The technical examination and conservation of blackened Delftware from anaerobic sites

Abstract

Cleaning methods are assessed for Dutch tin-glazed earthenware where the glaze has been blackened by burial in anaerobic environments, primarily cess-pits and canals. The blackening has been shown, by X-ray diffraction (XRD), electron diffraction and scanning electron microscopy (SEM), to result from the formation of crystalline lead sulphide particles within the glaze. The efficacy of chemical methods designed to transform or remove the lead sulphide blackening was investigated in laboratory experiments with AnalaR-grade lead sulphide and with excavated Delftware shards. By means of SEM, transmission electron microscopy (TEM), XRD and laboratory observations, it was concluded that: (i) mineral acids (notably nitric) can give visually acceptable cleaning but destroy the integrity of the ceramic body by removing calcite or dolomite; (ii) citric acid and tartaric acid, although less damaging to the ceramic body, are inefficient cleaning reagents; (iii) hydrogen peroxide partially restores the original glaze colours, by transformation of lead sulphide to lead sulphate, with no apparent detriment to the ceramic body; (iv) sodium hydroxide (5M) achieves effective cleaning with minimal apparent damage to the ceramic body. During these investigations the presence of carbon filaments in the earthenware matrix of one shard was observed by TEM. In preliminary experiments designed to assess alternatives to chemical cleaning, refiring of blackened shards at c. 950°C restored the original glaze appearance effectively. The potential of neutron autoradiography as a means of visualizing glaze designs is also discussed.