Scientific Articles

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.

This paper summarises the available information to date on the pigments used on Portuguese azulejos between the late 16th century and the beginning of the 19th century. Most references cited focus on 17th-century tiles, followed by the 16th century, whereas there is very little information on the 18th and 19th centuries. Regardless of chronology, the Portuguese azulejo palette uses cobalt, copper, manganese, and iron oxides to obtain blue, green, purple, and dark brown, respectively. Yellow is obtained through the Naples yellow pigment, which could be mixed with cobalt for obtaining green, or with iron oxide for the orange colour. Blue and yellow are the most studied colours and the currently available knowledge allows us to relate changes in their chemical composition to specific time periods.

Naples yellow was widely used across different types of artwork. Technical studies identified a binary Pb–Sb type as well as modified ternary variants with either zinc or tin in the structure. Although these variants were the object of previous experimental studies, a better understanding of the impact of the glazing procedure on the chromatic, chemical and crystallographic characteristics of the pigment is still lacking. In this work, several historical Naples yellow recipes were re-worked and subsequently applied and fired on test tiles, over a white glaze. The results show that the interaction between pigment and glaze produces important modifications to the colour, chemistry and structure of the pigment. Such modifications will strongly impact the reconstruction of historical recipes, with major consequences for identifying Naples yellow variants on artwork and investigating artistic practices.