Where is the term “bat” derived from?

This one is more tricky. It is not completely clear and there are many assumptions of its origin.

However, at Throw And Fire, we think the most plausible is this:

Bat: 1-flat slab of clay, sometimes called a clot, as beaten out or batted out either by hand or with a batting machine 2-thin piece of plaster of paris or board to place ware upon 3-A kiln bat is a refractory shelf used for placing ware in the kiln.

Source: Rick Hamelin on mon 21 feb 05 on http://www.potters.org/subject82701.htm

On the A-Z Of Ceramics at http://www.thepotteries.org/types/az.htm
modern day bat use is defined as
A flat disc made out of plaster, wood, or plastic which is affixed to the wheel head with clay or pins. Bats are used to throw pieces on that would be difficult to lift off the wheel head.