The colours of Ca-rich ceramic pastes: origin and characterization

The colour changes found in the wall thickness of archaeological calcareous ceramics are analysed. The main colour change from creamy to reddish is due to the incorporation of Fe3+ in several calcium silicates and aluminosilicates developed during firing. Iron appears as iron oxides (haematite and maghemite) and as structural iron, either in tetrahedral coordination related to the dehydrated clay minerals and to the melilites, or in octahedral coordination related to the pyroxenes of fassaite–diposide type. The creamy colour is related to the complete decomposition of calcite, a significant development of pyroxenes, and to the low presence of iron oxides. Haematite is determined across the creamy and reddish layers except in the outer surface, where maghemite is determined. The presence of maghemite and Fe2+ in the outer surfaces agrees with the use of a low reducing atmosphere at the end of the firing process.