steel the attached mold elements 5 6 7 and 8 of the left hand and right hand mold case are made from 100 fused silica material having a coefficient of thermal conductivity of 0 6 w m u00b0 c which is obviously quite low diluted refractory mixtures containing fused silica will have a higher coefficient of thermal conductivity of up to 10 w m u00b0 c as set forth above left and right hand mold elements 9 10 11 and 12 surrounding the mid portion of the piston head 22 are made from a refractory composition containing 70 fused silica fig3 the upper mold element 13 forms a portion of the upper surface of the piston head and is made from 100 fused silica refractory material hence the mold of the present invention is made up of elements comprising steel 100 fused silica and diluted fused silica refractory compositions each possessing a desired thermal conductivity depending upon the quantity of heat to be evacuated from the casting at each selected location it will be observed by those skilled in the art that the thermal conductivity of the mold elements will be lower in locations where the cross section of the casting is thin conversely in thick sections the thermal conductivity of the mold elements is made selectively higher so as to evacuate the heat more rapidly and solidify the metal in such thick sections thus by selectively providing the lowest thermal conductivity in the mold elements forming the.