Calx was produced by roasting a metal, most often in a shallow dish or crucible over sustained heat with free access to air, until the shiny, malleable metal transformed into a dull, often colored, brittle powder that had lost the defining metallic properties, luster, malleability and electrical or thermal conductivity as later chemistry would describe it, that had characterized the starting material. Every one of the seven classical metals could be calcined, producing calxes of quite different color and character, lead yielding the yellow litharge or, roasted further, the red minium, copper yielding a black or reddish calx, iron yielding a reddish-brown calx related to rust, and so on, a diversity the alchemical texts were well aware of even while grouping all these different products under the single umbrella term calx and the single umbrella operation calcination.
Calcination held real theoretical importance within the Great Work as one of the classical sequence of operations a substance was believed to pass through on the way to higher refinement, and some alchemical writers treated the production of a metal's calx as a necessary first step before that metal could be recombined, reduced, or otherwise processed toward a more perfected state. The calx was also, separately from its role in transmutational theory, a genuinely useful intermediate product in its own right for many practical purposes, pigments and glazes among them, entirely independent of whether the alchemist working with it held any transmutational ambitions at all.