This, down to the last paragraph, is good advice. I haven't made my own gunpowder, since schooldays in the 1960s anyway, but without speaking of this thread in particular, I sometimes feel I am not alone.
The main key to successful powdermaking is very, very fine powder for all the constituents - finer than you can buy them. I don't believe there is any risk of ignition if you ball-mill them very thoroughly while dry before mixing, although I suppose there is a slim possibility that that could happen with static electricity and airfloat charcoal, as sometimes happens in flour mills. I once worked in a Saudi university where the science department was stopped from showing students how to make the dreaded flour bomb in a cardboard box, so we are talking nothing like a gunpowder explosion here. I think a grounded copper wire trailing on the outside of a plastic mill drum would eliminate any such possibility.
Then lengthy milling after mixing the ingredients is required. This used to be done while slightly damp, but not so moist as to prevent the possibility of a very fierce ignition. I believe this was much reduced by the introduction of concave iron troughs in edge runner mills, rather than rollers, reducing the tendency for any to dry out on the edge of the floor, and the need for a human attendant to be around much of the time, shoving it back again.
The advantages of making the mixture into a soft paste were obvious, and yet good powder couldn't be made that way. It doesn't matter how wet you get the charcoal and sulphur, as they are insoluble in water. But if you dissolve part of your painstakingly milled saltpetre, it crystallises into much larger crystals when it dries out.
I believe the main purpose of that much slighter moistening (besides reducing flammability) was that it was just enough to coat each charcoal and sulphur particle with saltpetre. This is what binds the cake into a solid under pressure, ready to be broken into grains, rather than just fall apart again.
What I have not heard of being done, but find an interesting possibility, would be to use steam to produce that very slight moistening of the mixed powder. This picture is a derelict powder mill near my home. Note the flimsy roof and front wall, to be blown out without major damage if ignition did occur.
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