A clay slurry with a solids content of at least about 60 weight percent and a casting rate of at least 60 grams per hour, wherein said slurry contains from about 61.5 to about 71.5 weight percent of silica, from about 18.8 to about 26.8 weight percent of alumina, and less than about 0.6 percent of carbon, wherein said slurry has a loss on ignition of from about 7.4 to about 8.6, wherein from about at least about 9 weight percent of the particles in said slurry are larger than about 45 microns, wherein at least about 5 weight percent of the particles in said slurry are larger than about 75 microns, wherein from about 29 to about 36 weight percent of the particles in such slurry are smaller than 0.5 microns, wherein the ratio of such particles larger than 75 microns to such particles larger than 45 microns is from about 0.5 to about 0.7, and wherein at least 90 weight percent of such silica contains less than about 10.sup.17 Si and Si--O silicon-based radicals per gram.