A process for rapidly forming a ceramic shell mold on an expendable pattern is disclosed. The process entails use of refractory slurries which include a large particle size colloidal silica sol binder. The colloidal silica sol binder has an average particle size of about 40 nanometers, i.e., about 3-4 times larger than colloidal silica sol binders heretofore employed in manufacture of ceramic shell molds. The use of the large particle sols yields unfired ceramic shell molds which have about 40% to about 70% greater unfired strengths compared to ceramic shells made with prior art small particle size silica sols. Prime coats and refractory back-up coats which use the large particle size sol dry about 30% to about 40% faster than prime coats and back up coats which employ the smaller particle size silica sols of the prior art.