The聽place was Edinburgh, Scotland. The occasion, the Edinburgh Science Festival. There were a number of captivating presentations, but my biggest thrill came from looking out the hotel window. A light rail track was being constructed just outside and the workers were busy welding. My eyes popped when I saw what they were doing. I was looking at a live thermite reaction! I had talked about this reaction in class on numerous occasions and marvelled on it in videos, but had always deemed it too dangerous to perform.
A chemical reaction that produces heat is said to be 鈥渆xothermic.鈥� The most common example would be the combustion of a fuel. Light a candle and you can feel the heat that is produced. The hottest part of a flame, where the colour is a light blue, can reach a temperature of about 1400 degrees Celsius. But that is a low temperature compared to聽the 2500 degrees produced by the 鈥渢hermite鈥� reaction between aluminum and iron oxide. Essentially, this reaction involves the transfer of oxygen from the iron oxide to aluminum to yield aluminum oxide and metallic iron. At this high temperature, the iron is in its molten form and sets fire to any combustible material in its path, making the thermite reaction ideal for use not only in welding, but also in incendiary bombs and grenades.
Back in 1893, German chemist Hans Goldschmidt was looking for a way to produce pure metals from their ores. The classic method for extracting iron relies on heating iron oxide ore with carbon. The carbon is converted to carbon dioxide as it strips oxygen from the iron, leaving behind metallic iron. Some unreacted carbon, however, tends to contaminate the iron. Goldschmidt was looking for a way to produce iron without the use of carbon and hit upon the reaction of iron oxide with aluminum. He was impressed by the remarkable amount of heat produced and suggested that the reaction he had discovered could be used for welding. In 1899, the thermite reaction was put to a commercial use for the first time, welding tram tracks in the city of Essen.
It didn鈥檛 take long for the military to realize the potential of this extreme exothermic reaction in warfare. In 1915, the Germans terrorized England by using Zeppelins to drop incendiary bombs based on the thermite reaction. By the Second World War, the battle was on not only between Allied and German armed forces, but also between their scientists and engineers who sought to produce more effective incendiary devices. The Germans came up with the 鈥淓lektron鈥� bomb, named after Elektron, an alloy composed of 86聽per cent聽magnesium, 13聽per cent聽aluminum and 1聽per cent聽copper that was used for the casing of the bomb.
This alloy burns with a very hot flame, but requires a high temperature for ignition. The thermite reaction was up to the task. When an Elektron bomb hit the ground, a small percussion charge of gunpowder ignited a priming mixture of finely powdered magnesium and barium peroxide. This reaction produced the heat needed to ignite the thermite mix of aluminum and iron oxide, which in turn ignited the highly combustible casing. The Allies developed similar types of bombs resulting in the most destructive air raid in history, which was not Hiroshima or Nagasaki, but the firebomb raid on Tokyo in March 1945. An Allied bombing of Dresden the same year with incendiary bombs virtually destroyed the whole city. During the Second World War, the Allies dropped some 30 million 4-pound thermite bombs on Germany and another 10 million on Japan.
Thermite hand grenades were also used during the war to disable artillery pieces without the need for an explosive charge, very useful when silence was necessary to an operation. This involved inserting a thermite grenade into the breech of a weapon and then quickly closing it. The great heat produced by the thermite reaction welded the breech shut and made loading the weapon impossible. Alternatively, a thermite grenade was discharged inside the barrel of an artillery piece making it useless.
During the Vietnam war, thermite grenades found a different use. From the start of hostilities, putting a crimp into the enemy鈥檚 food supply was part of the U.S. military strategy. Since rice was a staple for the Viet Cong, destroying rice paddies was a primary goal. At first, attempts were made to blow up rice stocks and destroy paddies with hand grenades and mortars, but this proved to be maddeningly difficult. The next idea was to burn the rice paddies with thermite grenades. All this did was scatter the rice grains, which could then still be harvested. Another approach was needed.
Enter 鈥淎gent Blue,鈥� an arsenic-based herbicide, unrelated chemically to the more infamous聽Agent Orange. Agent Blue affects plants by causing them to dry out, and as rice is highly dependent on water, spraying Agent Blue on rice paddies can destroy an entire field and leave it unsuitable for further planting. The U.S. used some 20 million gallons of Agent Blue during the Vietnam war, destroying thousands of acres of agricultural fields and defoliating wooded areas that the Viet Cong used to ambush American troops.
Recently, the thermite reaction made the news in a different context. Conspiracy theorists purport that it was thermite explosives planted inside the World Trade Center that brought down the twin towers in a CIA coordinated plot. They also maintain that the moon landing was faked and that the U.S. government is hiding the bodies of aliens. Some also claim that the rise of Donald Trump was engineered by a Democratic conspiracy and that on the verge of being elected he will announce 鈥渇ooled you.鈥� Wouldn鈥檛 that be something? It would trump the thermite reaction for heat generated.
