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!):
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 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.
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.
