Metals were thought to be compounds of the metal's calx and phlogiston. For example, sulphur was thought to be a compound of sulphuric acid and phlogiston and phosphorus was thought to be a compound between phosphoric acid and phlogiston. Substances such as carbon which left little or no ash after burning were taken to be rich in phlogiston. And 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.