Ceramic Residues

Text by Julia Becher, Margherita Cantelli, Alice Di Muro, Oliver Craig, Shinya Shoda, Rebecca Stacey

Archaeological pottery is one of the most ubiquitous and durable archaeological materials and the study of ceramics is a well-established discipline in archaeology. In the past it was mostly focused on topological and technological features defining pottery styles, production sequences and chronology. In the last few decades archaeological scientists have expanded the research to ceramic residues with the purpose of providing direct evidence for the use of pottery. The use of novel analytical techniques and the development of extraction methods has permitted to study ancient biomolecules.

Residue could be potentially divided into visible, described as surface deposits on both sides of the potsherds, and non-visible, such as compounds absorbed by the porous surface of the ceramic. Foodstuff organic residues (e.g. charred foodcrust) are the most widely encountered, since ceramics are commonly associated with the processing or storage of foodstuff. Nevertheless, several deposits not related with culinary practices are also common (e.g. adhesive materials, sealants).

Sampling

From the field to the lab - ideal sampling considerations

Handling of sherd/sample the least as possible
Wearing nitrile- and powder-free gloves
Collecting minimum of 3cm x 3cm x 3-5mm sherd
(ca. 1-5 g of ceramic powder, depending on analysis, few samples can also work with less than 1 g depending on preservation)
Collecting associated soil sample - if possible adhering to sherds
Collecting background soil sample from non-anthropogenic part of surrounding site area (e.g. dongas can be useful to receive soil samples from various parts of the palaeo-ecological layers)
No contact with plastics, creams, labelling substances and glues
No washing of sherd/sample, especially no acid washing (risk of losing attached crusts and destroying organic residues)
No drying in direct sunlight - only in cool/dry indoor storage area
Bagging in acid free paper or clean aluminium foil.

Check sherds for foodcrusts

Foodcrusts can comprise of calcite (white-ish) or charred (brownish-black) deposits (see example Figure 1). If crust is present, handle with care as crust can be brittle and easily lost.

From the collection/museum to the lab - sampling considerations for already processed samples

Water-washed samples are still suitable for organic residue analysis, with possible limitations
Glueing (re-fitted) - take notes on used glue (brand and ingredients)
Bagging - alternative for bagging is paper bags and cardboard boxes
Labelling - if necessary, close to breakage edges and not in the middle of artefact surfaces; take notes of stationeries used (brand and ingredients)

Handling considerations: how to avoid contamination and loss of compounds

Since lipid residue analysis is based on the survival and recognition of molecules being present in many sources belonging to both the ancient and the contemporary world, during and post-excavation contamination should be taken into account, and some measures should be taken in order to avoid this. The following table summarises the main contaminants which can arise during and post-excavation as well as guidelines on good practice for sample collection.

Submitting samples for organic residue analysis

Before submitting a sample for ORA:

good record of artefact, with description, techno- and typological analysis and photographic record

Sampling should take place in laboratory facilities only in order to avoid cross-contamination

Sampling steps - Charred foodcrusts

Place a clean sheet of tinfoil on a work surface and clean the area with acetone or dichloromethane (DCM)
Using a clean scalpel, scrape the surface of the ceramic to remove the carbonised deposit (see example Figure 2) onto the tinfoil. IMPORTANT: try to obtain complete chunks to preserve for further analysis, such as SEM (scanning electron microscope) to analyse macro-remains trapped in foodcrust
Decant the deposit into a clean glass vial, or wrap the deposit in the tinfoil and label with a sample number
The sample number should include: site code, vessel code, type of sample (interior/exterior)
Take examples of exterior soot as well as interior foodcrust, in order to act as a control
Amounts needed for analysis:
- Plant microfossils: 5mg
- Protein analysis: 10-20mg
- Lipid analysis: 50-100mg

Sampling steps - Drilled ceramic fabric

Place a clean sheet of tinfoil on a work surface and clean the area with acetone or dichloromethane
Using a clean drill bit, clean the interior ceramic surface within an area of approx. 3x3cm or 4x4cm (depending on amount required for analysis) to remove any depositional/post-depositional contamination
Using a Dremel fitted with tungsten carbide drill bits (prior cleaned with DCM), collect a minimum of 1g of ceramic powder into a furnaced glass vial labelled with sample details. Depending on analysis, 1-5 g of ceramic powder are needed for extractions (ca. 3-7 mm depth into the ceramic matrix). Few samples can also work with less than 1g depending on preservation (see Figure 3 for exemplary sampled sherds)
Depending on research question, 0.5-1g of ceramic powder from the exterior can be useful

Fig. 1 Right- exemplary unwashed sherd with attached carbonised foodcrust (© J. Becher). Left - exemplary sherds with calcified deposits (see Hendy et al. 2018).

Fig. 2 Sampling of foodcrust with sterilised tools and tin foil (© J. Becher).

Fig. 3 Example of drilled/ sampled ceramic sherd from ‘typical find processed’ museum collection: washed, glued and labelled. Picture shows impact of drilling (ca. 1 g of exterior and 2-3 g of interior) and sampling the furthest from glue and labelling traces (© J. Becher).

Storage

Before storage, ceramics should be completely dry to avoid molding and hence fungal and bacterial degradation of the organic residues. The drying process should take place in in a cool/dry indoor storage area and not in direct sunlight. Afterwards, the pottery sherds are bagged ideally in acid free paper or paper bags and stored in cardboard boxes in a dry place (cool, max. at room temperature). It is important to avoid direct contact with e.g. plastic bags which would result in plastic contamination relatively quickly (phthalate peaks will mask other relevant archaeological substances in the chromatography).

Applications

What we can learn from organic residue analysis

The importance of ceramic residues analysis lies in the incredible amount of information it can provide about several ancient aspects, such as ceramic use, typological and technological aspects (e.g. mending, sealing, decoration), past human behaviour, food preparation processes (e.g. cooking, storing, transporting), identifying common processed resources in comparison to rather unusual or rare ones (e.g. plant oils, beeswax, fish, ruminant fats, dairy products, resin and tars), processing of food stuffs (e.g. fresh milk versus processed milk), animal husbandry practices (taxonomic identifications), identifying used plant products otherwise invisible in the archaeological record and dating of organic matter.

Limitations

The main limitation of organic residue analysis applied to ceramic sherds comprises of preservation conditions. Regarding lipids, the compounds tend to preserve better in sherds originating from slightly acidic burial environments, whereas in alkaline burial environments, lipids degrade much faster. However, due to their hydrophobic nature, lipids cannot be accidentally 'washed in' or 'washed out' of the ceramic matrix and are hence often good to well preserved.

The preservation conditions of proteins within the ceramic matrix or foodcrusts is still under investigation. To date it is known, that proteins preserve a lot better in calcified deposits in comparison to charred crusts although also proteins could be successfully extracted from charred material.

When comparing the analysis of lipids and proteins applied to ceramics, lipids are often well preserved allowing a good data comparison, whereas proteins tend to be less preserved. On the other hand, the detail of information that can be gained from protein analysis is a lot higher in comparison to lipid data, such as species level identification or even identifying markers for blood, fresh or processed milk.