The casting process involves the melting of the metal and its introduction into the shell cavity. The melting is much more involved than just making a lump of metal liquid. Proper chemistry must be maintained if the ending cast part is to have the physical and mechanical properties that are desired. Almost all commercially cast metals are relatively complex alloys. Absorption of atmospheric gases, reaction of the alloying elements with the atmosphere and each other, the ceramic furnace lining and the ceramic shell must be taken into consideration. Process variables include metal superheat temperatures, pour temperature, shell temperature when poured, pouring rate, support of shells during pouring, cooling of the poured shell, etc.