The 'della Robbia blue': A case study for the use of cobalt pigments in ceramics during the italian renaissance

The morphologies and elemental compositions of cobalt blue glazes are studied in 56 dated and undated terracotta sculptures from the Florentine della Robbia school. The data allow tracing of the pigments through correlations amongst elements that are known to be constituents of the cobalt minerals. The supply of such minerals from the Erzgebirge region in Renaissance times is compatible with our data. A substantial change in the materials is marked by the presence of arsenic and bismuth in the glaze, and by reduced amounts of iron and nickel just before 1520. We speculate that this is due to procedures introduced to industrialize the production of the blue pigment. The exploitation of different cobalt minerals in association with different production procedures may explain the variability of compositions that we observe in this unique ensemble.