Beginning towards the end of the 16th century, the chromatic palette of Portuguese decorated tiles (azulejos) expanded dramatically. Besides white and blue, the colours now include purple, brown and different shades of yellow, orange and green. Although the lack of local sources severely limits our knowledge on how the colours were achieved, existing analytical data suggests that lead antimonate (Pb2Sb2O7) is one of the most frequently recurring compounds in pigment mixtures. Knwon as Naples yellow, lead antimonate represents the main component of yellow pigments and, mixed with a number of other oxides, formed the base for olive green, orange and orange-brown.

Using a technical historical approach, this paper addresses the role of lead antimonate within the colour technology of Portuguese azulejos. First, an ensemble of historical azulejos (16th-18th century) were investigated through a multi-analytical approach, to obtain information on their chemical composition (EDXRF) and to identify the main colouring agents (UV-VIS FORS and µ-Raman spectroscopy). Moreover, colorimetric measurements of the different hues were performed to address the relationship between colour and chemical composition. All analyses were performed non-invasively on the surface of the tiles. Our results confirm the widespread use of lead antimonate, on its own (yellow) or in combination with zinc (orange), iron (brown) and cobalt (olive green).

Further to the work on historical samples, a number of lead antimonate-based pigments were replicated in the laboratory following contemporary recipes from the Italian majolica tradition. Once obtained, the pigments were applied on test tiles, over a white glaze, and fired again. The resulting tiles were analysed with the same suite of techniques. Micro Raman indicates that the molecular profile of test tiles matched that of the historical ones, confirming the role of lead antimonate as the main compound in many of the colours. Our replications also confirm that additional ingredients strongly affect the hue, as it is the case of alkali fluxes, zinc or tin oxide. On the other hand, the addition of smalt, a cobalt-based preparation, or copper oxide, gave olive green decorations, while the presence of iron oxide turned the pigment a dark orange to brown.

This article sheds new light on the role of lead antimonate as the main ingredient in three major colours found in Portuguese tiles. Our experiments show that by skilfully dosing a variety of reagents in the pigment recipe, painters could count on a rich palette of hues to accommodate their chromatic needs.