Cast Thermite




About: What's to say? I have a deep passion for all things electrical and mechanical, and the more of it I can get my hands on, the happier I am!

Intro: Cast Thermite

There are plenty of Instructables and other how-to's on making Thermite, the incendiary mixture that can not only raise, but also burn, eyebrows. (The concept for this Instructable came from 'the Anarchists Cookbook', and since it doesn't give many's how I do it.) Now, there's nothing wrong with powdered forms of Thermite, that's how I got started a few years back, but sometimes you just want something rock solid that burns nice and bright/hot.

That's where Cast Thermite comes in. And the best part is, If you have some thermite on hand already, it's pretty easy to make! If you don't, never fear, for I will be starting at the beginning with nothing but the raw materials.

Step 1: Obtain the Materials

So you made it through the intro? Good. There's something that must be said before we go any further. I'm not liable. For anything. Do your homework and be sure you know what safety precautions to take when handling something like thermite, such as not looking at it during ignition since it gives off harmful ultraviolet light! Be smart, treat this stuff with the respect it deserves. It's not flash powder but it's certainly not sugar either. ;-)

Now that that's out of the way, onto the fun part. The preparations!

The first thing to do is understand what thermite is made up of. Most commonly, homemade thermite is made up of iron oxide (rust) and granule or powered aluminum. When mixed together, these two metals create an incendiary capable of reaching temperatures of several thousand degrees. For more info check thermite's Wiki page. So:

What we need:

  • Fine (0000) Steel Wool (not pictured)
  • Aluminum
  • Some kind of scale
  • Plaster of Paris
  • Handling materials (spoon, cup, paper, coffee can...)
  • Some kind of mold to cast your thermite in
While I'm at it, I'll explain the best way to get a hold of the materials.

  • Steel Wool
Off to the hardware store! Honestly though, I buy my steel wool at Walmart unless I can find bulk fine grade steel wool at our nearby Harbor Freight .

  • Aluminum
Well, you could always go online and purchase some high grade Aluminum powder like I did. Before I thought to do that, I just ground strips of aluminum foil in a cheap coffee grinder. IMHO, the larger grains made from the coffee grinder method make for brighter and more enjoyable light shows.

  • Scale/Plaster of Paris
Ever heard of Michaels? It's a nice little place a lot like Hobby Lobby. Plaster of Paris and the scale can be bought at either of these places, and probably at Walmart as well.

  • The Mold
Use anything you want to, whatever suits your fancy. I use a small ice cube tray. Film canisters would probably work fairly well, as would larger tupperware containers for really big batches. Whatever floats your boat.

Finally, you say. I've got everything you told me to get. Let's get started!

Step 2: Refining

Time to start refining our materials.

Steel Wool

You did buy the right kind of steel wool, right? The best kind is usually labeled as "0000" which is the finest grade I've ever found. This stuff burns the easiest and is usually less wasteful than the heavier grade steel wool.

Take the steel wool out of it's packaging and place it in a container suitable for oxidizing substances...(burning things.) Pull them apart a little bit before you put them in the container. Don't worry about packing it full. As it burns you will be able to crunch it down and add more on top. After lighting it, (a regular match works fine), you'll have to move some air through it to make sure the fire spreads. If you're only doing this much, just blowing on it should be sufficient.

Once it has burned and cooled off, rub the tougher chunks and strands of steel wool along something with small holes to make sure the pieces you collect are fairly small. It also helps keep out the excess pieces that wouldn't burn so you can re-burn them or use them in your next batch.

I rub the burnt steel wool over an old window screen that I cut out and wrapped in duct tape. In doing this I end up with a much finer product.

Step 3: Refining Part 2

This next step is not necessarily required, depending on how fine your burnt steel wool, (now called iron oxide), is, but as I am something of a perfectionist I like my chemicals to be as fine as I can get them.

If you have a ball mill similar to the one below, this step will be simple for you. Toss your iron oxide into whatever container you use along with some grinding media and away you go!

If, on the other hand, you do not have a fancy shmancy ball mill, no big deal. A Mortar and Pestle will get the job done just as well.

So will a coffee can full of iron oxide and some grinding media. All you have to do is shake it... A lot. By grinding media I mean something small, hard, and round, that is capable of breaking up the particles that you are trying to refine.

So, if you opted to go through with this step, use one of these methods to crush the iron oxide. Unless you get it ground to a powder it clumps together a little bit, but don't worry about it, because that's about the consistency that I use it at, and it still works. It doesn't have to be perfect by a long shot, just do what you can and move on.

Step 4: Mixing

Now, assuming you either ground your own aluminum or bought some off of the internet, it's time to start mixing our materials.

What we are looking for here is a ratio, (by weight, not volume), of 3 parts iron oxide, 2 parts aluminum, and 2 parts plaster of paris.

