|Monthly Tech-Tip |
The cost of plaster bats prevents many from buying them. But that is a non-issue if you make your own. Using this rubber mold I have just made 8 - 12" bats and I still have 20 lbs of plaster left in the bag! The first rule for a good result: Lay it on a level level table (so the bat does not come out thicker on one side than the other). I 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 I dump the next batch of plaster in the water and repeat (by the time the next plaster is ready to pour I can remove the last bat from the mold). You can buy one of these rubber molds at Plainsman Clays for $75 plus shipping. Or, there are many ways you can make your own mold.
To achieve more even drying. As soon as was practical after throwing (a few hours), I covered the piece with a cloth and then put a garbage bag over it. While that put the upper section a little ahead of the base in drying, over night the base caught up (the plaster sucks the water out of it, even the walls of the piece stiffen). In the morning I remove the plastic and within an hour or two it is ready to lift off the bat and turn over, shortly after that I can trim it. I secure each plaster bat to the wheel-head using a "Batmate", that works extremely well. To stick the clay to the plaster well I apply a thin layer of slip, round off the piece of clay and firmly slam it down onto the plaster (if it is not rounded it will not stick as well or may break the bat). This is quick and effective to achieve the even drying needed to avoid a drying crack. This method is especially important for large plates and bowls, which often suffer s-cracks.
There is a problem that prevents many people from using a plaster bat for throwing on their potters wheel: Getting them to stick. The traditional method is to glue them down using a clay slip, but bats often detach during throwing. A BatMate you sticks them much better. The material resembles a leather chamois, like those used for cleaning windows. To attach the bat first assure that the plaster is not too dry, run it under the tap for a second or two if needed. Then immerse the BatMate in water and lay it on the wheelhead, put the bat on top, center it and press it down. Within seconds it sticks hard. No amount of pushing during throwing is enough to release it. To remove it pry up an edge using a putty knife. On removal the BatMate releases from the wheelhead first, so it must be peeled from the bottom of the bat. It can be used repeatedly all day.
Although there are reasons to use various types of bats in ceramic production (e.g. wood, plastic, masonite), for throwing pieces that are too big or fragile to lift immediately, a plaster bat is the best solution. This is especially so for porcelaineous clays that are difficult to dry. The main reason for this to minimize drying cracks. The plaster pulls water out of the base of the piece in the hours it sits after throwing. That solves a fundamental problem, for example, with making large platters and bowls. On a wooden bat the rims of these need to be dried until the walls are stable enough to support the weight upside down. By the time that happens the rim is well ahead of the base and a gradient has been set up that can case a drying crack across the base later. These are fairly expensive, but it is easy to make your own.
Plaster bats are a key tool used by many potters. But their true value is not understood by most: They produce ware with much less cracking problems