Abstract: Fusion-cast articles of refractory material are formed with cavities or hollow portions by means of apparatus and method employing a core formed of material more refractory than the fusion-cast refractory material, which core is disengaged from the refractory material solidifying therearound when such solidified portion has the capability of maintaining its integrity and shape absent support of the core, but before cooling shrinkage of the solidified portion onto the core has caused cracking of such portion. A stored energy device is connected to the core through an aperture in the mold which is sealably engaged by the core, and upon release of the stored energy, the disengagement of the core is effected, e.g. by at least partial removal from the mold through the aperture.