Microstructural evolution in bone china

Phase and microstructural evolution in model bone china bodies was determin ed by XRD and electron microscopy of quenched samples fired for 3 h at 600- 1500 degreesC. Unfired but shaped bone china comprised bone ash and clay ag glomerates (less than or equal to 70 mum) in a matrix of smaller (from subm icron to 10 mum) mixed clay, feldspar, and bone ash particles. The unfired microstructure and subsequent phase evolution is believed to be strongly de pendent on the extent of prior mixing. On firing, the clay component dehydr oxylated to metakaolin at similar to 550 degreesC. Metastable sanidine form ed from decomposition of the feldspar component above 600 degreesC and diss olved at 1100 degreesC. The bone ash component decomposed into beta -TCP an d lime (and/or Ca2+ and O2- ions) beginning at similar to 800 degreesC. CaO from the bone ash reacts with the clay decomposition products forming liqu id and anorthite at similar to 900 degreesC. Liquid formation is due to rea ction of CaO with feldspar and clay relict grains and is discussed in terms of the CaO-P2O5-Al2O3 ternary phase diagram. Above 1200 degreesC pure bone ash relicts contained small (5-10 mum) beta -TCP crystals, CaO penetrated clay relicts contained anorthite, and mixed clay-bone-feldspar regions cont ained both anorthite and larger (> 50 mum) beta -TCP crystals in calcium al uminosilicate glass. The major phase in the clay relicts was anorthite alth ough a few elongated (similar to 100 nm) needles resembling mullite in comp osition and morphology also crystallised in samples fired to 1100 degreesC and grew to similar to 30 mum in length at 1300 degreesC.