A reader writes:
Why is "black powder" gray when I make it, but black when I buy it?
I live in the Land of the Free and the Home of the Well-Regulated Militia and can buy gunpowder over the counter to, uh, use in my collection of historic muzzle-loading muskets, Officer. The stuff you buy, though (and the stuff I've seen on Mythbusters too, actually) is sort of shiny black, in various different particle sizes depending on burn rate and whatnot.
But when I MADE gunpowder as a kid, the recipe was three-quarters saltpeter, and saltpeter looks like salt. It's white. So the powder comes out pale gray.
As you say, the recipe for standard black powder for firearms is 75% potassium nitrate, 15% charcoal and 10% sulfur, by weight. And this does indeed create a grey powder, not a black one. Everything associated with making and combusting black powder tends to end up pretty darn black, thanks to the charcoal in the mix and the copious smoke and other solid residue created by the powder's inefficient combustion, but the powder itself isn't black.
Commercial black powder looks black because the little lumps of the stuff are coated with graphite. In the manufacturing process, the powder's mixed with water or some other liquid binder, pressed into cakes and dried, then crushed and screened into powders of various particle size, larger particles producing a slower burn. The graphite serves no chemical purpose, but it lubricates the particles, and also makes the bulk powder electrically conductive enough that it's unlikely to initiate proceedings unexpectedly because of a static-electricity spark.
(You can't, by the way, make decent black powder using graphite, or any other high-purity carbon, in place of charcoal, because the leftover wood impurities in charcoal make it ignite at a lower temperature. Pure carbon makes a black powder that burns slowly; it might eventually push a bullet out of a rifle's muzzle.)
Black powder remains a somewhat excitable substance, even with graphite on it; it is, for instance, still pretty impact-sensitive, if not by the standards of substances of which only lunatics prepare more than a gram.
It's now apparently becoming harder to find black powder even in the gun-happiest parts of the USA; instead, there are various black-powder substitutes. All of the substitutes are safer than black powder, and many of them have other benefits, like not fouling your muzzle-loader with corrosive sulfur compounds.
And then commenters will, I hope, correct at least the most obvious flaws in my answer.