Provenance of the transparent gypsum crystals (Lapis specularis) and gypsum mortars in the windows from the churches of Rome: S. Sabina sull'Aventino and S. Giorgio al Velabro

The latest example of the traditional Roman use of lapis specularis crystals in windows panels instead of glass are represented by the Paleochristian- Early Medieval churches of Rome. The churches of S. Sabina sull’Aventino and S. Giorgio al Velabro were characterized by some of the most ancient examples of windows frameworks built using gypsum mortar. The light was penetrating the panels throughout openings covered by two or more embricated gypsum cleveage fragments less than 15 cm across and up to 7 mm-thick, a technique unknown in older Roman examples, which is clearly related to the availability of small and low-quality crystals. The petrography of the gypsum mortars and the strontium and sulfur isotope analyses of the crystals indicate two diff erent supply sources. A large group of window frameworks were produced using selenite rocks and lapis specularis crystals probably quarried from Tuscany, Sicily or Cyprus, whereas a window in S. Sabina was built using alabastrine gypsum and lapis specularis crystals quarried in Emilia-Romagna, Sicily, Cyprus or Southern Turkey.