Thermit, or Thermite, a mixture of powdered aluminum and iron oxide that is used mainly for welding iron or steel; Thermit is a trade name. Thermit consists of about 25 per cent aluminum and 75 per cent iron oxide by weight.

Thermit welding is used primarily for repairing large objects such as ships' propellers. In the welding process, a sand mold is placed around the parts to be joined and the Thermit is placed in a crucible directly above. A fuse of magnesium wire is used to ignite the Thermit. A violent chemical reaction immediately takes place, the aluminum combining with the oxygen of the iron oxide to form aluminum oxide and iron. This diemical reaction produces a great amount of heat (the temperature rises to about 5,000 F. [2,760 C.] within a few seconds), so that the iron is melted. The molten iron is poured into the sand mold, where it melts the edges of the objects to be joined, causing them to flow together. Thermit is also used in incendiary bombs.

The method of igniting Thermit by a magnesium-wire fuse was developed in the 1890's by the German chemist Hans Goldschmidt. The chemical reaction that takes place in Thermit welding also forms the basis for the Goldschmidt process, in which powdered aluminum is used to extract metals with high melting points (chromium, for example) from their oxides.