Playing with my RPG group last night (don’t you love ones that start like that?) a couple people insisted that very old barrels of gunpowder would change into nitroglycerin. I called B.S., but none of them believed me.
I’m fairly well versed in explosives, having worked for a fireworks distributor and being a licensed lead pyrotechnician, and I can’t see how it would happen.
So, could black powder, or any gun powder, left alone in a wooden barrel for a very long time turn into nitroglycerin?
Easy one: no. They are different things with different chemical composition.
They could well be confusing gunpowder with dynamite. Nitroglycerin crystals can appear on old sticks of dynamite.
No. Black powder is a mixture of charcoal, sulphur and saltpeter.
All Dynamite is, is Nitroglycerine suspended in an absorbent, inert binder, such as diatomaceous earth. This stabilizes it to greatly reduce NG’s infamous, if exaggerated, propensity to detonate on shock or impact.
With time, however, the glycerine can ooze out of suspension in the filler, just like the pigments in paints settling to the bottom, and form “tears” or droplets of more-or-less pure nitro on the casing. This, as you might imagine, is terribly dangerous.
However, black powder is chemically and physically very much different than nitroglycerine or Dynamite. Over time, black powder remains fairly stable, and is pretty much only affected by variances in moisture content. (Which doesn’t mean “boom” or “no boom”, it just means slight changes in pressure per volume.)
IIRC black powder and nitroglycerin also work in different ways; black powder combusts; nitroglycerin detonates.
Your rpg friends may be confusing black powder with guncotton, or smokeless powder. Both are made with nitrocellulose, but some smokeless powders actually do contain nitroglycerin:
Modern ‘gunpowder’, i.e. the stuff that’s in any bullet nowadays, is ‘smokeless powder’, or ‘guncotton’. It’s completely different from ‘blackpowder’, the ‘gunpowder’ of the 19th century and earlier. As others have said it is based on nitroglycerin and is much, much more powerful than blackpowder. It is considered a ‘high’ explosive (like TNT) whereas blackpowder is a ‘low’ explosive.
There is no way one can ‘turn into’ the other.
You said you were playing an RPG game, maybe your friend got enough magic fairy dust from the wizard’s chamber to insist that he’d done it! 
How much shock does it take to detonate NG?
Thanks for that link, Squink! I’d been wondering for a while now what the technical difference between an explosive (which, Hail Ants, blackpowder is and smokeless powder isn’t) and a merely `fast-burning’ powder is, and that link explained the technical difference nicely.
Mmm blow up stuff
Smokeless propellant is not classified as an explosive at for legal or shipping purposes. That is not to say you cannot make an expolsive device out of it. I saw some cable channel program with the BATF making pipe bombs from common red dot pistol powder in order to do forensic testing. The trick is confining the pressure to accelerate combustion as with the pipe bomb or in the chamber of a gun. If smokeless propellant in normal metal or plastic cans is ignited it will burst the container and continue to burn but typically not explode. A loose pile of smokeless lit on fire will burn very rapidly but it will be a “fwoosh” rather than a boom. A large enough pile of black powder will explode if lit.
Bit of a hijack - well no I guess it’s a total hijack. So excuse me, but I gotta know.
Where does C-4 fit into the picture? What is it? Does it contain NG?
Thanks, Squink, that helps. I know how to make both, and I certainly couldn’t see a way that black powder would metamorphsize into a high explosive.
SandyHook, C-4 is a high explosive, it will detonate. It’s composed of the explosive, a binder, a plasticizer and sometimes marker or taggant chemicals. It’s 90% explosive, called RDX, which is cyclotrimethylene trinitramine. Then there’s about 5.5% of a binder, 2% of a plasticiser, and the rest is various taggants, binders, and other junk.
It’s off-white and has the texture of clay, if you ever find yourself needing to identify it. I hope you don’t.
Semtex is the other common plastic explosive, popular with terrorists due to easy availability and difficulty in detection. It’s a mix of RDX and PETN, another explosive, plus the plasticisers and binders and such. 700 tonnes of it was exported to Libya between 1975 and 1981; 250 grams could knock an airliner out of the sky.