What's a ball mill? How isit used in making glazes?

In the post on Ken making glazes, I mentioned a ball mill.  What's a ball mill I was asked.  A ball mill is a jar or large drum that opens at one end. A substance is placed in the mill with porcelain balls and some water. 

The ball mill is closed, and rotated so that the porcelain balls grind the substance.

At first the mill is quiet, but as the substance is ground finer and finer, the porcelain balls hit each other and the sound of their hitting each other gets louder.

Then the  mill is opened, and emptied into a colander and then a strainer .

Voila.  Glaze