The potassium nitrate is the oxidizer, the charcoal is the fuel, but what does the sulfur do? A catalyst? Wiki has a nifty article including the classic proportions of 15:3:2 and where to get ‘saltpeter’ but on the why of the third ingredient, it is mum.
Any pyrotechnic chemists out there?
It adds a lot to the smell.
I’m not a pyrotechnic chemist, but I have your answer anyway. Sulfur (the correct IUPAC-approved spelling) is a desiccant used to prevent the marginally less hygroscopic (water attracting) powdered charcoal from becoming saturated with humidity and incombustible. It is possible to make black powder without sulfur–although not in the manner displayed in Star Trek–but it’ll have a very short shelf life even in nominally arid environments. It is important to use sulfur of high purity, as small impurities can cause it to form unstable side products and spontaneously combust.
Making black powder at home is definitely ill advised, as many amputees and burn victims can attest. Once the mixture dries you are left with a layer of finely concreted dust which has to be ground up; too much pressure or a tiny spark is sufficient to ignite an entire batch. Even the professionals like DuPont or Hercules occasionally blow up a mixer or lose an entire plant.
Stranger
I hate to disagree with you, but I’ve found several credible cites which say the main purpose of the sulfur is to lower the ignition temperature of the mixture, making it more easily and reliably touched off. It may also act as you describe, but it’s apparently not significant enough to bear mention, as far as I can tell.
W00T! I beat Q.E.D. by 4 minutes and the same cite.
Hmm…well, I’ll have to retract my claim, or at least partially. From your first cite:*A recipe without sulfur yields only little less energy (Joule/g). But its advantage lies in its little smoke produced. Its fog then consists of potassium carbonate (potash). Unfortunately its ignition temperature is 100 °C higher than usual. For cap lock arms no problem, but a nuisance to flint and match lock sportsmen.*I had always understood the reaction resulting in the K[sub]2[/sub]S product to be incidential and inconsequental in terms of energetic yield, but I hadn’t considered its effect on reactivity. But then, like I said, I’m not a pyrotechnic chemist, or indeed, any kind of chemist other than the kind behind the bar. Color my ignorance fought for today.
And you can disagree with me anytime I’m wrong and you’re right, or at least have good reason to think so. I’d like to think that I still have a few things to learn. 
Stranger
Oh, this was strictly intellectual curiosity. I like my fingers right where they are, and unburnt.
Interesting that the sulfur also seems to be a major contributor to the smoke of “non-smokeless” powder.
This cite states that the reaction is:
KNO3(s) + C(s) + S(s) -----> N2(g) + CO2(g) + K2S(s)
I haven’t had to balance a redox reaction in years, but this looks to me like the potassium nitrate (oxidizer) splits into K+ and NO3-. NO3- will then react with the carbon (in a very energetic reaction) to produce N2 and CO2. The K+, on the other hand, will be taken up up by the sulfur. According to Le Chatelier’s principle, by removing product, you will drive the reaction to the right. By sequestering K+, you’d skew the reaction towards the NO3- + C, rather than NO3- + K+.
The reaction is roughly 2 KNO[sub]3[/sub] + 3 C + S —> K[sub]2[/sub]S + 3 CO[sub]2[/sub] + N[sub]2[/sub]
Without the sulfur you form KO rather than K[sub]2[/sub] and the reaction is less energetic.
Black powder at Numericana. I’ve never heard of this particular site before, but it looks not unreasonable.
Sure, you could probably use selenium rather than sulfur (ie one row down on the periodic table), and get toxic potassium selenide smoke instead of stinky potassium sulfide, but sulfur does have a nice low melting point.
I’ve never thought about this but I know a few things that may help. Assuming the above reaction is correct, the sulfur is also being oxidized. Sulfur is one of the more easily oxidized elements. This strongly suggests that the first step in the mechanism is oxidizing sulfur from S(0) to S(+2). Therefore, the initial source of the electrons that end up on nitrogen is probably sulfur. That would explain why sulfur lowers the activation barrier (ignition temperature). Once the reaction gets going, there is enough energy to oxidize the carbon directly. This is why a stoichiometric amount of sulfur is not needed. (I did not calculate that a stoichiometric amount of sulfur isn’t used just guessed.)
Yep, it lower the ignition temperature.
Right to the point. But how does it lower the ignition temperature? I’m curious too.
Not at all. It’s a different reaction if you don’t put the sulfur in there. The end products are NOT THE SAME. Folks should learn some chemistry before trying to answer questions like this.
That said, yeah, sulfur’s got a nice low melting point, and it ignites easily. Once melted it probably starts to dissolve the carbon and KNO[sub]3[/sub] particles, which vastly speeds up the reaction. If you make gunpowder with calcium nitrate instead of potassium nitrate, you’ll actually see little liquid globs of molten sulfer/nitrate/charcoal spitting fire. I’ve never made the mixture where selenium replaces sulfur, but sulfur melts at 239°F, while selenium requires 430°F.
Then please educate me as to what the products are if they aren’t essentially CO2 and N2. I suppose the potassium would prevent complete reduction of the N2. (gotta have an anion for a cation like that.) But since you apparently know, please tell me. I’m here to learn.
So you are suggesting that sulfur does not lower the activation barrier? I know some chemistry, so please help me understand what you are saying.
Actually, that really helps answer the question. Without the sulfur, the nitrogen can’t be completely reduced. The potassium has to go somewhere as a cation (Otherwise you end up with potassium metal as a byproduct and that doesn’t seem likely.). The sulfur provides a nice anion for potassium. This suggests a stoichiometric requirement for sulfur. (Fine I’ll do that calculation some time.)
I suppose that potassium oxide could be a product without sulfur, but that still leaves a lot of oxygen unaccounted for. I still say incomplete reduction of the nitrogen would be a result of not having sulfur around. Squink Let me know what you know. This is an interesting problem. I’d like to work it out with another chemist.
Could you not just get atomic oxygen from the nitrate, and be left with potassium nitrite?
Yeah, that and the whole part where you’re trying to kill the other guy and not yourself! 