Some of these calxes form metals when heated with charcoal. With only a few exceptions, the calx is found in nature, not the metal. These observations led Stahl to the following conclusions. Wood and charcoal are particularly rich in phlogiston because they leave very little ash when they burn. Because they are found in nature, calxes must be simpler than metals.
Metals form a calx by giving off phlogiston.
Because charcoal is rich in phlogiston, heating calxes in the presence of charcoal sometimes produces metals. This model was remarkably successful. It explained why metals have similar properties they all contained phlogiston. It explained the relationship between metals and their calxes they were related by the gain or loss of phlogiston. It even explained why a candle goes out when placed in a bell jar the air eventually becomes saturated with phlogiston.