Plant-ash (soda ash) glass

Plant-ash glass uses the ash of soda-rich plants as its flux instead of mineral natron, and by the tenth century it had become the dominant recipe of glassmaking in the Islamic world. ² Nadine Schibille's book notes that it is harder to classify than natron glass because the ash component is highly variable by nature, requiring new criteria to separate regional groups. ^{3 4}

Schibille suggests the new recipe in the Levant may have been inspired by Mesopotamian glassworkers brought to places like al-Raqqa, where the Raqqa 1 and Raqqa 4 groups were produced together after the city became the caliphal residence in 796, Raqqa 1 being the earliest known Islamic plant-ash glass of the Syro-Palestinian type. ^{5 1} In Mesopotamia plant-ash glassmaking continued on old Sasanian traditions and local materials. ¹ In Egypt the Fatimid glass weights of the late tenth century are the earliest securely dated plant-ash groups, and the author links systematic production to the founding of the Fatimid caliphate and Cairo. ¹ By the tenth century it was established in al-Andalus and Sicily as well. ⁶

Schibille proposes six chemical ratios that separate Egyptian, Levantine, Mesopotamian, Central Asian, Iberian and Sicilian glass, among them the chromium-to-lanthanum ratio that marks Mesopotamian glass, and thorium-to-zirconium and lanthanum-to-titanium ratios that separate Egypt from the Levant. ⁷ She stresses that no single criterion suffices: all must be used, both to exclude and to support an attribution. ⁵