brown fused alumina BFA95% refractory gradecalcined bauxite 88%calcined bauxite 80%artificial corundum white color fused alumina 99%minWFA artificial corundum white color fused alumina 99%minfused zirconia mullite for refractoryfused mullite white colorblack silicon carbide 98.5%minflint clay 45%mincalcined kaolin low iron