Most iron alloys are steels, with carbon as a major alloying element. The ductility of steel varies depending on the alloying constituents. AA 6063 is an aluminium alloy, with magnesium and silicon as the alloying elements. Corrosion and heat resistance makes iridium an important alloying agent. A further steel, HY-130, also includes vanadium as an alloying element. An increase in alloying agents or carbon content causes an increase in retained austenite. They are also used as alloying additives for enhancing the properties of other metals. Some of the main alloying elements are optionally listed after the alloy names. It also measures the content of the other alloying metals present. Tungsten is one of the oldest elements used for alloying steel. This was stabilized at room temperature by alloying it with gallium. Oxide may also be grown with impurities (alloying or "doping"). Thorium as an alloying compound is used when electric arcs have to be established. Aluminium is the most common alloying element. In the Middle East, people began alloying copper with zinc to form brass.