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.