Microscopic-Scale Examination of the Black and Orange-Yellow Colours of Architectural Glazes from Assur, Khorsabad and Babylon in Ancient Mesopotamia

Three major corpora of architectural glazed bricks from Ancient Mesopotamia dating to the Neo-Assyrian (Assur and Khorsabad sites) and the Neo-Babylonian (Babylon site) Periods have been submitted to an in-depth comparative study of the orange-yellow and black glazes. Distinct hues in the orange-yellow range were observed according to the archaeological site. They appear to have been well mastered by the glassmakers, consisting in the ex situ preparation of the antique lead antimonate pigment and its mixing with transparent soda-lime glass frit or with the glass-forming components. The intentional addition of hematite or of Cu2+ colouring ions in a controlled amount is suggested in two cases. SEM-EDX and Raman analysis of the lead antimonate pigments have pointed out different chemical substitutions in their pyrochlore structure, mainly Fe3+ in the Sb5+ site and Ca2+ in the Pb2+ site, the proportion of which being correlated to the pigment shade (from pale yellow to orange-red). Part of these substitutions arises from the chemical reaction of the pigment with the hematite and glass melt during firing. Regarding the black glazes, an unexpected colouring technique involving copper sulphide nanoparticles together with the chromophore Fe3+-S2- is highlighted for Khorsabad (8th century BC) and for Babylon (6th century BC). For Assur blacks, the study reveals a change in their colouring technique between the 9th and 8th centuries BC, from a colouration with Mn oxides to an enigmatic one that could also have involved copper sulphide nanoparticles.