Susan D. Costello, Katherine Eremin, Georgina Rayner, and Arthur McClelland
Abstract
A fifth-century BC Greek red-figure terracotta pelike at the Harvard Art Museums exhibited areas of black manganese dioxide accretions from burial, making interpretation of the painted design difficult. A research project was undertaken to develop a safe method to reduce the bound accretions. Based on the results, a novel treatment was carried out on the pelike using a poultice of bentonite clay, deionized water, and ethanol. The clay’s high ion exchange capacity was able to break the accretion’s bond to the ceramic, allowing it to be removed. The manganese accretions were characterized by x-ray fluorescence spectroscopy, Raman spectroscopy, x-ray photoelectric spectroscopy, and scanning electron microscopy with energy dispersive microanalysis. The latter was also used to analyze the bentonite poultice and the terracotta before and after treatment. Results showed that the terracotta surface was unchanged and no bentonite was left behind. The treatment of the pelike significantly reduced the manganese accretions and achieved the desired outcome of a clearer interpretation of the painted design. The results of this research project can inform future treatments of ceramics with manganese accretions.
KEYWORDS: Terracotta, Greek ceramic, Manganese dioxide, Bentonite, Stain reduction, Rock varnish