Molding sand, also known as foundry sand, is a sand that when moistened and compressed or oiled or heated tends to pack well and hold its shape. It is used in the process of sand casting for preparing the mould cavity.
Green sand is an aggregate of sand, bentonite clay, pulverized coal and water. Its principal use is in making molds for metal casting. The largest portion of the aggregate is always sand, which can be either silica or olivine. 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 surface 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 (like other casting sands) is usually housed in what casters refer to as casting 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.
Not all Green sand is green in color. But considered "green" as in the sense that it is used in a wet state (akin to green wood). 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.