Organic residue analysis
In archaeology, Organic Residue Analysis (ORA) refers to the study of micro-remains trapped in or adhered to artifacts from the past.
By analyzing these residues, ORA can reveal insights into ancient dietary behaviors, agricultural practices, housing organization, technological advancements, and trade interactions.
ORA's broad applicability encompasses a variety of amorphous materials such as substances used in mummification, pastes, glues, binders, and colorants.
These materials can be preserved in pottery, stone tools, the mineral matrix of bones, dental calculus, as well as in habitation floors or pits.
The unique value of ORA lies in its ability to provide direct evidence of the materials and substances utilized by ancient peoples, often offering insights that other archaeological techniques cannot.
Moreover, ORA plays a crucial role in uncovering ancient medical knowledge, cosmetic usage, and the processes involved in creating artworks and handicrafts.
Utilizing modern chemical analysis techniques, ORA offers archaeologists a powerful tool to directly explore and understand the daily lives, cultural practices, and technological progress of ancient societies.
The formation of organic residues can occur during a wide range of pre- and post-depositional events, including food preparation, cooking, storage, transportation, reparation and sealing.
[3][4] Given the complexities of organic residues, they can be defined at five nested scales: tissues, cells, macromolecules (e.g. lipids, proteins, metabolites, DNA and starches), molecules (e.g. fatty and amino acids) and atoms (e.g. carbon, nitrogen and hydrogen).
[15] During the 1950s and 1960s, the emergence of chromatographic methods, especially those linking Gas Chromatography (GC) and Mass Spectrometry (MS), resulted in a methodology used to resolve and recognise molecules.
One of the earliest papers using GC analysis applied to archaeological material was published by Thornton et al. (1970) and, investigated the composition of ancient bog butter.
A popular way to get these materials out involves using special liquids like chloroform and methanol to pull out fats that aren't tightly attached to the pottery.
[29][30] Modern studies of plant resins, consisting of di- and triterpernoids, show confident identification of these sources to the genus level and, sometimes even the botanic species.
[40][41] Alkylresorcinols (ARs) are amphiphilic phenolic lipids characterised by a non-polar odd-numbered alkyl side chain attached to a polar resorcinol (1,3-dihydroxybenzene) ring.
[42][43] These cereal biomarkers were previously found in a well-preserved Bronze Age wooden container from Switzerland,[38] and coarse ware vessels from a Roman cavalry barrack at Vindolanda.
However, if recoverable, analysis of these phenolic lipids in archaeological contexts is valuable as it can help explain the uptake and spread of cereal processing of past communities in particular regions.
This is achieved using Gas Chromatography-Mass Spectrometry (GC-MS) alongside other methods, including isotope ratio mass spectrometry (IRMS), various spectroscopies, such as infrared (FT-IR), Raman, nuclear magnetic resonance (NMR), and ultraviolet (UV), scanning electron microscopy (SEM) for imaging and elemental analysis by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray fluorescence (XRF), and high magnification light microscopy [63] The main chromatographic technique utilised for the identification of lipid molecules from food crusts and absorbed residues is GC-MS.
[64] Larger lipids, such as triacylglycerols (TAGs) and wax esters, are analysed using high-temperature GC-MS (HTGC-MS) without any prior hydrolysis step.
One observed limitation of GC-based analysis is its inability to identify polar lipids (e.g., glycerophospholipids and D-galactosyl diacylglycerols),[66] which are mainly found in cereals.
Furthermore, the preservation is affected by the material that the organic compounds are derived from, e.g. food crust, calcified deposit or ceramic matrix.
However, in general food crusts are understood to form through heating, and subsequent charring of foods, the organic preservation can be impacted by thermal alteration as well as microbial action due to the exposure to the burial environment...[75] Calcified deposits are another form of residue found on ceramic vessels, which mainly consist of calcium rich compounds, such as calcium carbonate.
[3][11] Predominantly, chemical alterations occur prior to the burial of a vessel via a wide range of processes such as cleavage,[78] hydrolysis, oxidation,[79] thermal decomposition and ketonic decarboxylation.
[90] Hence, depending on pore size, microorganisms can access the ceramic fabric to a greater or lesser extent, favouring or hindering degradation processes.
When applying lipid residue analysis to archaeological and ethnographic material, one must always keep in mind that interpretations are limited, and certain food resources can still be 'hidden'.
Hence, meat will appear to dominate over vegetables [93] In 2019, Dunne & Grillo et al. published a study combining ethnoarchaeological research and chemical and isotopic analyses of lipid residues from ceramics made and used by modern Samburu pastoralists in northern Kenya.
The goal of the study was to investigate whether the extracted lipid compounds represent the relative importance of different processed foods within a society, especially within daily life.
Nevertheless, the results reflect the significance of meat and fat for social and ceremonial activities of the Samburu culture [74] A similar study by Drieu and colleagues (2022) focused on diversity of organic residue absorption patterns via an ethnoarchaeological approach.
