Microstructural evolution in bone china

Phase and microstructural evolution in model bone china bodies was determined by XRD and electron microscopy of quenched samples fired for 3 h at 600–1500°C. Unfired but shaped bone china comprised bone ash and clay agglomerates (≤70 μm) in a matrix of smaller (from submicron to 10 μm) mixed clay, feldspar, and bone ash particles. The unfired microstructure and subsequent phase evolution is believed to be strongly dependent on the extent of prior mixing. On firing, the clay component dehydroxylated to metakaolin at ∼550°C. Metastable sanidine formed from decomposition of the feldspar component above 600°C and dissolved at 1100°C. The bone ash com ponent decomposed into β-TCP and lime (and/or Ca2+ and O2- ions) beginning at ∼800°C. CaO from the bone ash reacts with the clay decomposition products forming liquid and anorthite at ∼900°C. Liquid formation is due to reaction of CaO with feldspar and clay relict grains and is discussed in terms of the CaO–P2O5–Al2O3 ternary phase diagram. Above 1200°C pure bone ash relicts contained small (5–10 μm) β-TCP crystals, CaO penetrated clay relicts contained anorthite, and mixed clay–bone–feldspar regions contained both anorthite and larger (>50 μm) β-TCP crystals in calcium aluminosilicate glass. The major phase in the clay relicts was anorthite although a few elongated (∼100 nm) needles resembling mullite in composition and morphology also crystallised in samples fired to 1100°C and grew to ∼30 μm in length at 1300°C.