Shedding Light on Medieval Turkish Pottery

Aim

It's not just modern-day materials that need analysing to understand how they are made, ancient materials found in archaeological dig sites are great samples for analysis, as the results will shed light on long-forgotten techniques and practises. This application note will look at glazed ceramic tesserae from Kubad-Âbâd palace in Turkey, to understand the techniques used and if there are any differences between samples taken from across the palace.

Solution

Energy Dispersive X-ray Spectroscopy (EDS) is such a powerful tool to analyse what things are made of. EDS can be used to ascertain what minerals were used in the clays to make the tiles, thus showing us how they might have been made and possibly where their origin was. This technique can be used with large area mapping (LAM) in AZtec, whereby individual EDS and backscattered electron (BSE) maps are automatically collected across an entire sample and stitched together to form one large map. This allows for chemical analysis across an entire sample down to several microns of resolution.

Results

All tiles mainly contained particles of quartz, typically up to 0.5 mm in diameter, shown in the three maps in figures 1–3.

The results also showed that all tiles are of the stonepaste type, which were produced from a mixture of ca 80% crushed quartz, 10% clay and 10% crushed glass. Due to the absence of composite quartz grains and feldspar, it was assumed that the quartz was derived from crushed quartz pebbles. However, one group did contain small amounts of feldspar and fine-grained iron oxide, which suggested that it was added to make the unfired body plastic. There was considerable variation in the interstitial material, which may suggest that either the firing temperatures were different, or the body recipe varied significantly. The glazes across the tiles were also found to be rather different, with one group of tiles having a well-defined underglaze of cobalt pigment, comprising fine particles of cobalt-bearing iron oxide mixed with fine quartz; this may have been an attempt to stabilise or fix the cobalt pigment and stop it from bleeding into the glaze.

Given the high alumina contents, the low iron oxide contents are surprising, as high iron oxide contents are characteristic of many clays. The levels of alumina and iron oxide show that the clays are likely to have been white-firing kaolinitic clays. The results also showed fine grains with high alumina:silica ratios which is characteristic of kaolinite. This contrasts with many other stonepaste production centres in the vicinity, which appear to have used pale-firing calcium carbonate-rich clays.

Figure 1. EDS LAM of sample 1 showing the distribution of elements. Al is most apparent in the lower half of the sample towards the back of the tile.

Figure 2. EDS LAM of sample 2 showing the distribution of elements. Cr is seen to be below the glaze layer and is likely due to a paint.

Figure 3. EDS LAM of sample 3 showing the distribution of elements. This sample shows Pb within the glaze.

Conclusion

The results demonstrate that individual commissions were made for different rooms in the palace and the differences in style, technique and technology suggest that they were produced by different groups of tile makers. Along with archaeological evidence, most of the tiles were made within the vicinity of the palace, however one group was most likely imported due to having a significantly different body composition along with the presence of calcareous clay, compared to the kaolinitic clay found in all the others. Furthermore, groups defined by thin-section analysis may incorporate many subgroups that can be elucidated only by EDS. Analysis with EDS has the potential to be a powerful tool in investigating the production and procurement of stonepaste ceramics from archaeological excavation.

Acknowledgments

We would like to thank Tom Gregory and Ian Freestone from UCL for supplying the data and background to this study.

References

This app note was based on Freestone, I., Yegingil, Z., & Arik, R. (2008). Scientific analysis of glazed tile from the Seljuq palace of Kubad-Abad, Lake Beysehir, Turkey. 4th Forbes Symposium, 3–8.