b-TCP, anorthite, and a-quartz (SiO 2) in a calcium aluminosilicate glassy matrix.5,12,13 Iqbal et al.5 used scanning electron microscopy (SEM) and TEM to show that commercial bone china microstructures consist of lath-like

saturate the clay with CaO, with the result that little of the b-TCP needs to break down and form low melting phases. Commercial bone china microstructures are far fromThe b-TCP crystals, which now act as a filler, confer equilibrium consisting of distinct regions of lath-like refractoriness on the body. The proportions of clay, flux, (0•4-10 mm) anorthite and spheroidal (1-3 mm) and bone may be varied from the typical 25 : 25 : 50 ratio b-tricalcium phosphate (b-T CP) nodules embedded in depending on the properties desired and other components a heterogeneous composition glass along with isolated may be substituted. For example, ball clay and gum arabic irregular (∏30 mm) a-quartz crystals. T he composcan be included in the body recipe to increase green strength ition and morphology of the phases formed on firing and bentonite clay (predominantly montmorillonite) can be suggest that anorthite crystallised in clay relicts by the added as a plasticiser to increase workability and green reaction of metakaolin with CaO, b-T CP crystallised strength.3 f rom decomposition of bone ash, and that the liquidIn the UK bone china is manufactured using the following formed on vitrification has variable composition general processing route. Hard raw materials are crushed depending on the composition of adjacent phases. P 2 O 5 by jaw and roller crushers, weighed, wet ball milled, sieved, was never detected in the glass suggesting that any and passed over magnets to remove metal particles. Clays P 2 O 5 that dissolves in the liquid on firing is transient. (with a particle size of ~0•2-6 mm) are blunged. Bone ash T he cracks sometimes observed in and around b-T CP is milled to a median size of ~3-5 mm and fluxes to clusters arise from thermal expansion mismatch ~10 mm. Weighed proportions of clay, flux, and filler are between b-T CP and the surrounding glass and then mixed in mixing arks, again sieved, and passed over anorthite. BCT /381 magnets and stored. De-watering may be carried out by filter pressing or spray drying. After filter pressing, the T he authors are in the Department of Engineering bodies are aged and passed through a pug mill to de-air Materials, University of SheYeld, Mappin Street, the body and homogenise the water content. Shape forming SheYeld S1 3JD, UK. Manuscript

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In this study the effect of size and distribution of leucite crystals on the biaxial flexural strength of five leucite reinforced glass-ceramics used in dentistry for the construction of veneers was investigated. Three of these were found to have a non-uniform distribution of leucite crystals, which were associated with a considerable degree of microcracking in the glassy matrix around the leucite crystals. The mean values of biaxial flexural strength of these ceramics ranged from 56 to 70 MPa. The other two ceramics exhibited a more uniform distribution in their leucite crystals, which were also finer in size. The latter ceramics had a significantly higher biaxial flexural strength which ranged from 120 to 137 MPa. When the particle size of the starting powders of the three low strength ceramics was reduced, a more uniform microstructure with less evidence of microcracking was produced in the fired ceramics and the biaxial flexural strength values were significantly increased. We conclude that uniformity of microstructure and leucite crystal size are major factors in determining the biaxial flexural strength of leucite reinforced glass-ceramics.

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