 For example, metals were thought to be compounds of the metal’s calx and phlogiston, sulphur was thought to be a compound of sulphuric acid and phlogiston, and phosphorus was thought to be a compound of phosphoric acid and phlogiston. Substances such as carbon which left little or no ash after burning were taken to be rich in phlogiston. The preparation of metals from their calxes with the aid of wood charcoal was understood as the transfer of phlogiston from carbon to the metal.