The invention of lead glass in al-Andalus

Résumé En

Different types of high lead glasses appeared in the ninth to tenth century CE across Europe, including lead silica glasses in the strict sense with very high lead (PbO  60%) and silica as the main constituents, as well as potash lead (several variants) and soda-ash lead glasses with lower lead oxide contents (PbO typically between about 20% and 60%) and considerable amounts of alkali and alkaline earth elements.* Since their identification (Duckworth et al., 2015), Islamic lead and soda-ash lead glasses have turned up in the archaeological record throughout the Iberian Peninsula, dating to the eighth to twelfth century CE. They constitute for instance 70% of the tenth-century glass assemblage from Madinat al-Zahra (Cordoba) . One of the most striking features of these glasses are the exceptionally high chlorine concentrations (up to 2.5%). Recently, we have identified a second type of high lead glass among the eighth- to early ninth-century finds from the suburb of Saqunda (Cordoba) that appear to be a precursor of soda-ash lead glasses.The chemical compositions of the two types of lead glasses were determined by LA-ICP-MS, and lead isotope analyses were performed on a subset of samples by thermal ionization mass spectrometry (TIMS). The results revealed that the lead raw material was likely sourced from mining districts in al-Andalus, and identified significant differences in the nature and processing of the raw materials. The trace element characteristics of the high lead glasses from Saqunda suggest the use of lead slag, whereas a purer source of lead underlies the soda-ash lead glasses from Madinat al-Zahra. Replication experiments based on Theophilus’ De diversis artibus yielded clues about the production processes and possible source of the high chlorine concentrations in these soda-ash lead glasses. Based on our analytical, isotopic and experimental results, we