Copper: a chemical element with symbol Cu and atomic number 29; a soft, ductile and malleable reddish metal; melting point 1085°C, boiling point 2927°C; very high thermal and electrical conductivity. Together with silver and gold, copper belongs to group 11 of the periodic table. Copper and copper alloys (such as bronze and brass) have been known since at least 10 000 years. In Roman times, the most important copper ores were the ones mined in Cyprus; the Latin name cyprium – later cuprum – bears witness to this. In mythology and alchemy, copper is associated with Aphrodite/Venus. As a trace element in humans, copper is mainly found in liver, muscle and bone. Copper salts produce a range of pigments, mainly within the blue-green part of the spectrum. In nature, copper salt pigments give colour to minerals such as azurite, malachite and turquoise.
Aluminium disc; photo credit Theodore W. Gray.
Aluminium: a chemical element with symbol Al and atomic number 13; a silvery white, soft, ductile and malleable metal; melting point 660°C, boiling point 2470°C; does not easily ignite; nonmagnetic. Aluminium is the third most abundant of all elements in the earth’s crust (after oxygen and silicon). In spite of its common appearance, it does not seem to have any biological function.
Aluminium is, together with copper, one of the chemical constituents of the mineral turquoise. With sulfur and sodium, it produces ultramarin blue and violet pigments found in nature. It is also a constituent in potassium alum, a compound known since around 2000 BC and widely useful for its astringent, blood coagulating, flocculant, fire retarding, preserving and purifying properties. Alum is also efficacious for fixing pigments, textile dyes and making ceramic glazes. The multiple uses of alum is mirrored in antique treatises and alchemical practices. Metallic aluminium wasn’t extracted until the 18th century.
Pure aluminium can be used for metallic paints and pigments, as it keeps its silvery reflectance even in finely powdered form.
Zinc: a chemical element with symbol Zn and atomic number 30; a bluish-white, lustruous metal, brittle at most temperatures; melting point 420°C, boiling point 908°C; it burns with a bluish-green flame, is moderately reactive and tarnishes quickly. Impure zinc together with copper was used to make the alloy called brass as early as 14th century BC. Later, alchemists burned zinc metal and collected the resulting zinc oxide, calling it lana philosophica (“philosopher’s wool”) or sometimes nix album because of its woollen or snow-like texture.
Zinc oxide is used as a white pigment.
Zinc sulfate produces another white pigment, lithopone, while zinc sulfide is used to make luminescent pigments.
Zinc chromate makes a yellow pigment, which however deteriorates into brownish.
Zinc is an essential trace element in animals and humans, functional in the cells of the brain, nerves, muscles, bones, kidney, liver, eyes and reproductive parts.