Geochemical and mineralogical distinctions between Bonnin and Morris (Philadelphia, 1770-1772) porcelain and some contemporary British phosphatic wares

The major element compositions of 15 ceramic sherds from the Bonnin and Mor ris factory site were determined by electron microprobe. Thirteen samples a re phosphatic; the others consist of (a) "soapstone" (magnesian/plombian) a nd (b) true porcelain, and are interpreted as exotic artifacts, as is one c ompositionally distinct (relatively SiO2-poor, P2O5+CaO-rich) phosphatic sa mple. Although long considered to be virtually indistinguishable from Bow p orcelain (London: ca. 1747-1776), the phosphatic Philadelphia wares have a relatively low mean CaO/P2O5 ratio (3.3 versus 3.8; molecular proportions) and high alumina content (6.6 versus 5.4 wt % Al2O3). Furthermore, unlike B ow, the Bonnin and Morris samples contain calcic plagioclase (bytownite), a nd in some instances, an orthoclase-rich ternary feldspar. The preservation of calcic plagioclase indicates that Philadelphia porcelain was fired at ( rather than above) the thermal minimum in the An-SiO2-C3P system, although the presence of Na (and other fluxes) in these wares precludes the exact de termination of the maximum firing temperature from this phase diagram. Thes e wares are also distinctive insofar as the phosphate and melt phases can c ontain small amounts of leads they have bulk lead contents of approximately 0.1-1.2 wt % PbO. This component has not been detected in the body of Bow or other contemporary British phosphatic porcelains. Their principal simila rity lies in the fact that both wares contain sulfate. In addition, the gla zes an Bonnin and Morris porcelain (e.g., PbO similar to 35-50 wt %; SnO2 s imilar to 1-2%) compositionally resemble those used at Bow. If feldspar is formed at all, then Al-poor phosphatic porcelain (or those with low CaO/P2O 5 ratios) will have comparatively low modal calcic plagioclase contents, th ereby allowing the rapid depletion of this mineral via. resorption by the m elt phase during vitrification. Such appears to have been the case for anal yzed Bow porcelain, which is therefore interpreted to have been overfired ( sensu lato) relative to its Philadelphia counterpart. Conceivably, calcic p lagioclase could be preserved in low-Al wares that were fired only briefly at vitrification temperatures. Given the role of firing history in governin g the mineralogy of porcelain, compositional criteria are more reliable for distinguishing these wares. (C) 2001 John Wiley & Sons, Inc.