Heat; the most essential part of any foundry. In building and exploring your backyard foundry, you might have heard of a nifty little material called thermite. Thermite is a substance near and dear to metalworkers as it holds the title of the hottest-burning man-made substance on Earth.
But what exactly is it? What is it used for? How can you apply it to the projects you’ve envisioned in your own foundry? This article will give you some of the basic, essential information every metal worker should know about thermite.
Important: Thermite is incredibly dangerous. This article is for informational purposes only. You should only consider making thermite if you’re an experienced metal caster and fully understand and accept the risks.
What is Thermite made of?
At its most basic component level, thermite is simply a powder mixture of aluminum and the oxide of a metal, typically iron. But it can have different compositions.
Metals such as magnesium, zinc, titanium, silicon, and boron have been used as fuels instead of aluminum. The latter is usually preferred because of its high boiling point and cheap availability.
Metals used as oxidizers include bismuth, chromium, manganese, copper, lead, boron, and silicon.
How Thermite Works
When thermite is ignited, some simple chemistry results in a very impressive amount of heat. However, the ignition part is not so simple. Thermite typically requires a temperature over 3,000°F just to initiate the reaction; so using a simple flame just won’t cut it.
The best way to ignite thermite is using what is called Thermite Ignition Mixture. It is easily ignited with a flame and subsequently burns hot enough to start the Thermite Reaction.
The thermite is lit and an incredible amount of heat and light is produced. But what is happening beneath all the sparks? Answer: an exothermic reduction-oxidation (or redox) reaction. For those of you who are versed in chemistry, here is a formula:
Fe2O2+2Al⟶2Fe+Al2O3 ΔG=−840 kJ/mol
What results in this chemistry is temperatures reaching even well over 4000°F (roughly half the temperature of the surface of the sun). Molten metal, depending on the oxide used, is also produced in a ratio of roughly half the weight of thermite mixture used.
It is important to understand that thermite reactions are exothermic, meaning they release lots of energy onto their surroundings. Therefore, thermite ignitions must be carefully contained.
How is Thermite used?
Because thermite reactions are so energetic, and potentially even explosive, they are not really used as an ambient heat source, despite being able to reach such high temperatures.
Thermite reactions are usually contained within a covered ceramic container with a hole in the bottom to allow the molten metal to pour out. The resulting molten metal is typically used for welding purposes.
Thermite welding is especially useful for its speed and efficiency. The short time frame in which thermite reactions reach their peak temperatures is perfect for creating molten metal quickly.
One of the most common places thermite welding is used is on railroad lines. To connect large steel rails to each other, a portable ceramic foundry fitted for rail lines is filled with thermite and ignited. Here is a video of the process in action.
What can Thermite burn through?
It would almost be more appropriate to ask what thermite can’t burn through. The reaction produces so much heat that it can burn straight through steel, concrete, asphalt, household ceramics, and can even burn underwater.
In order to control thermite reactions, advanced ceramics and tungsten (the most heat-resistant metal) are used.
How to make Thermite
The two main ingredients that you will need are iron oxide and aluminum powder. The more finely ground the powders, the hotter the burn and the easier it will ignite. These ingredients are pretty easy to find in-store or online, but it is also possible to make your own if you prefer.
For safety reasons, we aren’t going to share the specific formula here. If you’d like to consult an authoritative source, take a look at the University of Minnesota, which has a page devoted to thermite reactions, along with proper lab safety procedures.
It is no surprise that working with thermite is very dangerous. With such high temperatures comes lots of light energy and pressure changes which can be potentially explosive. Here are a number of precautions to take when working with thermite.
Again, do this at your own risk! Thermite is dangerous, and you shouldn’t use it unless you have the experience and equipment to stay safe.
- Make sure your thermite powder is completely dry. Any moisture will quickly vaporize and potentially cause an explosion. (heat your powder to boil any water away)
- The same is true for other metals with low melting points, may sure they don’t contaminate your thermite (ie. zinc, lead)
- Wear eye protection, the reaction gives off a lot of UV radiation that can damage your eyes
- Wear clothes that covers your skin completely
- Handle materials with tongs
- Best to carry out the reaction outdoors, away from flammable materials where sparks may fly and catch light
- Avoid using glass or household ceramics as a vessel. These can shatter under the intense heat.
If you are using thermite to cast metal, here are some defects you could look for to evaluate your results.
Thermite is a very useful substance to have a working understanding of. The incredible heat can be a self-made metalworker’s best friend. But if not handled with care, thermite has the ability to spell disaster for your foundry.
Whether you’re casting, welding, or just want to watch some sparks fly, hopefully this article has provided the essential information for you to make informed decisions about your backyard foundry projects.