Why is there sulfur in black powder / gun powder?

Question

Black powder is composed of charcoal, sulfur and a nitrate salt. The charcoal and sulfur serve as fuel and the nitrate is the oxidizing agent.

However, wouldn't it be cheaper to leave out the sulfur, thus having only charcoal as fuel?

Wikipedia says about this (emphasis added):

Black powder is a granular mixture of

• [...]
• sulfur (S), which, while also serving as a fuel, lowers the temperature required to ignite the mixture, thereby increasing the rate of combustion.

I guess after this initial research my question boils down to: Why is sulfur required as an ingredient in gun powder, and how does it lower the ignition temperature of the mixture?

Answer 1

Black powder is a mixture of solids. As solids, they are not particularly inclined toward fast reactions between each other. Elevated temperature generally liven things up, because it allows diffusion of solids into each other. However, both carbon (charcoal) and potassium nitrate are relatively stable compounds, so even when potassium nitrate is melted, they still do not react immediately, further heating is required.

Sulfur, on the other side, is quite easy to melt. Nitrates are not very powerful oxidizers, but potassium nitrate in particular is a low-melting compound. And at the moment both potassium nitrate and sulfur are molten, their interdiffusion becomes quite easy, so they ignite, and charcoal follows.

Summing it up: charcoal is almost impossible to melt and potassium nitrate is a relatively weak oxidizer and also hard to melt, so the mixture is hard to ignite. Sulfur is easily flammable, easy to melt, so it is much easier to react with potassium nitrate solid, and its presence makes mixture much more ignitable.