Exploring 500 years of Early Islamic Glass Technology from the Settlement of Kush, in Ras Al-Khaimah, UAE
Authors: Swan Carolyn , Kennet Derek , Rehren Thilo 
Published: July 2019

Introduction

Investigating Glassmaking in the Early Medieval World

UCL-Qatar has embarked on a rigorous scientific project that aims to understand the production and trade of glass objects within the Byzantine Empire and Abbasid Caliphate—two contemporary and often rival regional powers. “Glass from Byzantium to Baghdad: Trade and Technology from the Byzantine Empire to the Abbasid Caliphate” is a QNRF-sponsored research project that combines archaeological and scientific evidence to compare the technological traditions and economic networks in place within the lands of the eastern Mediterranean and Arabian Peninsula during the early medieval era.

This paper presents some of the very first chemical analytical data ever produced for glass artifacts excavated from archaeological sites within the GCC region. This data enables us not only to identify the type of raw materials and aspects of the manufacturing technology used to make glass, it also allows scholars to better contextualize this material by addressing various economic aspects related to the production, consumption, and trade of glass objects in the early medieval world. In addition to shedding light on how glass was made and traded, the chemical analysis of excavated glass artifacts helps scholars elucidate what material- and knowledge-interactions people in the medieval world may have had, and whether such interactions went beyond cultural, political, or geographic boundaries.

Case Study

Islamic Glass from the Site of Kush, UAE

The first major case study in this investigation are the glass artifacts excavated from the archaeological site of Kush. Located in the Shimal area of Ras al-Khaimah (Fig. 1), Kush is one of the largest archaeological tells in the UAE. The tell was first noted by scholars in 1977, and was excavated from 1994 to 2000. Archaeological investigation shows that the settlement was occupied from the 5th century to the 13th century CE; situated just inside the Gulf on the edge of what was once a lagoon, the site was involved in the long-distance trade of goods with both the East and the West during much of its history.

Among the artifacts that were recovered from excavation were more than 5000 fragments of glass, which suggests glass vessels played an important role in the daily lives of people living at Kush. Because there is as yet no evidence for glassmaking or glass working in Eastern Arabia, it is likely that most of the glass was imported from the major contemporary glass producers that were located to the north and northwest in the Levant, Mesopotamia, and Iran.

Small samples were removed for analysis from 43 fragments of glass vessels excavated from Trench A at Kush. These glasses are translucent and come in a variety of colors including turquoise, aqua, light green, green, yellow green, dark yellow, amber, and purple hues as well as colorless glass. Over the course of time, the glass artifacts have deteriorated, leaving flakey white and black weathering layers that obscure the surface.

The chemical composition of the glass samples was determined using electron probe microanalysis (EPMA); 23 glass forming compounds and elements were measured, the results given as weight percent oxides. The chemical data sheds light on three aspects of the glass from Kush: 1) the raw materials used to make the glass, 2) the technological tradition of glassmaking and possible provenance or origin of the glass objects, and 3) changes in glass chemistry over time.

The Kush glass objects are all made from soda-lime-silica glass, a type which dominated ancient and medieval glass production and continues to dominate production even today. In terms of the raw materials used to make the glass, the relatively high magnesia and potash content of the sampled vessels (with an average of 3.9 wt% MgO and 2.5 wt% K2O) indicates that the glasses were made using plant ashes rather than mineral soda as a fluxing agent to lower the melting temperature of silica. Because mineral soda was the dominant flux used by glassmakers in Syro-Palestine and Egypt until the 9th–10th centuries CE, it is likely that the Kush glass vessels dating to this period were instead imported from Mesopotamia, where plant ash glass technology was in continual use since the Bronze Age. This suggests that at this time, Kush was primarily involved in the trade of raw glass or finished glass objects not with the main glassmaking regions within the former Byzantine Near East, but rather with the central Mesopotamian regions of the Abbasid caliphate of which the Arabian Peninsula was a part.

Plant ash is a chemically complex source of soda, and typically has a high concentration of other chemical compounds, making plant ash a rather “dirty” material. However, the Kush samples have lower phosphorus content than might be expected from a plant ash-based glass, which could suggest the plant ash soda was purified in some way during processing. In many of the samples, trace amounts of antimony, manganese, copper, cobalt, tin, and lead could indicate some degree of glass recycling was also taking place. In some cases the addition of metals to the raw glass was intended to colorize the glass, as in the case of the unique bright turquoise glasses that have high traces of antimony, copper, tin, and lead. The manganese content of the Kush samples is also notable; manganese is typically added to create purple or colorless glass, but manganese is present in relatively high levels in some of the green, aqua-green, and yellow-green glasses found at Kush.

The glass from Kush was also sampled to assess change over the course of time. The Kush glass can be divided into chronological periods based on their typological features and excavation context, and the samples were taken from vessels dating to four different periods of Kush's Early Islamic era of occupation history: the 7th–8th centuries CE (Period II), the late 8th-early 9th centuries CE (Period III), the 9th-late 11th centuries CE (Period IV), and the late 11th-early 12th centuries CE (Period V). However, the chemistry of the glass does not appear to reflect any major compositional changes over time. The glass samples from the 9th–11th centuries are of particular interest, as they coincide with a time of flourishing population and economy within the region; there may be some degree of standardization reflected in the 9th–12th century glass samples, as is suggested by the narrower range of potash levels.

Conclusions and Future Work

In comparison to glass made using mineral soda, plant-ash based glass is more chemically complex. As such it requires a more rigorous analytical program. The data provided in this preliminary study using electron probe microanalysis (EPMA) gives a good general impression of the glassmaking technology of the Kush samples, but it is expected that a better technological assessment and interpretation will result from the application of a new chemical analytical technique: laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). This technique goes beyond the detection capabilities of EPMA and enables measurement of the trace element composition of glass to parts per million (ppm). Trace element analysis of the Kush glass will thus allow us to discriminate between discrete glass groupings, and to better interpret the economic networks and patterns of trade that took place in the Gulf region during the Early Islamic era.

Acknowledgments

We would like to thank Philip Connolly for providing the EPMA data for the Kush samples discussed above. This presentation was made possible by NPRP grant 7-776-6-024 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.