Analysing the glaze of a medieval ceramic fragment from the Durres Amphitheater in Albania

The paper analyses the glaze of a ceramic sherd found in the southern sector of the Durres amphitheatre. Specifically, the sherd was found in a layer datable to the late 12th to early 13th century, which can be interpreted as a dismissal layer of a pottery kiln in use between the early and second half of the 12th century. The glaze was analysed using SEM-EDS and Total XRF techniques. The green-ocean glaze with a blue-sky decoration of the fragment has As-Co and Pb-Sn-Si compounds as pigments and phosphorous as a modifying agent and a flux. The glaze composition is SiO2 47.6 wt.%, TiO2 0.22 wt.%, Al2O3 4.08 wt.%, FeOtotal 0.22 wt.%, MnO 0.08 wt.%, MgO 0.23 wt.%, CaO 2.51 wt.%, Na2O 1.55 wt.%, K2O 5.16 wt.%, P2O5 3.01 wt.%, SnO2 4.13 wt.%, As2O5 4.13 wt.%, PbO 25.4 wt%. Fe is expressed as FeOtotal. The trace elements composition (ppm) is Co 3684, Ni 1023, Cu 819, Zn 3070, Bi 3172, and Sr 205. We introduced a robust glaze classification scheme based on chemistry. This scheme categories the glaze as alkaline-lead SnO2-opacified. We examined uncommon compounds formed in various textural contexts to establish the production origin and technique peculiarity. The glaze glasses form three different compositional domains: one represents the parental high-temperature initial glass composition, and two are related to immiscible segregations forming at lower temperatures. Five phases of the apatite supergroup were identified, along with other phases distributed throughout the glaze. The compounds present, such as Pb and Sn silicates, leucite, and k-feldspar and their balances, constrain the firing temperature to 720 celcius and 900 celcius, respectively.