plaster mix was cast therearound the thermocouple was sufficiently close to the cast metal in the sprue that the cooling pattern of the metal after casting could be accurately monitored a plaster mix containing by weight 60 dense plaster 28 85 talc 10 wollastonite 1 boric acid and 0 15 portland cement was mixed dry for 10 minutes the dry mix was formed into a slurry with water in proportion 1 8 parts water per 1 0 parts dry mix and the slurry was mixed for 8 minutes water temperature was 80 u00b0 85 u00b0 f the slurry was poured against a polyurethane pattern of a computer component face and allowed to set thus forming a two part mold the mold was calcined at 600 u00b0 f for 16 hours whereupon a permeability of about 1 or higher resulted using the standard permmeter test while the mold was still hot from calcining the mold cavity was pressurized with a gas mixture containing about 4 volume percent of sf 6 in carbon dioxide for about 3 minutes at a supply pressure of 25 psig and flow rate of 30 scfh molten magnesium of commercial purity was then poured into the hot mold.