UCLA

One of the earliest examples of synthetic pigments is ‘cobalt blue’ or ‘Amarna blue’, which was produced in Egypt 3500 years ago during the New Kingdom (16th – 11th century B.C.). It is a cobalt (Co) aluminum (Al)-spinel blue compound with an AB2O4 (i.e.CoAl2O4) structure, consisting of two metallic cations A2+ and B3+ in tetrahedral and octahedral arrangement. This cobalt-based ceramic pigment was produced through processing of natural materials at high temperatures (1). Cobalt blue was a product of great value and significance and of highly specialized technological knowledge (1). The localized geographical development and short life-span of its production of 200+ years, however, raised many questions about its raw material selection and production technology. Previous research conducted primarily on a small number of surface-collected ceramic sherds, suggested that cobalt blue was produced using cobaltiferous alum from the Dakhla and Kharga Oases of Egypt, via precipitation or solid state sintering (2). Owing to the importance and significance of this pigment both from an archaeological materials science perspective as well as from a materials engineering perspective, this ancient spinel was investigated both experimentally and theoretically. Using materials science principles and reverse engineering, and modern reproduction experiments to recreate the ancient pigment, the composition, structure and variability, as well as, the operational sequences for the production of this ancient pigment have been established. Findings from the systematic analysis of cobalt blue from the archaeological collections of the Metropolitan Museum of Art in New York and the Petrie Museum in London using electron microscopy and X-ray diffraction, indicated compositional and morphological homogeneity, with an interconnecting network of nanocrystals of various spinel phases, together with other crystalline production byproducts and remnants of the production process. Reproduction experiments producing end-products with similar composition and structure to the ancient pigment suggested a solid-state synthesis. Potential model parameters for CoAl-spinel was developed for the first time and produced similar results to reported experimental data (3). Theoretical modelling of the structure and properties of CoAl-spinel indicated an equilibrium lattice parameter of 8.1127 and a bulk modulus of 212GPa.