Production methods for the elements less frequently recognised as metalloids involve natural processing, electrolytic or chemical reduction, or irradiation. Carbon (as graphite) occurs naturally and is extracted by crushing the parent rock and floating the lighter graphite to the surface. Aluminium is extracted by dissolving its oxide Al 2 O 3 in molten cryolite Na 3 AlF 6 and then by high temperature electrolytic reduction. Selenium is produced by roasting the coinage metal selenides X 2 Se (X = Cu, Ag, Au) with soda ash to give the selenite: X 2 Se + O 2 + Na 2 CO 3 → Na 2 SeO 3 + 2 X + CO 2 ; the selenide is neutralized by sulfuric acid H 2 SO 4 to give selenous acid H 2 SeO 3 ; this is reduced by bubbling with SO 2 to yield elemental selenium. Polonium and astatine are produced in minute quantities by irradiating bismuth.