Millstone

Millstones are blocks used to turn Rock Salt into processed Salt.

Usage
To use a Millstone, you must right-click on it to open its GUI. Then, you must put the item you want to grind (rock salt) into the top slot of the GUI. Once you have done that, you need to leave the GUI, and place a power source (a lever, presumably) next to it, and turn the power source on. The Millstone will start to turn, and soon your item will be ground.