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Hans Goldschmidt and the Invention of Thermite

Re-create the incendiary reaction that welded the world’s modern railways.

Bremen

“A blast furnace that fits in a vest pocket.”

That’s what Wilhelm Ostwald, a Nobel Prize-winning chemist, called thermite, which is a mix of two common chemicals — iron oxide, better known as rust, and powdered aluminum. When combined and ignited, the stuff burns hot enough to melt iron. One of the most powerful chemical processes used in industry, the thermite reaction had a major role in the building of railroads.

Through the 1920s, railroads were built by joining tracks using steel connectors, called fishplates, and several thick metal bolts. However, mechanically joined tracks made an irritating “clack-clack” sound as the trains rode over them. More importantly, bolted connections become loose, requiring a great deal of ongoing and expensive maintenance. Executives of the day were actively searching for a way to reduce the costs of maintaining the thousands of miles of track they owned.

In 1893, Dr. Hans Goldschmidt, a German chemist, accidentally developed the process for welding thick sections of steel together in the field. While searching for a method of purifying metal ores in his laboratory in Berlin, he discovered that a mixture of iron oxide and aluminum would generate 3000°C heat, which was more than hot enough to weld steel track. He quickly switched his attention to refining this process, which he named thermite welding. The process was first used to weld streetcar track in Essen, Germany, and within a few years thermite welding was being used nearly everywhere there were railroad tracks to be joined.

Thermite is still commonly used to repair existing track and in some cases, build new track (makezine.com/go/thermite-welding). Additionally, the military frequently uses thermite in the form of hand grenades to disable captured weapons, such as artillery pieces and truck engines.

Thermit_1988

The Chemistry of Thermite

One aspect that makes thermite so interesting is the incredible simplicity of the chemical reaction:

Fe2O3  + 2 Al → 2 Fe + Al2O3 + lots and lots of heat

Iron oxide and powdered aluminum are stable at room temperature, and even if you mix purified reagents together, they will just sit there as a mound of inert gray powder. But if you can initiate the chemical reaction with an extremely hot flame (hotter than a propane torch) the stuff burns wildly, in an intense exothermic reaction hot enough to burn through dirt.

When the thermite reaction occurs, oxygen atoms are ripped out of the iron oxide, which then becomes pure metallic iron. It takes a great deal of energy to accomplish this as the oxygen is bound tightly within the rust molecules. The energy comes from aluminum, a powerful reducing agent that provides enough energy to heat the iron far past its melting point — enough to spew out sprays of spark and melt adjacent hunks of iron.

Making and using thermite is not normally a project suitable for amateurs, but this project is safe enough for a junior high science student to undertake with supervision.

Related

Steps

Step #1: Prep the iron.

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Hans Goldschmidt and the Invention of Thermite
  • Clamp the threaded rod portion of the hitch ball into a vice. Using an angle grinder (faster, easier) or sandpaper (slower, harder) remove the rust resistant plating. The ball surface doesn’t have to be particularly smooth, but all of the plating should be removed.
  • CAUTION: Angle grinders throw out a lot of hot metal, so wear eye protection, leather gloves, and a face shield.

Step #2: Bring on the rust.

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Hans Goldschmidt and the Invention of ThermiteHans Goldschmidt and the Invention of ThermiteHans Goldschmidt and the Invention of ThermiteHans Goldschmidt and the Invention of Thermite
  • Immerse the balls in salt water for several days and allow them to air dry in a dark, warm spot.
  • OPTIONAL: You can try to jump-start the oxidation process using chemical means. Don rubber gloves and eye protection, then paint a thin coat of acid on the iron.

Step #3:

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Hans Goldschmidt and the Invention of Thermite
  • Let the acid evaporate, then apply the hydrogen peroxide solution to the surface with a brush or rag. The iron will begin to oxidize almost immediately, but this coat of rust is too thin to enable the reaction. Continue to apply the hydrogen peroxide to the iron. A hot air gun can help speed the process — just don’t touch the metal until it has cooled.
  • NOTE: I attempted several methods for speeding up the rusting process (electrolysis, oxygen rich environments, and so on) with middling success. In the end, the most successful method was simply waiting a week for the rust to build up.

Step #4: Apply the foil.

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Hans Goldschmidt and the Invention of Thermite

When the balls are thoroughly coated in rust, wrap one in heavy-duty aluminum foil, taking care to make it as wrinkle-free and smooth as possible.

Step #5: Create the reaction.

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Hans Goldschmidt and the Invention of Thermite
  • Don heavy gloves. Grasp the threaded rod of the rusted ball in one hand, the aluminum foil-wrapped ball in the other.
  • Carefully but forcefully, bring the unwrapped, rust covered ball down smartly on the aluminum foiled-wrapped ball. Glancing blows produce extraordinary sparks and a loud, firecracker-like snap. If you examine the balls after striking, you will see that the aluminum foil has actually welded to the iron.
  • NOTE: Watch your fingers when you bring the balls together!
  • To get a fresh surface of iron oxide, rotate the rusted iron ball after each strike. When you strike, aim for the areas with the thickest coating of rust.
  • Producing sparks may seem a bit tricky at first, but once you get the hang of it, you can put on quite a show, especially if you dim the lights!
William Gurstelle

William Gurstelle

William Gurstelle is a contributing editor of Make: magazine. His new book, Defending Your Castle: Build Catapults, Crossbows, Moats and More is now available.


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