Milling Jars Paul O AbbéThe mill jars are rotated with the aid of a jar rolling mill which has two rollers to support and rotate the jars at the proper speed The milling jar is half filled with grinding media or balls from ¼ to 1 and as the mill rotates the solids are trapped between the ball and are milled down to as little as 1 micron by the cascading and tumbling action of the ball charge