920 971 the following schedule was used 100 u00b0 c hour to 600 u00b0 c hold for 2 hours 100 u00b0 c hour to 725 u00b0 c hold for 2 hours 60 u00b0 c hour to 1075 u00b0 c hold for 1 hour and allow the resulting sagged and cerammed mold to cool for approximately 36 hours within the firing kiln although it will be understood that a variety of glass ceramic compositions may be utilized to form the shell type sagging mold of the present invention it is necessary for operability of the mold that it be thermally stable and highly refractory so as to withstand sagging cycles of up to at least about 730 u00b0 c without failing and that it have a relatively low coefficient of thermal expansion so that it will remain dimensionally stable during the heat up and cool down of repeated sagging cycles a relatively low coefficient of thermal expansion of less than about 20 u00d7 10 7 u00b0 c such as shown in table ii of u s pat no 3 582 371 is deemed sufficient to provide the necessary degree of dimensional stability generally a highly crystalline glass ceramic normally results in a relatively low coefficient of thermal expansion although not limited thereto a preferred glass ceramic composition is one comprising a beta spodumene solid solution dispersed within a glassy matrix with a crystallinity of at least about 80 such as those compositions disclosed in table i of u s pat no 3 582 371 the master mold may be formed either by a casting process or by machining a master mold blank when it is necessary.