High-fired early English porcelains of the ‘A’-marked group, east London (c. 1744): A Raman spectroscopy and electron microscopy compositional study

Two rare, slip-cast and highly translucent very early English manufactured porcelains, known as the ‘A’-marked group, have been nondestructively analysed to determine their bulk composition, the mineralogy of their body, glaze and overglaze decorative enamels using Raman spectroscopy and electron microscopy. The bulk composition of the body and the glaze have been found to have a Ca-Al-Si composition and therefore demonstrate congruency with the specification of the Edward Heylyn and Thomas Frye patent of 1744. The patent specification required the porcelain paste to be prepared using unaker clay from the Carolinas and a lime-alkali glass in varying proportions from 1:1 to 4:1, which when fired at temperatures now shown to range upwards of 1280°C vitrifies to form a refractory porcelain containing β-wollastonite, the calcic plagioclase feldspar anorthite, diopside and mullite. The glaze used was similarly formulated but required lower levels of aluminous clay and a higher proportion of the glass flux to enable vitrification to occur at a lower melting temperature and therefore acting as an optical pyrometer. Crystallites of wollastonite, diopside and cassiterite were recorded in the glaze. Compositions of the blue, black, brown, gold, green, red and purple decorations applied as overglaze enamels have been determined. It is concluded that this porcelain group is attributed to Bow manufacture in East London c. 1744–1745.