Calx is a residual substance, sometimes in the form of a fine powder, that is left when a metal or mineral combusts or is calcinated due to heat.

Calx, especially of a metal, is now known as an oxide. According to the obsolete phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in the process of combustion.

"Calx" is also sometimes used in older texts on artist's techniques to mean calcium oxide.

Etymology

Calx is the Latin for heel: the tarsus bone. Calx is also the Latin base for lime, calcium and to tread.

2008, Oxford Pocket Dictionary, Oxford University Press: Oxford, UK..

In Popular Culture

• UK Electronic music artist, Aphex Twin (Richard David James) named some of his tracks after different coloured Calx (green, yellow and blue).