SEM-EDS research on mineral inclusions found in the biscuit of azulejos as a tool for provenance studies

Abstract

During the year starting on July 2017, the Museu Nacional do Azulejo (Portuguese National Azulejo Museum), the Laboratório Nacional de Engenharia Civil and the HERCULES Laboratory of the University of Evora made a joint effort in the instrumental study of 16th century Portuguese azulejos, aimed at establishing their origin, technology used and eventual systematization of workshop productions. The microscopic observation of the biscuits did not reveal any signs of mixtures of clays, as are routinely perceived through the banded colours seen in the biscuits of 17th century Portuguese productions. Therefore, it seems likely that the workshops used plain marls with a suitable composition to grant compatibility of the biscuit with the glaze and thus their composition is a prime choice to confirm a local provenance. Azulejo samples are collected from panels on the walls and are necessarily superficial and very small. In such samples the biscuit composition as pertains to minor and trace elements is tainted by the penetration of the raw glaze and digestion phenomena over firing. However inclusions of minerals that are infusible at the kiln temperatures remain largely unaltered. A means to discriminate provenance based on a morphologic and compositional study of the small inclusions found in biscuit sections, often less than 5µm across, is proposed in this communication. These include mineral inclusions, both with and without repetitive morphologies, and micro-fossils. This communication reviews the inclusions repetitively found in Portuguese 16th century azulejos as a first step for the construction of a database aimed at exploring their potential as provenance markers.