Why should potters who make wide platters or plates throw on plaster bats rather than wood or plastic? To avoid drying cracks.
If your platters, plates or any other item with a wide foot ring are cracking think carefully about your forming and drying process. If at any stage the rim is drier than the foot then the stress this introduces cannot easily be fixed by simply retarding drying to re-even-out the water distribution. The gradient will be remembered and could crack the piece later during drying. Potters use much more plastic clays than industry, these have significantly higher drying shrinkage. To have success with these care is needed to make sure all parts of a piece have close to the same water content at all stages of drying. A key way to make this happen for platters and plates is to use a plaster bat. If a piece is put into a humidity room or covered in plastic (after the rim has stiffened a little) the bat will do its work of sucking water out of the base. The next day the walls will have also stiffened and the plate will lift off the bat and have an evenly distributed water content and can be turned over and trimmed. The stiffness the bat creates on the base is normall at the perfect stage for trimming.
Sticking plaster batts to the wheel: The Xiem Batmate Gripper is the answer. It is sold at most pottery supply stores (google 'batmate grippers" or "xiem batmate" to find it). The descriptions at online retailers can be misleading, they say it is the "solution for annoying bat jitter and warping and that placing Batmate below your problem bats on the wheel instantly makes them become stable". That may be true, but their real value is that they effectively attach bats to the wheelhead. It is a chamois (or shammy) that you wet and lay on the wheelhead and then press the bat firmly down onto it. The plaster sucks the water from the chamois and in doing so sucks itself firmly down onto the wheelhead. The only way to get it off is pry an edge upward using a putty knife.
To Buy or to Make?
Batts cost $10. A bag of plaster costs $25. A 12 inch by 1 inch thick bat weighs about 5 lbs so you can make 10 from a bag of plaster. The ones you buy may or may not be truer, more level or better finished than the ones you can make. See the picture below for instructions on making them.
Using this rubber mold I have just made 8 - 12" bats and I still have 20 lbs of plaster left in the bag! Just just weigh 1600 grams plaster, dump it in 1120 grams of water, wait 4 minutes, mix 4 minutes and pour. As soon as the water at the top disappears, dump the next batch of plaster in the water and repeat (by the time the next plaster is ready to pour you can remove the last bat from the mold). If you are in a drier climate and make wide shapes, especially with a porcelainous clay, using a plaster bat is an excellent way to get even drying and avoid cracking. Using a BatMate you can stick them down to the wheel very hard, yet they are easy to get off. I would never use any other bat than these. You can buy one at Plainsman Clays for $75 plus shipping.
By Tony Hansen