Ceramic Petrology

What is it?

Petrology is the study of rocks and minerals and is based on the identification of minerals and their associations. Ceramic petrology is a specialised sub-discipline which uses the same techniques and knowledge to study burnt and fired clay and archaeological artefacts made from this material.

The main differences between the two are:

Why should I want it?

If you study ceramics you will probably want to know what those ceramics are composed of. At the most basic level this may simply be in order to accurately describe the rock and mineral component of the ceramic body. Ceramic petrology can also be used to study clay preparation, an important part of ceramic production. Its techniques can be used to test the hypothesis that ceramics from two separate archaeological sites were obtained from the same source and, in the most favourable circumstances, it can be used to pinpoint the source of the raw materials.

Techniques

In some circumstances information can be gleaned on the identification of inclusions or clasts in a ceramic body simply by visual observation. More often, however, a sample will need to be examined using a stereo microscope or a broken or polished edge.

thin-sections

Should this not be sufficient to identify the rock or mineral content then a thin-section needs to be produced.


polished thin-sections Opaque material cannot be identified in normal thin-sections and requires a polished thin-section.

Various methods exist to make the thin-section easier to study. One such technique is Dickson's Method of staining. Another is Heavy Mineral Analysis.

Identifying inclusions

All pottery is made from a mixture of rock and mineral fragments (often termed collectively inclusions or - a more loaded term - temper) and clay minerals. When using the petrological microscope a distinction is made between inclusions and the clay matrix, often used to refer to all material less than 0.1mm across. This is a useful practical distinction but is to some extent a simplification since this matrix is itself composed of a mixture of clay minerals and other material.

Voids in a pot fabric are usually treated as a form of inclusion (and in many cases are actually left when an original mineral inclusion has dissolved or disintegrated).

Lenticular voids never contained any inclusions and are a by-product of the mixing process. They merge with laminae, which can themselves be relicts of the original stratification of the clay bed or created by the folding in of air during the preparation stage.

Inclusions over 0.5mm across can sometimes be identified by eye, or using a stereo microscope, on the basis of their mineralogical characteristics, These include:

Other large inclusions, despite being visible to the naked eye, can only be accurately identified in thin-section, using transmitted light or, if the mineral is opaque, reflected light, for which a polished thin-section is required.

Further precision may be required, for example in the case of limestones and carbonates Dickson's method of staining can be used to distinguish ferroan calcite from non ferroan calcite, and both of these from dolomite.

The precise chemical composition of a mineral may be a diagnostic feature and can be measured using Energy Dispersive X-Ray analysis on a polished section.

The use of ceramic petrology for inclusion identification is very cost effective, since it can be used to train the analyst to identify inclusions by eye and to confirm tentative visual identifications.

Sampling

A single sample may be sufficient to identify an unknown inclusion type.

Studying clay preparation

If the source of a potting clay is know then a comparison of the raw clay and artefacts made from it can reveal whether or not the clay was prepared before use. Ethnographic studies show that potters can dry the raw clay and crush it to a powder, mix it with water to form a slip, sieve it or dice it with a knife to find and remove large inclusions, add temper or mix clays from two or more sources. In addition, the potter can spend a variable amount of time kneading the clay to drive out air bubbles and generally lubricate the clay before starting work.

All of these processes ought to be recognisable in the fabric of the finished product.

Where the raw material is not known, then it becomes much more difficult to identify preparation processes. A fine textured body, without large inclusions, might have been formed by cleaning the parent clay but might have been obtained by finding a particularly clean clay source. Similarly, fabrics which look exactly as though they have been tempered with quartz sand can be produced by firing clays that have become mixed with overlying sand deposits, although here, of course, the potter would have no control over the quantity of inclusions. Even in these circumstances, however, useful conclusions about clay preparation techniques can be gained by comparing different products from the same source, for example building materials, tablewares, and cooking vessels might all be made in the same workshop but the requirements for each might have led the potter to prepare the clay for each product differently.

Sampling

Determining clay preparation processes is a comparative process. Therefore, at least two samples are required. These may consist of one sample of parent clay and one product or of two different products from the same workshop. In practice, one would want to analyse several samples of each group to be compared to establish the range of characteristics. The exact number will depend on the amount of variation found in the raw materials or products. Since most preparation processes affect texture it should be possible to gauge the amount of variation by eye and sample accordingly.

Sourcing

There are several different ways in which ceramic petrology can help to demonstrate the source of pottery. The simplest and most elegant is when the sampled object contains a distinctive rock or mineral type which only occurs naturally in a small area. There are very few examples of this method since geological processes take place on a wide scale and much of the earth's surface is covered with homogeneous tracts of land. For example, the glacial outwash sands of the North European Plain are extremely similar in character from Russia through to Jutland. Blown sands derived from these outwash sands, löess, is a widely-used raw material for European pottery from the Neolithic period onwards and vessels with löess tempering look depressingly similar.

Nevertheless, in the right circumstances, this method is extremely powerful.

A more common method is to demonstrate that a potsherd or other ceramic contains rocks and minerals that do not occur naturally in the area where the artefact was discovered but to offer a range of possibilities as to where the object might have come from.

A third method is to compare the petrology of material from a known source, such as a kiln site, with material from a consumer site. The trouble with this approach is that strictly speaking it cannot confirm that the consumer site was supplied from the kiln site. It can only show that there is no petrological reason why it couldn't do so.

Sampling

There are no hard and fast rules about the number of samples or how they should be taken for sourcing or characterisation studies. A single thin-section might be sufficient to demonstrate that a pot was tempered with Lizard Gabbro, from Cornwall, UK, whereas perhaps five or six samples might be required from a consumer site and the same number from the kiln sites to demonstrate that the range of inclusions in North Devon gravel-tempered ware was identical with that from the kiln-sites in Bideford, in the adjoining county of Devon. It is, therefore, important to know the archaeological and geological context of any work before settling on a sampling strategy.

Inter-Site Comparison

Perhaps the most common approach to the use of ceramic petrology in archaeology, and the most difficult, occurs when there are no known sources for any of the pottery used at a site and yet there is reason to believe that two or more sources of supply were exploited. One of the reasons that such work becomes difficult is that in the absence of any other data ceramicists will classify their pottery according to its visual appearance. These classifications often include a mixture of petrological, technical and decorational traits.

The ideal approach is to characterise each petrologically-defined group and then subdivide any technical or decorational groups on the basis of fabric and characterise them too. A thorough approach on these lines can be very time-consuming and expensive and one of the main conclusions, almost always, will be that visual classification tends to identify more groups than can be seen using ceramic petrology. Within the archaeological ceramics world practitioners of the two approaches tend to be termed "splitters" and "lumpers".

It is important to remember that groups of ceramics defined through the use of ceramic petrology are not necessarily all from the same source, nor even necessarily made using the same raw materials. They certainly aren't necessarily all of the same date. These groups are simply "petrofabrics" which cannot be subdivided sensibly using ceramic petrology. Other methods, for example chemical analysis, might reveal significant subdivisions. Nevertheless, the use of a protocol grounded in the natural sciences does mean that the work should be replicable by different workers and provides a sound basis for comparing sites.

Sampling

For inter-site comparison it is necessary to show that the pottery classifications on one site are made using the same criteria as those on the next site. Attention is therefore given to the boundaries between classes with presence/absence criteria given a higher importance than frequency data. For example, a division into chert-tempered and quartz-tempered fabrics will be replicable by anyone able to identify chert and quartz whereas a division into Fabric A, which contains moderate chert, flint and quartz, and Fabric B, which contains moderate quartz and flint, and some chert, is only replicable if one knows what the original researcher meant by "moderate" and "some". Ceramic petrology can be used to bring some rigour into the classification by the use of point counting or, more recently, automatic image analysis.

The Ceramic Petrology Group

A research group exists to further the use of ceramic petrology in archaeology by holding meetings, publishing a newsletter, The Old Potter's Almanack: Joint Newsletter of the Prehistoric Ceramics Research Group and the Ceramic Petrology Group. The newsletter has three issues per annum.

Subscription information may be obtained through the editor:

Andrew Middleton,
British Museum,
Department of Scientific Research,
Great Russell Street,
London WC1B 3DG;
telephone: 0171 636 1555,
fax 0171 323 8276.


icps sample containers

See Also

Chemical analysis of pottery



Digital Archaeological Reports

© Alan Vince 1999