I just went by what would fit best in my mixing container. 60 grams of iron oxide, followed by 40 grams of both aluminum and plaster of paris. (Divide 60 and 40 grams by 20 and you get the original ratio, 3:2:2.)

Once you have the right amount, mix the three together thoroughly. The metals tend to sink and 'stick' to the bottom, so make sure to mix it as well as you can until it is one uniform color. In most cases it should look solid gray.

Step 5: Just Add Water!

Now that your cast thermite powder is mixed, make sure you have your molds ready, then go ahead and add some water.

You'll want to add plenty of water. As you mix be sure to scrape the bottom of the container you are using, as some of the thermite will be rather stubborn, and won't want to join the rest of the crowd. If you think you added too much water because your thermite is really soupy, don't worry. As it dries the extra water will be pushed to the top and you can just drain it off.

If all goes well, it should end up looking something like the picture below.

Oh yea, one more thing. Once you've done this, you have less than ten minutes to pour the thermite into the mold, so be SURE you are ready to continue before you mix in the water.

Step 6: Casting

This is it! The final step!

Take that nice gooey thermite that you worked so hard on and carefully slop it into whatever you are using as a mold. This ice cube tray worked very well as I wanted something small and reusable. Since the shape of the ice cube tray is larger at the 'bottom' I didn't have any trouble removing the cubes.

Keep in mind how the thermite will harden inside an object when you are deciding what you want to use as a mold. Something like a bottleneck vase for flowers would never work unless you planned on breaking the vase off of the cast thermite once it had dried.

Once it has solidified, (it should only take about a half hour to 45 minutes to), pour off the water and remove it from the mold. According to the anarchist cookbook, these will have to dry in the sun for a week, more or less depending on the size of the cast. Alternately you can place them in a oven or dehydrator to dry them much faster, probably in a matter of a few hours, which is the method I use.

Whatever you do, dont put them in your microwave. I doubt they would ignite, but as they are basically chunks of metal, I don't think your microwave would appreciate it. Once again, I'm not liable! Be smart!

As for ignition, thermite requires extremely high temperatures. If it was normal powdered thermite that we were talking about a magnesium sparkler would do the trick, but not for cast thermite. This requires something like the heat from a propane torch, (don't try it, you would have to be far too close to ignite it.) I use something called First Fire Mix. Google it or check back in the near future, as my next instructable will be on making a basic igniter for thermite.

That's it, you're done! If you need any help, check back or comment/pm me, I would be more than happy to help. This is my first instructable, so let me know how I did!

*Edit* For those of you who still need help lighting thermite and other difficult to ignite materials, check this out.

Enjoy your new cast thermite!



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    307 Discussions


    Question 4 months ago

    So in looking in the cook book, years ago, why are you using steelwool and not rust?

    Why not sugar in equal amounts for the same thing?

    a better way to get iron oxide is by the electrolosis of a salt water solution with some iron rods. Not graphite. Then filter the muck and let it dry. Grind it up in a pestle and mortar and there powdered iron oxide.

    11 replies

    That is a 'better' way to get iron oxide, if you're looking for efficiency from cost/time/end-product mass Vs. beginning mass. There is one thing that you did not consider, though, and that is the fact that when you make iron oxide via electrolysis you do not have an end product of Fe2O3 which is the best iron oxide to use for thermite mixtures.

    The Ideanatorchriskarr

    Reply 8 years ago on Step 2

    Whats wrong with Fe3O4? it has more oxygen, shouldn't that let the thermite burn better?


    Reply 1 year ago

    It exists in nature, and is called "magnetite".

    I don't know that you _can't_ use it in an aluminum oxidation reaction, but I know that it typically isn't.


    Reply 1 year ago

    Correction: magnetite (Fe₃O₄) is used in thermite as well, though less often than hematite (Fe₂O₃).


    Reply 1 year ago

    Thermite types:


    Reply 6 years ago on Introduction

    Can someone help me out here?


    2 years ago

    How did you grind the aluminum, or can you use aluminum oxide like the kind used for polishing stone?

    1 reply

    Reply 1 year ago

    The thermite reaction is Fe₂O₃ + 2Al -> 2Fe + Al₂O₃ ... basically, you're oxidizing the aluminum by stripping the oxygen off iron. So Al₂O₃ wouldn't work, since you'd basically be trying to burn ash.

    dhooks111490Canuk science

    Reply 1 year ago

    I already have a reel of magnesium ribbon for this purpose. So I'm assuming if I let a piece of it dry with the mold so that it sticks out like a fuse it will work.


    2 years ago

    Will the putty stick to surfaces?

    Canuk science

    2 years ago

    Easiest place to get both iron oxide and aluminum powder is a website called United Nuclear,

    Canuk science

    2 years ago

    Could you use the powdered thermite(which is easier to ignite) to ignite the solid could even make some sort of thermite based fuse,where you tightly wrap thermite in a paper tube,then put a magnesium ribbon or powder in the end to start the reaction