Go read the book "Guns, Germs, and Steel" by Jared Diamond. Or at least read its Wikipedia page and the list of references / associations. The book is a little wonky in places, but the basic premise is useful for first-order effects.

It's not enough to have saltpeter, charcoal, and sulfur. You have to have a reason to combine them and cook them until they accidentally light. Maybe you're trying to make an elixir of immortality or something... Nah, you'd have to be a medieval Chinese philosopher or something to try that.