ESTIMATION OF THE BULK COMPOSITION OF FINE-GRAINED MEDIA FROM MICROCHEMICAL AND BACKSCATTER-IMAGE ANALYSIS: APPLICATION TO BISCUIT WASTERS FROM THE BOW FACTORY SITE,LONDON*

Microprobe and digitized electron-backscatter image data reveal that eighteenth-century Bow porcelain consists of a silica polymorph(s) (25–43 vol. %), a calcium phosphate phase (42–55%) with homogeneous and stippled textural variants, and a silicon-, aluminium-, and calcium-rich, former melt phase represented by discrete domains (14–20%) and micron-scale blebs that occupy 20–70% of the stippled calcium phosphate. Bulk compositions calculated from these data are deficient in calcium phosphate components and more siliceous than those determined by microprobe analysis of fused powders of the same samples. This reflects difficulties in determining representative values for the composition and volume of the melt phase, and underscores the limitations of the method where applied to media containing compositionally- or texturally-heterogeneous phases.