An Investigation of Discoloured Oil-bound Copper Green Glazes in a Seventeenth-century Harpsichord Lid, With Reference to Reconstructions

This study focuses on copper green glazes on the front and reverse schemes of a seventeenth-century harpsichord lid painting. The aim was to investigate the materials and techniques of the copper-based green paints and their brown discolouration, a phenomenon commonly encountered in glazes made with the pigment verdigris (basic and neutral copper acetate). The study summarises the historical materials and recipes for copper green glazes, and includes a review of hypotheses that have been proposed for discolouration, focusing on findings from investigations that have been conducted in laboratory environments into the chemical interactions between verdigris and linseed oil. Analyses, using optical microscopy, SEM-EDX, XRD, THM-Py-GC-MS and ATR-FTIR, were conducted on paint samples from the browned copper green glazes from the front and reverse of the painted lid. To link the laboratory studies with the information known from historical texts and technical analysis of the object, analysis was also conducted in tandem on reconstructions, comprising neutral copper acetate in linseed oil. These were made and previously studied as part of the MOLART project (1995–2000), based on historical paint recipes. The analysis of the MOLART reconstructions supports the idea of a correlation between pigment-medium interaction and oxidation. Results from analysis of the painted lid support the idea that artists used a range of copper-based pigments for green glazes besides copper acetate, and illustrate different techniques for the glazes used within the interior and exterior painted schemes.