Despite a fall in popularity among machine shops, cast iron remains the most popular material for master surfaces (different use from a surface plates) 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, including by the Whitworth three plate method, without the need to send them out to be reconditioned. Despite its high stability, cast iron remains unsuitable for use as a normal surface plate in high-tolerance production applications because of thermal expansion. The nature and use of a master surface, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable.
