Steam Incorporating Mills

Here the gunpowder ingredients were crushed together in a pan under the rims of two vertical mill wheels running in a circular trough. They were originally powered by a central waterwheel and may have looked like this:

From the Waltham Abbey Special Collection.

Worn millstones were stood on their side and covered in earth to provide protection from accidental blasts.

Millstones standing on their sides. AndyScott, CC BY-SA 4.0, via Wikimedia Commons

Some of these date from the 1600s. The maker’s mark can be seen on the back of the one on the far right.