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Calcination (combustion)

Term denoting the alchemical-chemical operation of exposing a solid substance—generally metallic or mineral—to the action of fire with the aim of producing a change in quality. Also described different operations through which fire released volatile principles from solid substances. Calcination involved a destruction of some properties of substances that was attributed to the loss of one or more essential components. The opposite of calcination was reduction or revivification, i.e., the restoration of the substance to its initial state. According to the theory of Georg Ernest Stahl (1660-1734), the metal, thanks to calcination achieved by fire, lost an earthly volatile substance, called phlogiston, which was the source of metallic qualities. In reduction, the phlogiston was returned to the lime, restoring it to its original metallic form. Carbon was used for reduction, as it was supposed to be rich in phlogiston. Stahl's model conflicted with experimental measurements, since a loss of phlogiston was supposed to produce an increase in weight, while a recovery of phlogiston was supposed to cause a loss in weight. Antoine-Laurent Lavoisier demonstrated the non-existence of phlogiston; he showed that calcination entailed a combination of the metal with oxygen, whereas reduction freed oxygen previously combined with the substance. Calcination was thus redefined as a combustion process involving a chemical reaction of oxygen.