The most common composition is iron thermite. The oxidizer used is usually either iron(III) oxide or iron(II,III) oxide. The former produces more heat. The latter is easier to ignite, likely due to the crystal structure of the oxide. Addition of copper or manganese oxides can significantly improve the ease of ignition.
The density of prepared thermite is often as low as 0.7 g/cm. This, in turn, results in relatively poor energy density (about 3 kJ/cm), rapid burn times, and spray of molten iron due to the expansion of trapped air. Thermite can be pressed to densities as high as 4.9 g/cm (almost 16 kJ/cm) with slow burning speeds (about 1 cm/s). Pressed thermite has higher melting power, i.e. it can melt a steel cup where a low-density thermite would fail. Iron thermite with or without additives can be pressed into cutting devices that have heat-resistant casing and a nozzle.
Oxygen balanced iron thermite 2Al + FeO has theoretical maximum density of 4.175 g/cm an adiabatic burn temperature of 3135 K or 2862 °C or 5183 °F (with phase transitions included, limited by iron, which boils at 3135 K), the aluminium oxide is (briefly) molten and the produced iron is mostly liquid with part of it being in gaseous form - 78.4 g of iron vapor per kg of thermite are produced. The energy content is 945.4 cal/g (3 956 J/g). The energy density is 16,516 J/cm.
The original mixture, as invented, used iron oxide in the form of mill scale. The composition was very difficult to ignite.