1893

Thermite

Hans Goldschmidt (1861–1923)

As mentioned in the entries on the Hall-Héroult process and stainless steel, aluminum is an odd metal. Thermodynamically, it is far happier as an oxide. This (stated more scientifically) means that the energy state of a mixture of iron oxide (rust) and elemental aluminum, for example, is much higher than that of a mixture of aluminum oxide and iron. If you could just start a reaction between the elemental aluminum and the iron oxide, you’d liberate a lot of heat along the way.

And that’s thermite. Nothing more, in its original recipe, than powdered iron oxide mixed with powdered aluminum, but they will not start reacting without the addition of an outside energy source. A fuse of burning magnesium metal is the standard, but any sufficiently hot flame (greater than 4,000 degrees Fahrenheit, 2,200 degrees Celsius) will do. The effect is dramatic and dangerous, and once the reaction starts, nothing will stop it until it goes to completion. Aluminum has a relatively low melting point (1,220 degrees Fahrenheit, 660 degrees Celsius), letting it react very quickly as it soaks into the iron oxide, but it has a relatively high boiling point for such a reactive metal, so it doesn’t evaporate away. Enough heat is generated to produce nearly white-hot liquid iron at temperatures up to 4,500 degrees Fahrenheit (2,500 degrees Celsius), and the aluminum oxide product is so light that it tends to float on top of it. You can also run the thermite reaction with metal oxides other than iron (copper, manganese, chromium), because the energy payoff from aluminum going down to aluminum oxide is enough to drive a wide range of reactions.

German chemist Hans Goldschmidt discovered the thermite reaction in 1893 when he was trying to find a new way to refine metals. It can do that, albeit uneconomically and violently, but thermite’s real uses have to do with all that heat. A portable source of molten iron (initiated on demand with added oxygen) turned out to be very useful for welding steel rails, for example (or cutting them, if you’re in wartime sabotage mode). It’s instant liquid fire.

Welding railroad tracks with thermite.