In the late Middle Ages and Renaissance period, gunpowder makers often worked in the nude, and (later on, as the process became more industrialised) wore wooden sandals, so as to avoid generating static electricity and setting off the gunpowder…
According to Wiki, a simple mechancial mixture of ground sulfur, charcoal and saltpeter is a green mix, and usually performs poorly.
Apparently the big advance in producing black powder was ball milling, which not only was finer, more thorough, and uniform than hand grinding, but also mashed the sulfur and saltpeter into the pores of the charcoal, producing a much better reaction.
Apparently ceramics aren’t spark-proof enough for grinding black powder, except for soft, low silica stone like limestone or marble.