An investment casting method involves producing a casting shell by applying a hardenable refractory material to a sacrificial pattern. The casting shell having a plurality of phosphate bonds in the hardenable refractory material, which provide increased structural integrity during casting and improved frangibility during shell removal. The casting shell may also have a plurality of gaseous pockets suspended in the refractory material, which do not degrade the structural integrity during casting and provide improved frangibility during shell removal.