Make: Projects

Hans Goldschmidt and the Invention of Thermite

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


“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 ( Additionally, the military frequently uses thermite in the form of hand grenades to disable captured weapons, such as artillery pieces and truck engines.


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.


Step #1: Prep the iron.

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.

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:

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.

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.

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.

  • Serai

    The link to your book (in the biography) is down. Thanks for the tutorial!

    • alrui

      Maybe they pulled it? I dont see why but its nowhere to be found in the Maker book listings. Its a great book, I have it & is still available on Amazon. There is no recipe for Thermite in the book at least that I recall or see in the index:-(

  • Rudy R

    I see some humor in presenting safety concerns about an article for making thermite, but it might save someones hand. Particularly as this project is “safe enough for a junior high science student to undertake with supervision.”

    [b]Don’t wear gloves when using rotating power tools![/b] No loose clothing either. And contain long hair. Also, if it is too cold to work without gloves, it is too cold to work with power tools. As seen on TV, be sure to “read, understand, and follow all instructions which come with your power tools.”

    I have extensive experience with power tools, and have various scars to prove it. If a rotating tool grabs a glove, the hand will follow faster than you can see or react. Any number of my scars would be much worse if I had worn gloves during my lapses.

    Without gloves, you may catch hot chips, maybe even get scratches, may even get some pieces of grit embedded under your skin. If the rotating part kisses your skin (mistakes do happen), it will hurt, but you will probably heal ok.

    My prescription safety glasses are not safe enough for a grinder. In general, use (at minimum) safety glasses with side shields, better is well fitting impact-resistant goggles, or even better, a full-face shield. Refer to your owners manual for best practice with your tool. The danger here is not just the grit coming off the business end of the grinder, Those rotating discs can and do explode once in a while. Keep track of the plane the disc spins in, so pieces go in safer directions, and be aware of where the sparks are going. A spark in a pile of dust may smolder for hours before burning down a house. And wear closed-toed shoes; a grinder does not stop spinning just because it is dropped. Need to respect the tool, but don’t fear it. Arm yourself with knowledge, and let the sparks fly!

    With that said, a grinder is extremely efficient at removing metal. Doing this with sandpaper will take a long time. A temping alternative is to use a file. But chrome plating is tough, and may ruin a file.

    Thermite has it’s own oxygen source. While lighting it wet might be a problem (there is a solution in the Periodic Table of the Elements), don’t count on water to put it out once lit. No flammable clothes either, and watch out for hair. And pet fur. And dust under the bench in the garage…

    This looks like a great project. So go make the boom-boom, just be safe! And always have fun!

  • dbarak

    In addition to the safety considerations, there may also be legal issues to worry about. As I understand it, something being pyrotechnic means the oxidizer is incorporated with the fuel, at least as told to me by a motion picture pyrotechnician (things like propane, wood and gasoline don’t have oxidizers in any large quantity). Pyro usage, at least in California, requires a license. I’m know there are exceptions such as road flares, incendiary pest control devices, safety smoke devices and fireworks.

    In California, it’s only legal for an unlicensed person to purchase fireworks (which are pyrotechnics), and only those certified as “safe and sane,” between June 28 and July 6, and only from “established” (whatever that means in this case), California-licensed retailers, and there are restrictions on the storage of fireworks by the end-user. San Diego County is an exception – no private fireworks at all. Don’t ask me how I know, but I have a CalFire fire marshal to thank for looking out for me. ; )

    Anyway, I digressed a bit. Thermite is a pyrotechnic compound, so be careful of the legalities of making, storing and using it.

    And please, if I got anything wrong, correct me here, I won’t be offended.

    • alrui

      This is simply a demonstration project not a recipe to make vast quantities of Thermite though being a CA native I can understand your concerns, just another of the many reasons I left CA:-)

      • dbarak

        Sure, I understand that a rational person making rational amounts of thermite for rational uses is rational in a rational world, but the laws here aren’t necessarily rational. ; )

  • Dexter Wong

    I first heard of thermite in college chemistry when the instructor put together a thermite demonstration in a plexiglass partition equipped with a fume hood. It was spectacular, lots of sparks and red hot liquid pouring into a graphite crucible. Later I read that it was also used to weld rail joints.