Microstructures of phosphatic porcelain from sintering to vitrification: Evidence from sherds excavated in Charleston, South Carolina

Abstract

Eight phosphatic porcelain sherds recovered from various historical sites in Charleston were analyzed by electron microprobe. Some sherds contain sulfur (2.3-3.1 wt.% SO 3 ); others contain only traces of this component. The analytical data suggest that the sulfurous sherds are Bow porcelain (London, Bowcock period, ca. 1755-1769). The origin(s) of the low-S samples remains unidentified; one compositionally resembles "gold-anchor period" (phosphatic) Chelsea porcelain (London, ca. 1756-1769) but its decoration is inconsistent with known wares produced by this factory during that era. The degree of vitrification is highly variable, particularly among the SO 3 -poor samples. The melt phase is strongly enriched in incompatible elements (Ti, Fe, Na, K). The phosphate phase [calcined bone ash (hydroxyapatite)] in poorly vitrified samples hosts minute melt blebs, but remains porous. With increasing vitrification, these melt blebs increase in size and begin to coalesce, ultimately forming ameboid patches up to ~10 mm in diameter. In the most vitrified samples, the coalesced melt "leaks" into the matrix, leaving behind a phosphate phase that lacks pores and melt and has a lower CaO/P 2 O 5 ratio (ϭ2.7, molecular proportions) than either hydroxyapatite (3.3) or b-whitlockite (3.0). The two varieties of phosphate occur in some poorly vitrified samples, suggesting the recycling of highfired wasters (as "grog") in their ceramic pastes. Melt compositions vary with contiguous mineralogy, accounting for their divergence from the ternary eutectic in the Ca 3 (PO 4 ) 2 -CaAl 2 Si 2 O 8 -SiO 2 system. The resorption of phosphate by the matrix melt virtually precludes recognition of anorthite formed by the "non-phosphate glass equation."

Geochemistry of sauceboats excavated from Independence National Historical Park (Philadelphia): evidence for a Bonnin and Morris (c. 1770–73) provenance and implications for the development of nascent American porcelain wares

“…The relict bone ash lacks pores or melt blebs (Fig. 4), suggesting that the sample was thoroughly soaked at high temperature (Owen et al, 2011). This is consistent with the abundant matrix melt phase, which averages 25 wt.% anorthite components.…”

“…Unlike the melt in phosphatic sherds from the Bonnin and Morris factory site, the melt in the sauceboats is lead-free, showing that potash-lead glass was not used in the manufacture of these artifacts. The melt contains between 0 and 1.5% phosphate, reflecting the role that the partial dissolution of contiguous minerals can have on the composition of this phase (Iqbal et al, 2000;Owen et al, 2011). The phosphate phase contains up to 2% sulphate.…”

“…Note the relatively aluminous character of the sauceboats (here averaged) from INHP (see text). Diagram is simplified afterOwen et al (2011).…”

Igneous Rock Associations 12. A Geologist’s Look at Archaeological Ceramics and Glass

“…Melts, however, rarely plot on or near eutectics and cotectics on these diagrams, probably because of the influence of contiguous phases on their compositions (i.e. equilibrium is approached only on a domainal scale; Iqbar et al 2000;Owen et al 2011b). It is, however, the simplicity of the chemical systems on which these diagrams are based more than evidence for lack of equilibrium that can limit the usefulness of phase diagrams in determining firing temperatures.…”

Why not let them rest in pieces? Grog‐temper, its provenance and social meanings of recycled ceramics in the Baltic Sea region (2900–2300 BCE)

“…Grog‐tempered ceramics have been used globally in various cultural and chronological contexts. Sporadic occurrences of grog are recorded in ceramic technologies from the Neolithic period to modern times, for example, in various parts of Europe, the British Isles, the Americas, the Caribbean, the African continent, the Near East and Oceania (e.g., Babetto et al, 2021; Borgers et al, 2020; Braekmans et al, 2017; Cantin & Mayor, 2018; Cau Ontiveros et al, 2019; Di Prado et al, 2020; Dickinson & Shutler, 2000; Fitzpatrick et al, 2003; Heath‐Stout, 2019; Heydarian et al, 2020; Lyne, 2015; Maggetti et al, 2011; Owen et al, 2011; Thompson, 1982; Ting et al, 2018; Wallis et al, 2011). In archaeological ceramics, grog‐temper appears in a wide variety of ceramic forms: from delicate bowls to large containers as well as water pipes, bricks and tiles (Cuomo di Caprio & Vaugnan, 1993).…”