Chemically speaking gunpowder is not a compound where molecules of different elements join together to form new molecules, but rather it is a physical mixture or "combination" of three substances: sulphur, charcoal, and saltpeter (otherwise known as potassium nitrate). They were combined not in a chemical reaction, but in a physical process in a combining mill like this one, which ground the ingredients down into the correct, uniform size and mixed them together. A few safety features are worth noting: the wheel is iron, but to prevent sparks the bowl is made from wood, strengthened where necessary with copper fittings (copper is a non-ferrous metal that does not generate sparks when in contact with ferrous metals). Despite these and other precautions, explosions did occur, for example when the ingredients were accidentally contaminated with ferrous metal such as iron nails.