### When it all started

One of the great things about being "grown up" is that one day you wake up and realise there are a lot of fun things which you can now do without asking anyone for permission. Take the thermite reactions for example. I first heard about these when I was at school. Back then several of us were keen on having a go at our own thermite experiments but there was no way we could get our hands on the materials and we would never have been given permission by our parents anyway. So we never had any fun with thermite. Well that's all changed now. In April 2009 I got talking to somebody about thermite and suddenly it dawned on me: why not visit eBay, buy the materials and have a go? So I did.

I gathered together a few other interested souls and, loaded with ingredients for making thermite, we headed to an appropriately remote location for our pyrotechnic experiments. It was a lot of fun! It was also very dangerous but fortunately we had thought carefully about how to do things safely so there were no accidents. One day I may get round to trying some more thermite or thermite-type experiments. If I do, I'll make more of an effort to document and record my efforts. Also, it occurred to me that it might be satisfying to try to cast something using the molten metal produced by the thermite reaction; it would be nice to have an interesting memento. Below is a picture of some of the results of the thermite reactions which we carried out (think how much better these could be if they were cast!):

Some of the results of our thermite reactions. The shiny piece of metal on the below right is a piece of manganese. The fragment of flower pot at the top has been coated with copper.

### A serious warning

THERMITE IS DANGEROUS. NEVER TRY IT WITHOUT THINKING THROUGH EVERY STEP FIRST AND EDUCATING YOURSELF APPROPRIATELY.

I should probably also mention that in Ireland there are very strict (and to be honest, pretty sensible) laws regarding fireworks (and unlike the years of my youth, the laws are enforced quite a bit these days). I don't know if these cover the thermite reaction (e.g. I've seen the workers laying the Luas lines thermite welding and I doubt they needed a firework licence). In any case, let me emphasise that I am not disclosing where our reactions were carried out and in particular which country they were carried out in or who was present, other than myself.

### The thermite reactions

The thermite reactions are popular and well documented; see for example wikipedia, Amazing Rust and Pyroguide. For this reason, I don't think there's any need for me to say much other than identify which reactions we carried out and to say that they varied in size from using about 200g to a little over 1kg of thermite.

The standard thermite reaction is of course aluminium powder being burned using iron III oxide (red rust) as the oxidization agent. This works well (very well!) but it is also possible to substitute other metal oxides in place of an iron oxide. Copper oxides and mangansese oxides are also fairly common so-called "exotic" thermite reactions. For our experiments we used iron, copper and manganese oxides; more specifically we used iron (III) oxide Fe2O3, copper (II) oxide CuO and manganese (IV) oxide MnO2. All of these worked well (the copper worked "too" well - see the video below!).

In each case the reaction is carried out by mixing the aluminium powder with the powdered metal oxide in the appropriate proportions. The reaction isn't terribly sensitive to the proportions used but it's worth making some attempt at getting them right. The proportions should be calculated by mass, not volume, and depend on the metal oxide being used. You can calculate the correct proportions from the stoichiometry of the reactions. For example the net reaction using rust is dominated by: $$Fe_2O_3 + 2Al \to 2Fe + Al_2O_3$$ Typical relative atomic masses of Fe, Al, O are 55.85, 26.98, 16.00 respectively. Thus for a mass 159.69 of rust (in any units), we need 53.96 of aluminium (in the same units). Thus this simple model for the reaction suggests proportions rust : aluminium of about 2.96:1, which is conveniently close to 3:1.

Of course you can just look up the recommended ratios using the above links. Once the powders are mixed, you're ready for ignition.

### Ignition

Fortunately there is little chance of the reaction kicking off by accident as the required ignition temperature is very high. A burning piece of magnesium ribbon is a standard way to start the reaction. This works well since the magnesium ribbon works like a fuse and gives you time to get far away from the thermite reaction before it starts. The fuses do have a habit of going out but once they touch the thermite, things kick of instantly. Note that we tilted our magnesium fuses out over the side of the pots containing the thermite so that if the fuse broke and fell early (as happened once) then the fuse would not land in the thermite unless it had been burning for long enough for us to be some distance away. Below is a picture of one of our magnesium fuses burning:

Magnesium fuse

Another standard way of igniting thermite is to use the reaction of potassium manganate (VII) KMnO4 (i.e. potassium permanganate in old-fashioned non-oxidation number naming) with glycerol C3H5(OH)3 (also often called glycerine). This works well but the potassium permanganate needs to be sufficiently finely ground (again, I don't know the required mesh size but caster sugar size particles seemed about right). Incidentally, glycerine is used as a sweetener and is available over the counter in most pharmacies and potassium permanganate is an old-fashioned disinfectant (apparently sometimes still used to disinfect wooden surfaces) and is available over the counter in about 20% of pharmacies (in my experience). I would not recommend trying to buy both substances in the same shop! To start the thermite reaction this way, place about a teaspoon of potassium permanganate on top of the thermite, pour on the glycerine and get the hell out of there! Incidentally I had experimented extensively with my potassium permanganate and knew how long it took for the reaction with glycerine to get started: about 10 seconds. This meant I knew I had about 10 seconds to get away from the thermite. Even still, this was not the safest way to ignite. I recommend the magnesium fuse instead.

### Last words

The dominant net reaction in each thermite reaction is that the oxygen atoms are transferred from the less reactive metal (e.g. iron/copper/manganese) to the alumium. The finer the powders, the greater the suface area and so the faster the reaction will take place. Unfortunately I don't know the exact mesh size of the powders we used but I do know that they were all very, very fine, e.g. they were finer than icing sugar. One of the powders, the CuO was probably too fine (in my fingers it felt more like a cream than a powder) and both times we carried out this reaction it exploded POWERFULLY instead of burning. This was a bit of a shock but fortunately we were far enough away on each occasion.

### Some pictures and videos

As I said above, if I ever got round to trying this again I'll try to document it more carefully and in particular I'll make more of an effort to get good quality pictures and videos. Still, we did get a few reasonable pictures and videos which I hope some people will be interested to see. Here are a few:
Copper thermite explostion (video)
Standard iron thermite reaction (video)
(I had to mute the above videos to preserve attendies' anonymity which is a shame as the thermite sounds awesome!)

That's one hot pot!

The camera which took the above two photos did not have an IR filter and so the colours look strange (but quite nice, I think).

A thermite reaction in full swing.

We got some satisfying sparks when we attempted (unsuccessfully!) to melt an old hard drive.

A frame from a video of the copper thermite just as it's starting to explode.

A frame from the video of the copper thermite explosion

Another from the video of the copper thermite explosion

And another

And another

And finally one more

I thought I'd finish with a picture of this rather civilized little reaction.