Petrology of Gilbody, Pennington and Christian/Pennington (18th Century Liverpool) Porcelains and their Distinction from some Contemporary Phosphatic and Magnesian/Plombian British Wares

Abstract

Sherds recovered near the sites of three 18th century porcelain manufactories in Liverpool were subjected to electron microprobe and backscattered-electron image analysis to determine the composition and proportion of their constituent phases and bulk, major element compositions. Samples collected near the John Pennington (Copperas Hill manufactory,c. 1770–1779) and Gilbody (c. 1755–1761) works have phosphatic compositions, and contain silica polymorphs, calcic plagioclase, a tricalcium phosphate phase and a former melt phase (i.e., the “glassy matrix”) that is anomalously silicious compared to compositions predicted using phase diagrams. Melt compositions appear to be adulterated by minute, high-T silica polymorph crystallites. Samples from near the Christian/Seth Pennington (formerly Chaffers) works (c. 1765–1799) have magnesian/plombian compositions, and contain silica polymorphs, diopside and a variably magnesian, glassy matrix whose composition appears to be governed by the proportion of entrained enstatite crystallites formed by the dehydration (melting?) of talc (steatite). The presence of diopside indicates the use of calcite in the recipe and helps to distinguish these wares from early Worcester porcelains. Production rates for the Christian/Seth Pennington works are estimated at ≤135,000 objects/year, based on calculated recipes and historical data concerning the provision of steatite to the manufactory. Discrimination diagrams derived from these analyses and literature data show that Liverpool porcelains can be distinguished from various contemporary (and later) wares by their major element compositions. The use of these diagrams, supplemented by mineralogical criteria, led to the recognition of a possible Christian/Seth Pennington porcelain sherd from an archaeological site in Atlantic Canada.