Phosphatic alteration of lead-rich glazes during two centuries of burial: Bartlam, Bonnin & Morris, and Chelsea porcelain

Abstract

Discolored lead-rich glazes on phosphatic porcelain sherds from the sites of the Bartlam (Cain Hoy, SC), Bonnin & Morris (Philadelphia, PA), and Chelsea (London, UK) factory sites record the effects of alteration after two centuries of burial. The alteration presents as a dark brown to black scale on the American samples, and as pale brown crazing of the Chelsea glaze. Backscattered electron images of this material show the development of Liesegang rings and a sharp corrosion boundary where in contact with (relatively) unaltered glaze material. Compared with their unaltered counterparts, the altered glazes are variably but in some instances massively (≥ 90%) depleted in SiO2 and alkalis, and enriched in P2O5, CaO, PbO, and various trace elements, notably V. Some of the Bonnin & Morris samples have had bone ash components—especially CaO—leached from the now-porous phosphatic paste, so their CaO/P2O5 (molecular proportions) ratios (~ 2) are much lower than those of the relatively unaltered Bonnin & Morris samples (3.1–3.5). The ceramic body is not, however, the source of phosphate (and calcium) enriched in the altered glazes because phosphate enrichment characterizes glaze alteration even when there is no evidence of bone ash dissolution. Anthropogenic sources are likely for this and other components enriched in the altered glazes. Glaze alteration is interpreted in terms of leaching (de-alkalization) and silica-network dissolution in the presence of subsurface alkaline aqueous fluids (pH ≥ 9). Unaltered glazes on both creamware and phosphatic porcelain sherds from Cain Hoy have overlapping trace element concentrations, supporting the hypothesis that John Bartlam manufactured porcelain there, despite the apparent absence of biscuit wasters at this site.