One of the most powerful chemical processes used in industry is called the thermite reaction. Thermite is a mix of two common chemicals—iron oxide, better known as rust, and powdered aluminum. When these chemicals are combined and ignited, the mixture burns hot enough to melt iron. Wilhelm Ostwald, a Nobel Prize–winning chemist, called the reaction “a blast furnace that fits in a vest pocket” because you can use it to produce overheated liquid steel within a fraction of a second wherever and whenever you have the need.

Goldschmidt Discovers Thermite

Dr. Hans Goldschmidt, a 19th-century German chemist and a student of Robert Bunsen (the man who invented the eponymous Bunsen burner), discovered the powerful thermite reaction in his laboratory. An able chemist, Goldschmidt was given credit for several important contributions to modern chemistry. His most important discovery was almost certainly thermite, which was applied to building railroads in several ways.

Through the 1920s, work gangs built railroads by laying 39-foot-long rails end to end, and then joining the tracks using steel connectors called fishplates and several thick metal bolts (see Figure 6-1). However, mechanically joined tracks made an irritating “clack-clack-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 actively searched for a way to reduce the cost of maintaining the thousands of miles of track they owned.

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

Thermite is still commonly used to repair existing track and, in some cases, build new track. In modern thermite welding, workers butt two pieces of track together, then cover the joint with a heatproof fixture called a joint mold. A worker pours thermite powder into a funnel-shaped crucible atop the track. A pan positioned next to the mold catches slag or waste products from the reaction. A combination of chemicals is inserted into the crucible to start the reaction, and then everybody stands back as the thermite does its thing (see Figure 6-2). When the welding is complete, the mold is removed and any excess weld is knocked off with a hammer.

In addition to its uses in welding metals together, thermite has another use. Because of its ability to melt steel quickly, the military frequently uses it to disable captured weapons such as artillery pieces and trucks by melting key metal parts such as cannon barrels and engine blocks.

Producing the Thermite Reaction

As you might have surmised from the earlier description of a thermite reaction, making and using thermite is not normally a project suitable for amateurs. But in this chapter, we will explore a way to make it in a manner that is safe enough for a junior high science student to undertake with supervision.

The general process is this: 1) form an iron oxide surface layer on two large iron balls; 2) cover one of the balls with a layer of aluminum foil; and 3) clack the two balls together briskly to initiate the chemical reaction between the iron oxide and aluminum.

To run your thermite reaction, follow these steps:

1. Clamp the threaded rod portion of one of your hitch balls into a vise.
2. Using an angle grinder (faster, easier), a hand file (see Figure 6-3), or sandpaper (much slower and harder), remove the rust-resistant plating that covers the iron.

   

   The ball surface doesn’t have to be particularly smooth when you’re done, but all of the plating should be removed.

3. Repeat steps 1 and 2 for the second ball.
4. Rust the surface of the iron balls by immersing them in saltwater for several days (see Figure 6-4); then allow them to air dry in a dark, warm spot.

   

   (Steps 5–8 are optional.) To speed up the oxidation of iron, you can attempt to jump-start the rusting process using chemical means.

5. Don rubber gloves and eye protection; then paint a thin coat of acid on the iron to clean it (see Figure 6-5).

   

6. Let the acid evaporate and then apply the hydrogen peroxide solution to the surface with a brush or rag.

   Almost immediately, the iron will begin to oxidize and a thin coat of rust will appear.

7. Unfortunately, the rust layer is too thin to enable the reaction. Since the goal is to obtain a thick, even layer of rust on the ball, continue to apply the hydrogen peroxide to the iron.
8. You can speed up the oxidation process by applying heat from a hot air gun (see Figure 6-6). If you do this, don’t touch the metal until it has cooled.

   

9. When the balls are thoroughly coated in rust, wrap one iron ball in heavy-duty aluminum foil (see Figure 6-7).

   

10. Don heavy gloves. Grasp the uncovered rusted iron ball by the threaded rod in one hand and hold the aluminum foil-wrapped ball, also by the threaded rod, in your other hand.
11. 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 you strike them together, you will see that the aluminum foil has actually welded to the iron.

12. 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 be 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 (see Figure 6-8)!

    

How the Thermite Reaction Works

You can’t just lay one ball atop the other and expect to get the spark and crackle of the thermite reaction. Besides the iron oxide and aluminum, you must add energy by banging the balls together in order to start the reaction.

In 1889, Svante Arrhenius of Sweden first used the term activation energy to describe the minimum energy that must be added to a collection of chemicals in order to start a chemical reaction. When using a wooden match, for example, the friction of the match sweeping across a rough surface provides the activation energy needed to start the reaction between the phosphorus and sulfur, which causes the match to burst into flames.

The thermite reaction, like a match head, requires some energy to get it started (see Figure 6-9). It’s like investing in the stock market: you invest a little of something in order to get a bigger something later.