 Another substance concept from roughly the same period is phlogiston, which served as the basis for 18th century theories of processes that came to be called oxidation and reduction. Georg Ernst Stahl (1660–1734) introduced the theory, drawing on older theoretical ideas. Alchemists thought that metals lose the mercury principle under calcination and that when substances are converted to slag, rust, or ash by heating, they lose the sulphur principle. Johann Joackim Becher (1635–82) modified these ideas at the end of the 17th century, arguing that the calcination of metals is a kind of combustion involving the loss of what he called the principle of flammability. Stahl subsequently renamed this principle ‘phlogiston’ and further modified the theory, maintaining that phlogiston could be transferred from one substance to another in chemical reactions, but that it could never be isolated.