Grinding reduced the tin ore into much finer particles- about the size of grains of sand.
Grinding was considered key to good processing. It was very difficult to get right, too little made it difficult to separate the tin particles from the rock, and too much grinding could make it too fine.
Machines called Stamps were used for grinding. The stamps were heavy and would move up and down, hitting the tin ore underneath and grinding it down. Stamps were replaced by Ball Mills in 1937 at Geevor.
Ball mills act like a washing machine with steel balls inside. The tin ore tumbled with the balls to grind it down.