The Inside (and Outside) Scoop: Scientific Analysis of Food Residues Inside the Jars from Old Edgefield, South Carolina
The exceptional and exciting occurrence of organic food residues inside large nineteenth century alkaline-glazed stoneware vessels from the Old Edgefield district, some also signed by enslaved potter and poet David Drake, has offered a unique opportunity to study those residues as anthropological evidence of the jars’ contents (e.g., preserved meat, fat, eggs, etc.), potentially offering information on the use of the jars and the lifestyle of the people that used them. Residues scraped from the jars’ surfaces were analyzed in the Department of Scientific Research of The Met to evaluate their composition, assess contaminations and clues to their condition (e.g., presence and extent of degradation products), all informing of the next process to further characterize the residue. A selection of samples was also sent for analysis by our ARCHE collaborators at the University of Bordeaux, so that tissue or species sources could be identified, and if age or processing information was attainable.
When tackling analysis of historic food residues, analysts must grapple with variables potentially affecting the residues’ composition and integrity. Where on unglazed earthenware, organic residues can be preserved, absorbed into the porous ceramic fabric, residues on pottery glazed or fired at high temperature remain exposed on the surface and are more vulnerable to compositional changes due to washing and cleaning, as well as contamination.
Materials deposited from smokerooms, animals (e.g., insects and other critters)-, and human- interaction (skin cells, hair, clothing fibers), including those from reuse, contribute to environmental clues, but contaminate evidence of the jars’s historic content.
The samples varied in homogeneity and texture (viscous, jelly-like, gummy), color (clear, pale yellow, orange and black) and solids (quartz, black particles, fibers, animal hair, paint, insect parts). At the microscope, the solids which would have interfered with analysis were separated.
Fourier transform infrared micro-spectroscopy (micro-FTIR), revealed that the samples are predominantly of lipidic (fat/oil) composition, confirming results obtained in situ with the portable FTIR at the McKissick Museum, Columbia, SC.
Within the miniscule samples used for this micro-analysis, only very few contained protein, in addition to lipids: gummy residues, such as those sampled from the lower areas of the jars’ interiors and under the handle of a jar. Most likely protected from the regular washing of the jar’s interior, the residues under the handle are also most contaminated from moving the jars with bare hands.
Lipids from fats or meat, compared to water-soluble proteins, could better survive the jars’ periodical washings in scalding water. However, they are altered by moisture and air, through pathways of hydrolysis and oxidation, with some of these processes having already taken place in the processing of lipids (e.g., fat rendering) and preservation of meat (brining, dry-curing).
Free fatty acids from hydrolysis of triglycerides, such as palmitic and stearic acid (both very abundant in animal fats), were indeed detected in the clear jelly-like and crystalline clusters inclusions, but also in the gummy black accumulations under a jar’s handle, and bottom of the jars.
Brownish inclusions in some of the residues were rich in metal oxalates. Oxalic acid, a degradation product of air-exposed organic matter (including proteins and lipids) and a metabolite of fungal activity, had formed salts with calcium, iron and other reactive ions likely originating from the residue, environmental particulate, and/or the jar’s exposed fabric and glaze components.
The extent of lipid hydrolysis in the residues was assessed by Evolved Gas Analysis-Mass Spectrometry (EGA-MS). By this technique, the residues were heated in a miniature furnace over a temperature gradient (100 to 700oC) and their constituents detected by the mass spectrometer in function of the temperature and time ranges at which they evolved from the sample. The temperature profiles (thermograms) obtained show the distribution of the volatile fractions (e.g. free fatty acids) and less volatile (triglycerides/ diglycerides) fraction within the sample. Their relative ratios give a sense of the hydrolysis undergone by the residue.
The identity of the individual fatty acids, free or bound in glycerides, as well as the products of oxidation of the unsaturated fatty acids, was assessed by Thermally assisted Hydrolysis and Methylation Gas – Chromatography-Mass Spectrometry (THM-GCMS). Here the lipid residue is hydrolyzed and hydroxyl groups (-OH) from fatty acids and glycerol are methylated and identified by GCMS, providing distributions of fatty acids, some of which attributable to animal sources. In addition, oxidized fatty acids derived from unsaturated fatty acids, as well as dicarboxylic acids, although not specific to origin of the lipids, are helpful clues to the oxidation of the residue analyzed.
With minimal sample sizes required for the analyses described, the organic composition of the residues can be interpreted as likely including animal and possibly plant lipids with occasional protein, as well as degradation products, but there still leaves unanswered the question: what exactly was stored in these jars? To begin to unravel the question of animal species source or food processing, we have to turn to more sophisticated “omics” techniques with our ARCHE collaborations at the University of Bordeaux, France.
The organic residues are an invaluable source for insight into these jars’s content and use, with their analysis and interpretation challenged by the chemical effects of curing, rendering, and preservation methods of meats or fats followed by aging during storage, and after the jars became collectable objects. Fat-rendering and meat preservation processes were replicated in the lab in small scale to eventually address these questions, but there remain the challenges, both analytical and logistical, to pursue such investigation in a museum lab and underscores the necessity for collaborative efforts. Nevertheless, it is hoped that broader discussions about the importance of this research focused on Old Edgefield’s jars stimulate further studies on these and other historical residues, to the benefit of the field of science, anthropology and food history.
Related Content
SCIENTIFIC RESEARCH
Cutting Through the Fat
Following analysis of food sources in stoneware vessels from the Old Edgefield District, South Carolina, an innovative approach probes the composition of trace food residues through close collaboration of The Met’s Department of Scientific Research and the University of Bordeaux through ARCHE.
PERSPECTIVES
Examining Storage Jars from the American South
Get an introduction to scientific research on food residue found in nineteenth-century stoneware vessels produced by enslaved artisans sheds light on the contents once stored inside.