The colour palette of 16th-18th century azulejos: A multi-analytical non-invasive study

This work provides an overview of the pigments used by Portuguese azulejo painters through the study of 34 glazed tiles belonging to the National Azulejo Museum in Lisbon, Portugal. The tiles are dated from the late 16th century to the early 19th century, with most samples attributed to the 17th and 18th centuries. Building on a previous study where the chemical composition of the 17th-century colour palette was characterised [1], we used a non-invasive methodology to further identify compositional differences among groups of colours spanning a wider timeframe. Blue, white, yellow, orange, purple, green and brown decorations were analysed by EDXRF to obtain qualitative and (in some samples) quantitative information on their chemical composition. µ-Raman spectroscopy was used to identify the main colouring agents. Finally, colorimetric measurements of the different hues of each colour were performed to address the relation between colour and chemical composition. Our results show that greens could be obtained through a copper-based pigment or by mixing lead-antimonate with cobalt-blue pigment. Although cobalt is the colouring agent of all blue decorations, compositional differences suggest the use of different types of raw materials or the addition of specific reagents to modify the hue. On the other hand, yellow hues ranging from lemon yellow to orange were manufactured by adding zinc, tin, or iron oxide to a lead antimonate base. Finally, a manganese ore was used to make the purple pigment. Overall, we noted that while the basic pigments remained the same, the colour palette was progressively widened during the 18th century. This was done by using more pure pigments (i.e., with less iron), by changing the ratio in pigment mixtures (i.e., blue and yellow), or by overlapping colour layers for artistic effects. Finally, the results of the chemical and colorimetric analyses are always influenced by the concentration of the pigment in the glaze, its thickness and the overlapping of pigments used by azulejo painters to expand the shades available for the final artistic composition.