds do preserve throughout their deposition in archaeological context either by being absorbed within the ceramic matrix of a potsherd, or by retention in the pores of a visible encrustation on the surface of a ceramic sherd or stone tool.

Absorbed

Absorbed residues often result from long-term use of the artifact, with the residue comprised of various components absorbed during the use-lifetime of an artifact. Absorbed residues occur primarily in pottery, though recent studies suggest that they may also occur in porous rocks. In order to become an absorbed residue, chemical components must be sufficiently soluble in water that they are able to absorb within the porous matrix in the first place, but not so soluble that they are washed out of the artifact by rainfall during deposition. As a result, absorbed residues are primarily made up of lipids, and occasionally proteins, which fulfill this condition.

Visible

Visible residues, as shown in Figure 1, may result from either a single use of an artifact - a major culinary disaster in the kitchen, or a very messy episode with a stone tool - or from a buildup from repeated use. Because the constituents of a visible residue do not have to have been absorbed within a porous matrix, a wider range of components can be preserved in this way, including starch grains and other components less water-soluble than lipids. Organic components, as well, may be preserved in a visible residue, most notably phytoliths and pollen (see Phytolith Analysis; Pollen Analysis).

Figure 1 A visible residue encrusted on a sherd of early stumpware from the George Reeves site in Illinois (11S650).

Environmental Degradation

In both absorbed and visible forms, only certain types of chemical components are able to survive archaeological deposition. Various experimental studies have been undertaken on this point, and have established that the overall result of environmental degradation of organic residues is the preferential disappearance of water-soluble, shorter-chained and polyunsaturated constituents. Longer-chained and saturated components tend to preserve proportionally longer.

Residue degradation takes several forms. First, water-soluble components such as sugars are washed out of the residue and into the soil. The components of the residue then begin a complex series of reactions: microbial, thermal, and oxidative. Microbial degradation is caused by the activities of microbes and bacteria. Thermal degradation results from chemical reactions produced by the exposure of residue constituents to heat. This type of reaction can be very useful to analysts. For example, the best indicator for the presence of marine processing in a vessel is the presence of ffl-(o-alkylphenyl)alkanoic acids, which are thermally produced from triunsaturated fatty acids. Oxidative degradation results from the reaction of oxygen with various residual components, and is responsible for the preferential degradation of polyunsaturated acyl lipids.