What glass is made from

Ask where each ingredient comes from, what it costs to obtain, and what happens when it is scarce or is reused. Sort what you find into social, environmental and economic impacts.

Read one find closely. Analyses of the glass from the suburb of Šaqunda at Córdoba revealed a local production of lead glass at the end of the eighth or in the first two decades of the ninth century ^2 3. Šaqunda yielded little glass compared with late antique and Visigothic sites, and of different kinds: large open tableware is largely replaced by unguentaria, bottles and beakers ⁴. About 60% is still recycled natron glass, with two Egyptian pieces; the second group is a dark yellowish-green or amber high-lead glass with about 52% lead oxide and 34% silica ⁴. The record says Nadine Schibille treats this as the invention of a novel technology in al-Andalus, using metallurgical waste to make lead and soda-ash lead glasses ^2 3. The dating and the word invention are hers.

It is easy to picture plant-ash glass replacing natron glass at one stroke. The records do not describe that. Natron glass production continued well into the Islamic period; the shift to plant-ash glass came only at the turn of the ninth century in Syria-Palestine, and considerably later in Egypt and the western Islamic world ⁵. In Egypt natron glass was still made until about 870 ^6 7. By the tenth century plant-ash glass had become the dominant recipe of glassmaking in the Islamic world ⁸.

The record reports a method with stated assumptions. Scholars divide Mediterranean natron glass into ten major compositional groups, each the likely output of a discrete primary centre active for a limited period, on the assumption that its composition reflects the silica source ^8 9. Plant-ash glass is harder to classify because the ash component is highly variable by nature, requiring new criteria to separate regional groups ^9 10. Schibille stresses that no single criterion suffices: all must be used, both to exclude and to support an attribution ¹¹. Her explanations are marked as hers: she suggests that the new Levantine recipe may have been inspired by Mesopotamian glassworkers brought to places like al-Raqqa ^11 7.

For the late change of recipe in Egypt, the record reports two kinds of explanation from Schibille: restrictions on natron extraction imposed by the director of finances Ibn al-Mudabbir, and Egypt's natural natron deposits ⁷. Which kind of cause do you think counts for more when a workshop changes its recipe, and what evidence, in the glass or in written records, would you use to test your choice?

• Natron glass: the ten compositional groups and the Damascus mosaics.
• Plant-ash glass: where it emerged and the six chemical ratios used to tell regions apart.
• Andalusi lead glass: Šaqunda, the metallurgical waste and the reasons Schibille suggests.
• Islamic glassmaking: the craft these recipes belong to.