[paul cotter (OP)]

Armstrong's mixture is an insanely sensitive explosive consisting of a roughly stoichiometric mix of red phosphorus and potassium chlorate. It can be triggered by a gentle tap and the mechanism behind such extreme sensitivity is a bit puzzling. I can understand highly sensitive molecular explosives such as nitrogen trichloride or ethyl perchlorate where bond weakness allows for low activation energy. Potassium chlorate needs about 300c to release oxygen. I wonder is it a case of embedded p4 molecules in the red phosphorus. Any ideas? PS I have never done this nor do I intend to-I find my hands to be useful.

[SeanB]

Probably a catalyst reaction, where the one decomposes the other with only a minor addition of energy to get over the catalyst activation energy, and then the released energy is enough to drive the reaction strongly, and very exothermically, to completion.

[Bored chemist]

I read that part of the reason why mixtures of  sulphur and chlorates are so unstable is something like this:
 the sulphur is attacked, even at room temperature, by oxygen.
That produces sulphur dioxide and that, together with water in the air is acidic.
That acid reacts with the chlorate releasing oxides of chlorine.
Those react with the sulphur to produce more acid(s).

The reaction is "autocatalytic".
The heat generated also speeds up all of the reactions.

I suspect something similar happens with phosphorus.

[paul cotter (OP)]

Interesting, however I think the main danger with the chlorate/sulphur occurs in storage where slow oxidisation causes a build up of acid and once it starts it becomes, as you say, autocatalytic. In my reckless youth I often made "chlorate gunpowder" with sodium chlorate which was freely available at the time and quite inexpensive but unfortunately hygroscopic. I found that to initiate this mix with impact it had to be put on a solid metal surface and struck very hard with a hammer. I don't know if any tests on the storage stability have been done with Armstrong's. It is or has been used in a phlegmatized (no idea how one could do that) form in toy cap guns.