GREEN SAND

'Green Sand' is an aggregate of sand, bentonite clay, pulverized coal and water. Its principal use is in making molds for the casting of metals. The largest portion of the aggregate is always sand. There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to degas. The coal, typically referred to in foundries as sea-coal, which is present at a ratio of less than 5%, partially combusts in the presence of the molten metal leading to offgassing of organic vapors.
Sand casting is one of the earliest forms of casting practiced due to the simplicity of materials involved. It still remains one of the cheapest ways to cast metals because of that same simplicity. Other methods of casting, such as those using shell molds boast higher quality of surface finish, but higher cost.
Green sand (and other casting sands) is usually housed in what casters refer to as flasks which are nothing other than boxes without a bottom or lid. The box is split into two halves which are stacked together in use. The halves are referred to as the top (cope) and bottom (drag) flask respectively.
Green sand is not green in color, but "green" in the sense that it is used in a wet state. According to the Cast Metals Federation website, an alternative casting method is to heat-dry the molded sand before pouring the molten metal. This dry sand casting process results in a more rigid mold better suited to heavier castings.

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Other meaning

Other meaning

Green sand also refers to the sand on beaches that are covered with olivine crystals (which are green) such as Pu'u Mahana, Green Sand Beach in Hawaii.