Silica- or alumina-containing granules are loaded into a mold of which the inner walls at least are formed of a nonbonding material constituted of ferrochromite, magnesiochromite, calciochromite, or trivalent-chromium oxide. The mass is then heated to a temperature above the softening point of the granules so as to fuse them together and thereafter the heated mass is hardened. As the mass can inherently not bond to the lining of the mold subsequent separation is relatively easy.