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 the slurry contains from about 68 to about 74 weight percent of silica and less than about 0.8 percent of carbon. At least about 9 weight percent of the particles in the slurry are larger than about 44 microns, and at least about 5 weight percent of the particles in said slurry are larger than about 74 microns.