The aluminium hydroxide is calcined to produce alumina. Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. The hydroxyaluminate or hydrated aluminate can be precipitated and then calcined to produce anhydrous aluminates. This precipitate is then thickened and centrifuged before being calcined to uranium oxide. Plaster of Paris is calcined gypsum (roasted gypsum), ground to a fine powder by milling. Carbon anodes are produced from calcined petroleum coke and coal tar pitch. Carbonate ores are also smelted with charcoal, but sometimes need to be calcined first. The carbonate is calcined in air to give a mixture of manganese(II) and manganese(IV) oxides. Such ostensibly simple procedures afford hydrated oxides that are calcined. This helps to concentrate the calcined gypsum at the surface. All kernels are precipitated from a sol-gel, then washed, dried and calcined. The powders are cooled, reground and calcined again. Montmorillonite can be calcined to produce arcillite, a porous material. Calcined petroleum coke (CPC) is the product from calcining petroleum coke. For manufacturing of the graphitized anodes, anthracite and petroleum coke are calcined and classified.