And after burial? Corrosion structures of high-lead glazed ceramics in Al-Andalus objects from Palmela Castle (West Portugal)

The study of the degradation of archaeological objects is essential for guiding their conservation and restoration, alongside understanding possible formal and aesthetic changes that may have occurred since their burial. Several difficulties hinder these studies as numerous factors, both intrinsic and extrinsic, are involved in the degradation processes. In the present work, we focused on a group of representative green and honeydew transparent glazed fragments from Pamela Castle, dated between the 9th and 11th centuries. A comparison of different coloured archaeological lead-glazes subjected to broadly comparable environmental conditions was possible, enabling a direct evaluation of corrosion patterns while accounting for potential microenvironmental variations. All objects were studied by OM and SEM-EDS to assess the glaze’s corrosion structures and investigate the associated degradation processes. Results allowed the identification of two corrosion layers on all fragments; however, these presented significantly different characteristics depending on glaze colour. The intermediate layer presents a large depletion of the major glaze constituents for the honeydew ones, while for the greens, this layer displays an apparent silica enrichment and a banded structure, identified as a Liesegang-like corrosion pattern. The findings revealed distinct corrosion structures associated with the different glaze colours, which were used as the basis for a schematic representation of the degradation structures identified in this case study.