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4000 BC and the first metal to be purposefully alloyed with another metal, tin, to create bronze, c. The commonly encountered compounds are copper(II) salts, which often impart blue or green colors to such minerals as azurite, malachite, and turquoise, and have been used widely and historically as pigments.
High-purity scrap copper is melted in a furnace and then reduced and cast into billets and ingots; lower-purity scrap is refined by electroplating in a bath of sulfuric acid.
Pure copper is orange-red and acquires a reddish tarnish when exposed to air.
The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells corresponds to orange light.
It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity.
A freshly exposed surface of pure copper has a reddish-orange color.