But most metals occurred in the form of oxides, or “earths.” Earths, he said, were sometimes called calxes, and these ores were known to be insoluble, incombustible, infusible, and to be, as one eighteenth-century chemist wrote, “destitute of metallic splendour.” And yet, it was realized, they were very close to metals and could indeed be converted into metals if heated with charcoal; while pure metals became calxes if heated in air.
