Traditionally the internal passages were represented with a sand and water-based adhesive core set in the major mold (also a sand and water-adhesive mold). Several passages between the outer major mold and inner mold were approriately placed. After the molten metal (iron and later aluminum) was poured in the mold casing and allowed to solidify, the major mold casing was broken and the outer mold broken/rinsed away. The passages alluded to earlier are then broken away and the resultant "holes" in the block are then rinsed with high pressure water to break up and flush the inner sand mold out of the block or head casting. The resulting cavities would be the various cooling and oil gallaries. The holes left over from the previously alluded to connecting passages are then capped with "core" plugs. In sub-zero conditions if the coolant freezes, with any luck the core plugs will pop out before the block or head cracks from the pressure of ice (water expands as it freezes). Hence the misnomer, "freeze plug", came into common usage. As mentioned previously, another casting method using wax has been in use for centuries for casting intricate shapes, too. It involves carving the shape in wax and placing the wax model in a casing and pouring plaster or other heat tolerant material around it in the case. Once set up, molten metal is poured over the wax, which instantly burns/melts away allowing the metal to form the same shape in the resulting cavity. The outer casing is broken away exposing a newly cast metal item identical to the original wax core. Great for jewelry, and this is how rings are made, but the wax was permanently lost in the process. G.M. looked at lost-wax casting and developed a method whereby styrofoam cores were used instead of wax. The weight savings were considerable when considering the weight of a cylinder head-sized chunk of parrafin wax. In the case of something like a cylinder head or block, the internal passages would need to be sand-filled as in conventional casting described above located between bonded halves of styrofoam since the internal passage core would need to be heat tolerant during the casting process. It's easy to recognize a lost foam casting. The "cells" are clearly visible on the surface of the metal just as they were on the original styrofoam core. Smooth castings formed in high temperature steel dies or lost foam castings require minimal machining, but sand cast aluminum blocks and heads are still made, too.