designed for the casting on hand. This sand will be built up around the pattern to a thickness of . The sand that fills in around the facing sand is called backing sand. This sand is simply silica sand with only a small amount of binder and no special additives. Types of base sands Base sand is the type used to make the mold or core without any binder. Because it does not have a binder it will not bond together and is not usable in this state. Silica sand Silica (SiO2) sand is the sand found on a beach and is also the most commonly used sand. It is either made by crushing sandstone or taken from natural occurring locations, such as beaches and river beds. The fusion point of pure silica is , however the sands used have a lower melting point due to impurities. For high melting point casting, such as steels, a minimum of 98% pure silica sand must be used; however for lower melting point metals, such as cast iron and non-ferrous metals, a lower purity sand can be used (between 94 and 98% pure). Silica sand is the most commonly used sand because of its great abundance, and, thus, low cost (therein being its greatest advantage). Its disadvantages are high thermal expansion, which can cause casting defects with high melting point metals, and low thermal conductivity, which can lead to unsound casting. It also cannot be used with certain basic metals because it will chemically interact with the metal, forming surface defects. Finally, it releases silica particulates during the pour, risking silicosis in foundry workers. Olivine sand Olivine is a mixture of orthosilicates of iron and magnesium from the mineral dunite. Its main advantage is that it is free from silica, therefore it can be used with basic metals, such as manganese steels. Other advantages include a low thermal expansion, high thermal conductivity, and high fusion point. Finally, it is safer to use than silica, therefore it is popular in Europe. Chromite sand Chromite sand is a solid solution of spinels. Its advantages are a low percentage of silica, a very high fusion point (), and a very high thermal conductivity. Its disadvantage is its costliness, therefore it is only used with expensive alloy steel casting and to make cores. Zircon sand Zircon sand is a compound of approximately two-thirds zirconium oxide (Zr2O) and