Residues Associated with Pottery

Fragments of broken and discarded pottery vessels are one of the most common classes of archaeological find. These sherds offer few immediate clues as to their original content and use - a significant point of enquiry in archaeology. During use, however, pottery vessels are known to accumulate residues of foods processed in them. If these residues survive long-term burial then they offer potential for determining artefact use. The residues occur as both charred or burnt deposits, which can be observed on the surface of the pottery, and as absorbed residues whereby organic components migrate into the pores of the vessel fabric. The porous microstructure of the fabric offers some protection to the residue from the effects of biodegradation and leaching during burial. The lipid constituents of these residues preserve rather well, and these can be extracted (by solvent washing of powdered sherd) from excavated sherds and analysed by GC and GC-MS.

A gas chromatogram from a typical degraded fat residue recovered from an archaeological sherd of Iron Age date (c. 100 bc) is shown in Figure 1B. The residue is rich in acylglycerols and free fatty acids and is typical of a partially hydrolysed lipid. This can be compared with the composition of fresh mammalian depot fat (Figure 1A), which is dominated by intact triacylglycerols. The monoacylglycerols, diacylglycerols and free fatty acids in the degraded fat result from the hydrolytic processes which begin as the pot is used (e.g. during boiling of food) and continue during burial. Furthermore, lipid residues are depleted in unsaturated fatty acids (such as oleic acid; C18:1). This illustrates the problems in making simple comparisons between ancient lipids and fatty acid compositions of modern fats and oils.

Fractionation of the lipid to obtain minor constituents, such as sterols, can assist in determining a plant or animal source (or indeed whether lipids from both are present). Odd and branched-chain fatty acids may also be present. These are characteristic components of bacteria. They are, however, also introduced into ruminant adipose tissue by bacteria in the rumen and migrate throughout the animal's body, contributing to all the tissues. The presence of appreciable levels of these components, both in the free state and as components of the acylglycerol fraction, supports the view that the predominant source of lipid in the example shown derives from a ruminant animal.

Identification of thermal degradation products may give clues to vessel use. The long chain ketones in Figure 1 are formed by a high temperature reaction of fatty acids that is catalysed by the mineral matrix of the pottery fabric. Thus vessel use may be further understood by linking the molecular composition of the residues with exposure to high temperatures during formation, for example, during cooking. Recent research suggests that animal fats (such as adipose tissue, dairy products and fish/ marine mammal oils) and plant tissues (notably the waxy compounds coating the surfaces of leaves) have the ability, under favourable burial conditions, to survive.

Pottery sherds may also exhibit the remains of organic surface treatments or sealants preserved as surface deposit. These are often resins, waxes or tars. GC analysis of one such deposit, a burnt surface residue on a neolithic potsherd, from Ergolding Fischergasse, Germany (mid 4th millennium bc), led to its identification as beeswax (Figure 2). The chromatograms shown compare wax ester distributions in fresh beeswax (Apis mellifera) with the fraction extracted from the surface deposit removed from the neolithic sherd. The principal wax esters in both samples are even-carbon-numbered aliphatic chains of saturated alcohols and fatty carboxylic acids with total carbon numbers in the range C40 to C50, with the C46 wax ester the most abundant. The unsaturated wax esters present in the natural beeswax are absent from the neolithic residue. This is due to the deleterious effects of burial, during which the double bond is rendered susceptible to oxidation or reduction reactions. Natural beeswax also contains a considerable alkane component (in the range C21-C33), yet this was severely depleted in the archaeological sample, suggesting its combustion when the beeswax was burned. The sealing and water-repelling properties of beeswax suggest that it may have been used to seal the vessel to enable it to hold liquids. It is possible, however, that the vessel was used to store the beeswax for other uses. The identification of this commodity also implies the availability of honey to

Figure 1 Partial gas chromatograms showing the compositions of (A) fresh beef fat and (B) the lipid residue extracted from an Iron-Age cooking vessel from Easingwold, Yorkshire, UK. The peak identities were established by GC-MS and are as follows: F14-F18 denote saturated fatty carboxylic acids with 14-18 carbon atoms respectively; F18:1 denotes a monounsaturated fatty acid with 18 carbon atoms; M16 and M18 are monoacylglycerols with 16 and 18 fatty acyl carbon atoms respectively; K31, K33 and K35 are mid-chain ketones with 31, 33 and 35 carbon atoms respectively; D34 and D36 represent diacylglycerols with 34 and 36 fatty acyl carbon atoms respectively. T46-T54 are triacylglycerols with 46-54 fatty acyl carbon atoms respectively. * Internal standard.

Analytical conditions: gas chromatography was carried out on a Hewlett Packard 5890 series II gas chromatography, equipped with a flame ionization detector. Samples were introduced by on-column injection into a 60 cm x 0.32 mm i.d. retention gap of deactivated polyimide-clad fused silica capillary tubing connected to the analytical column via a capillary connector. The carrier gas was helium at a constant flow of 1 mL min~1. The temperature of the oven was programmed from 50 to 340°C at a rate of 10°C min~1 following a 2-min isothermal hold at 50°C after injection, with the final temperature held for 8 min.

The combined GC-MS was performed using a Hewlett Packard 5972A quadrupole mass selective detector in conjunction with a Hewlett Packard 5890 series II gas chromatograph. Samples were introduced via a splitless injector at 340°C with a 3-min purge time. Helium carrier gas was at constant pressure of 10 psi. Mass spectra were recorded over a mass range of 50-700 ^m. The MSD interface temperature was 340°C, and the temperature was programmed from 50 to 340°C at a rate of 10°C min~1 following a 2-min isothermal hold at 50°C after injection, with the final temperature held for 12 min.

In both cases, the analytical column was a polyimide-clad 12 m x 0.22 mm i.d. fused silica capillary coated with BP1 stationary phase (immobilized poly(dimethylpolysiloxane), OV-1 equivalent, 0.1 ^m film thickness, SGE, UK).

neolithic communities in Europe. GC-MS has also been used to identify beeswax residues associated with medieval ceramics, and in lamps from late Minoan Crete where the wax was burned as a fuel.

Analysis of a large number of vessels from an archaeological site enables correlation between resi due type and pottery form and fabric, providing general assessments of use within assemblages.

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