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.