Copper was smelted from a range of ores, chiefly the carbonate malachite and the closely related azurite, in furnaces that reduced the ore's metal content with charcoal, a metallurgy old enough to define an entire archaeological period, the Copper Age, that in most regions preceded the later adoption of bronze and then iron. Copper is comparatively soft and ductile, easily worked cold or hot, and it takes a bright reddish luster that darkens and eventually turns green on long exposure to air and moisture, forming the same basic copper carbonate patina discussed under verdigris and malachite elsewhere in this atlas. Alchemically, copper was studied closely alongside its own vitriol, since dissolving copper in oil of vitriol or exposing it to acidic vapor yields blue vitriol directly, a transformation between metal and salt that alchemists watched as a visible demonstration of a metal taking on, and later potentially recovering from, a dissolved, salt form.
Beyond the laboratory, copper's practical importance as coinage metal, tool metal, and later as the essential ingredient of bronze and brass alloys made it one of the most economically consequential of the seven classical metals throughout the whole pre-modern period, its alchemical study never separate for long from its very practical, everyday uses across metalworking trades that had nothing directly to do with transmutation.