Tuesday, 12 March 2013

My own research


I study the chemical composition of samples of sheep wool found in archaeological deposits. These are mostly in the form of woven textiles, but there are occasional examples of un-spun raw wool. I am particularly interested in looking at textiles from the Middle Ages (approximately 600 to 1500 AD) in Europe.

3cm fragment of textile from Reykholt, Iceland
A tiny fragment of a textile from Reykholt, Iceland, dated somewhere between 1200 and 1400 AD. (c) Isabella von Holstein. I worked on samples from this site with Penelope Walton Rogers, by courtesy of Gudrun Sveinbjarnardottir [1].
This chemical analysis can show where the wool originally came from, because it is related to the local environment of the sheep that grew the wool. Because temperature, humidity, and rainfall change with distance across regions, so does the composition of wool from across the region. Composition can therefore indicate that an archaeological sample is made from wool which had been moved a long distance from a different environment. In order to do this work, I compare the regional patterns found in the composition of archaeological wool to those in archaeological sheep bone (same species, same time, different part of the body) and also to modern wool (same species, different time, same part of the body).

I compare the chemical information on origin to information derived from  the way the textiles are made. In the past, this changed over time and was different from place to place because of differences in technology (for example combing tools, spinning tools, loom type) or differences in tradition in different times and places. Now imagine at an archaeological site, you have found 50 textiles, of which two are very different: they are made using the same technology as the rest of the material at that site, but with different technical construction. They might both be imports. But they could both be local, and are rare either because of an archaeological accident (by chance, all other similar samples have decayed) or simply because they were only rarely made. How could you tell the difference? I hope that my chemical analysis method can help here, and distinguish between the movement of textiles themselves from movement of ideas about how to make and use textiles.

Tricky aspects

There are a number of difficulties with this work.  First of all, samples of wool in archaeological deposits are not intact. They are often stained brown and may well have been attacked by soil microorganisms or partially dissolved by soil water over their long period of burial. I have to be careful that changes to composition due to decay are not confused with differences in composition due to different origins.

3 archaeological textiles, all shades of brown
Textiles from the Museum of London

Secondly, environments, sheep and sheep farming have all changed between the past and now. Modern sheep farmers are likely to provide concentrate feeds during seasons when pasture plants don’t grow (winter in cold climates and summer in hot climates) and during lambing (because ewes can need better food in order to produce healthy lambs). These concentrates may include non-native species such as maize. Farmers may also use chemical fertilizer on fields. Changes in climate over time can also affect how sheep are managed, with farmers needing to respond to things like changes in the number of days of snow cover, types of plants in pasture fields, and periods of pasture growth. The breeds of sheep which exist today were mostly developed after the Middle Ages, so we don’t know how similar they were to the sheep that existed at the time [2]. So I need to be careful when selecting modern samples to reduce all of these factors as much as possible.

More about the chemical technique

The aspect of chemical composition that I am interested in is isotopic composition. In this section I’ll explain how this is different from other chemical measures of composition, and also how it relates to farming and origin.

All sheep wool from everywhere has very similar content of proteins, fats and pigment: this is called molecular composition. Each of these molecules is made of atoms, and so the overall proportions of atoms of the most common elements (carbon, hydrogen, oxygen, nitrogen and sulfur) in all wool everywhere is also very similar. This is called elemental composition. Therefore the molecular and elemental compositions of wool are not useful for identifying where wool comes from (though I do use them to work out whether decay during burial has significantly affected the fibres).

However some atoms have different versions of themselves which differ a little bit in weight but are still stable. For instance, carbon has 2 stable versions: carbon-12 (99% of all carbon atoms) and carbon-13 (1% of all carbon atoms). It also has 13 unstable versions, which are radioactive and decay, including carbon-14, which is used to date archaeological objects. The relative amounts of the stable versions of the elements, in this case carbon-12 to carbon-13, is called isotopic composition.

The isotopic composition of a sheep’s body depends on, firstly, the isotopic composition of the sheep’s diet, and secondly, the chemical reactions taking place in the sheep’s body as part of normal metabolism. The composition of sheep diet, which is largely pasture plants, depends on temperature, humidity, rainfall and vegetation type, and therefore varies with location. It can also be affected by farmers providing particular plants to sheep as fodder. We can therefore use the isotopic composition of wool to tell us about where it comes from and also something about how it was grown[3-4].

Stable isotopic analysis of the same elements (carbon, hydrogen, oxygen, nitrogen and sulfur ) is widely used in archaeology to study human diet in the past. However here, the focus is not so much on where humans lived as on what they were eating: how much meat? Any fish, and if so freshwater or marine? What sorts of plants? However, because human diets are much more complex than sheep diets, it’s more difficult to use this method to examine origin. Only oxygen isotopes are used in this way, together with the isotopes of other metals such as strontium and lead.

Why is this research interesting and important?

This research will help us to understand how textiles were made and moved in the past. Textiles today are not worth much because they are made by machine, but imagine if all of them had to be made by hand! A textile archaeologist calculated that it would take about 3 years for a single person to spin enough thread to make one Viking sail [5]. Once you add in the cost of the materials (the land and the livestock [6]) and the cost of weaving, it’s likely that the sail cost more than the boat’s hull. Therefore understanding archaeological textiles is much more than a hobby interest: these products required substantial investment, and could generate a substantial return: they made a significant economic difference. And because they were so laborious to produce, they also made a social difference. Being able to identify the ones which were moved long distances will not only tell us about how different places were connected in the past (often before the point where written records survive),  but also about how people organised themselves socially and economically to produce the local materials, which were themselves important goods.   

In addition, the same technique may be suitable to identify the origin of other objects from archaeological deposits which are also made out of the body parts of mammals: for example leather, parchment, fur, horn, bone or antler. The work on textiles is important because it is the first to try to work out whether this is possible. It would also be interesting to look at more textile fibres, including silk, flax and cotton.

1.              Sveinbjarnardóttir, G. (2012) Archaeological investigations at a high status farm in western Iceland. Reykjavík, IS: Þjóðminjasafn Íslands/Snorrastofa.
2.              Ryder, M.L. (1981) 'British medieval sheep and their wool types', in: D. Crossley (ed) Medieval Industry, 16-8. London, UK: Council for British Archaeology.
3.              Camin, F., Bontempo, L., Heinrich, K., et al. (2007) 'Multi-element (H, C, N, S) stable isotope characteristics of lamb meat from different European regions', Analytical and Bioanalytical Chemistry 389 (1), 309-20.
4.              Hedges, R.E.M., Thompson, J.M.A. and Hull, B.D. (2005) 'Stable isotope variation in wool as a means to establish Turkish carpet provenance', Rapid Communications in Mass Spectrometry 19 (22), 3187-91.
5.              Andersson, E. (1999) The common thread: textile production during the Late Iron Age-Viking Age. Lund, SE: University of Lund, Institute of Archaeology.
6.              Bender Jørgensen , L. (2012) 'The introduction of sails to Scandinavia: Raw materials, labour and land', in: R. Berge, M.E. Jasinski and K. Sognnes (eds), N-TAG TEN. Proceedings of the 10th Nordic TAG conference at Stiklestad, Norway 2009, 173-81. Oxford, UK: Archaeopress.