PHASE-EQUILIBRIUM CONSTRAINTS ON KILN TEMPERATURES AT THE EBY POTTERY (ca. 1857-1905), CONESTOGO, ONTARIO

Abstract

Samples from the Eby pottery, in Conestogo, Ontario, that have been distorted by a high degree of partial melting contain both relict temper (sand) grains, liquidus minerals (low-Ca pyroxene and calcic plagioclase with quenched morphologies), and coronal minerals formed by peritectic-like reactions between temper and melt. The coronal minerals include low-Ca pyroxene (enstatite or protoenstatite) on diopside temper, and ternary feldspar on K-feldspar temper. They only loosely constrain firing conditions (~1373, 990°C, respectively), as indicated by phase diagrams. Assuming relatively reducing conditions (QFM buffer) consistent with the dark grey color of the distorted Eby samples, modeling of melt compositions indicates that small amounts of liquidus orthopyroxene started to form at about 1150–1125°C, and was joined by plagioclase after the melt cooled to about 1050–1030°C, consistent with the results of plagioclase–melt thermometry applied to quenched crystals. In more aluminous samples, plagioclase was relatively calcic, and preceded pyroxene on the liquidus. The similarity of the bulk compositions of successfully fired and distorted Eby pottery shows that the distorted samples were casualties of uncontrolled temperatures in the kiln rather than a relatively non-refractory paste. Extensive melting of the Eby samples nonetheless occurred at temperatures near the upper part of the range commonly inferred for historical “low-fired” pottery.