e g placed atop a chill plate in the furnace the casting furnace may be pumped down to vacuum 54 or charged with a non oxidizing atmosphere e g inert gas to prevent oxidation of the casting alloy the casting furnace is heated 56 to preheat the mold this preheating serves two purposes to further harden and strengthen the shell e g by at least 5 more of ultimate mor strength and to preheat the shell for the introduction of molten alloy to prevent thermal shock and premature solidification of the alloy accordingly the preheat temperature and duration are advantageously sufficient to substantially further harden the shell above its prehardened condition this may involve sintering of the ceramic particles within the shell advantageous mor is in excess of 85 and more particularly in excess of 90 or 95 of ultimate mor this may be achieved with a preheat temperature of at least 1200 f more particularly at least 1400 f with an exemplary preheat temperature of about 1600 f exemplary preheat times are approximately one hour e g 0 25 4 0 hours more narrowly 0 75 2 0 hours after preheating and while still under vacuum conditions the molten alloy is poured 58 into the mold and the mold is allowed to cool to solidify 60 the alloy e g after withdrawal from the furnace hot zone after solidification the vacuum may be broken 62 and the chilled mold removed 64 from the casting furnace the shell may be removed in a deshelling process 66 e g mechanical breaking of the shell and the core assembly removed in a decoring process 68 e g a chemical process to leave a.