Cast iron
Cast iron is made from pig iron, which is the product of smelting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants. Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively. If desired, other elements are then added to the melt before the final form is produced by casting.
Surface plate
Despite a fall in popularity among machine shops, cast iron remains the most popular material for surface masters (different usage from a surface plate) among laboratory metrologists, machine builders, gauge makers, and other high-accuracy industries that have a requirement for gauging flatness. Cast iron that has been properly cast is more dimensionally and geometrically stable over time than granite or ceramics, is more easily worked to a higher grade of flatness, and provides a better bearing surface to assist the creation of other master standards. These specialized surface plates are produced in sets of three, by the company that will be using them, so the plates may be regularly verified and refined without the need to send the plates out for external rework. Despite the very stable structure, cast iron remains unsuitable even in high tolerance production applications for use as a normal surface plate because of thermal expansion encountered with regular use as an inspection tool. The nature and use of a surface master, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable.