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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.

Calx is a Latin word that is also the root for Calcium.