Calx

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.

Calx, examined through modern chemistry, corresponds in most cases to a metal oxide, the specific compound depending entirely on which metal was calcined: lead's calx is litharge (PbO) or, further oxidized, minium (Pb3O4), both covered as their own entry in this atlas, while other metals' calxes correspond to their own respective oxides, copper oxide, iron oxide, and so on, each a distinct compound with its own formula.

There is, deliberately, no single chemical-formula value that can be assigned to this entry, since calx was always a category term describing an operation's typical product rather than the name of one fixed substance, and treating it as if it named one specific compound would misrepresent how the alchemical sources themselves used the word. This entry exists in the table precisely because the operation and the category, calcination and its calx, were genuinely important and widely discussed alchemical concepts in their own right, even though no single modern formula can honestly stand in for the word. Readers looking for the specific chemistry behind any particular metal's calx should consult that metal's own entry in this atlas, litharge and minium for lead chief among them, where a real formula can be given with confidence.