The Roots of Provenance: Glass, Plants and Isotopes in the Islamic Middle East

Article excerpt

Introduction

By the mid-eighth and ninth centuries, the time of the 'Abbasid caliphate in the Islamic world, society was complex, dynamic and highly centralised. Urban life was dominated by the caliph, and at a local level, by regional governors. By the ninth century a traveller would certainly be able to recognise Islamic material culture. Indeed this was a period of technological innovation in which a wide range of advances in scholarship, arts and sciences occurred. This was especially true under Harn al-Rashid (AD 786-809/AH 170-193) and his son al-Mamun. Their domain covered an area from southern Spain to central Asia; trading links were created between the Indian Ocean and the Mediterranean Sea. Goods were traded and the movement of soldiers, ideas and craftsmen occurred over this vast area. At a more local level urban production centres for a range of metal, pottery and glass objects fed the suqs which formed important distribution nodes for both urban and rural populations.

In the Middle East, Islamic glass workers initially appear to have relied upon 'Byzantine' expertise for the production of raw unworked glass, until about the end of the eighth century. This glass was fused on the Levantine coast from beach sand containing shell fragments and a mineral flux (natron). A major change in glass technology occurred in the early to mid-ninth century, the end of a period of technological transition (Henderson 2002) when plant-ash glass largely replaced natron glass. Various soda-rich halophytic plants belonging to the Chenopodiaceae family would have been used (Barkoudah & Henderson 2006): the ashes provide the flux rich in sodium carbonate and most of the calcium.

Chemical analyses of Islamic vessel and raw glasses have made it possible to suggest the raw materials used to make them. However, a means of independently provenancing Islamic glass scientifically has not been possible. Such a provenance would provide a basis for distinguishing between glass made in different parts of the Islamic world and would provide direct evidence for trade in glass. Whilst the excavations of shipwrecks, such as that found at Serc Limani off the western coast of Turkey, has provided firm archaeological evidence for trade in Islamic glass on a large scale (Bass 1984) such evidence does not, in itself, confer an origin on the glass. This article presents new isotopic analyses of Islamic glass and plant ashes (from which they were made), building on scientific results of glass analysis from al-Raqqa, northern Syria as an essential first step. The study is set within an environmental context, thereby providing a provenance for the glass. It is the first such study.

Using isotopes to provenance Islamic glasses

Islamic glasses are thought to have been made from a mixture of plant-ash and ground quartz pebbles/sand; the chemical and isotopic ([sup.87]Sr/[sup.86]Sr) composition of the plant-ash used reflects the (geological) age and rubidium content of the bedrock on which the plants grew (Freestone et al. 2003; Montgomery et al. 2006). Strontium substitutes for the calcium that is taken into plants by transpiration, calcium being a major component of plant-ash. No isotopic fractionation occurs when the glass is made (Henderson et al. 2005a). Therefore, by determining the strontium isotope signatures of glasses and plants we have a means of linking the geological origin of the lime (in the plant) to the glass made from it. Assuming local plants were used this constitutes a new means of provenancing glass.

The success of this technique is therefore dependant on contrasts in the geological age of calcium sources used. For example, old marine limestones of the Middle Eastern steppe can be distinguished from sea shells (in sand) that take in 'modern' water.

By sampling plants that grow on contrasting geologies (Beydoun 1977) and by determining their [sup.87]Sr/[sup.86]Sr isotope signatures, an index of signatures reflecting variations in the geology across the landscape can be established. …