That's basically the same legend with salt peter.

Now I've got to find out why I can't make a forge bomb. My speculation that it could only be made after killing the smith (and scarring his helper smile ) was wrong - mixing the coal and salt makes an invisibility potion (well, not really; it just vanishes so nothing to mix the sulfur with). Darn! I know there's other ways out but blowing up things in a game is fun!