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Green sand is a mixture of sand with bentonite clay, pulverized coal, and water used in metal casting. It is not necessarily green in color, but is called green because it is used while wet, like the "green wood" sometimes used by wood turners. Green sand is one of the most commonly used types of molding or foundry sand.
In sand casting, the molding sand is placed around a pattern, the shape to be cast, in two halves. The two halves of the mold are housed in flasks, which are basically boxes with no top or bottom. The top half of the mold is called the cope, while the bottom half is called the drag. A gating system is incorporated to allow the molten metal to be poured into the mold, and after the metal cools inside the mold, the sand can be broken away to reveal the molded metal shape.
Sand casting is preferred for its low cost, and the simplicity of its materials. Other casting methods produce smoother products, but are more expensive. Shell casting, for example, which uses a fine resin-covered sand, is more accurate than sand casting, and is typically used for small, high precision parts. The green sand used in sand casting leaves a rough surface on the molded part, though it may be polished smooth after the molding process.
Green sand is used for sand casting because it holds its shape well when wet, creating a solid mold. In all forms of green sand, sand is the main component, while there are varying amounts of clay and coal. The sand in green sand may be silica or olivine.
Silica is made of quartz and is the sand found in most inland and non-tropical coastal settings, such as beaches in the United States and Europe. Olivine is an abundant light green mineral, also known as peridot when it is gem quality. Olivine appears as sand on so-called green sand beaches, notably in Hawaii, where the olivine crystals have eroded from lava rocks. If olivine is used as the main component in molding sand, the green sand may actually be green in color.