Nonferrous alloys contain no appreciable amounts of iron. The first alloys, bronze and brass, were used for thousands of years, along with lead alloys, pewter and others—but these were all made from metals that were fairly non-reactive and could be smelted over open flames. In the 18th century, Antoine Lavoisier helped to establish the oxygen theory of combustion, displacing the defunct phlogiston theory that had ruled since the late Middle Ages. The oxygen theory helped correctly explain the phenomenon of things like oxidation of metals (i.e., rust) and how rocky ores transform into metals when heated. Lavoisier predicted that many of the earths, salts, and alkalis—for example in alum, a salt used since antiquity—contained metallic bases that were too reactive to oxygen to smelt by the usual methods. His work eventually led to the periodic table of elements, which helped confirm the existence of these "missing metals."