|Monthly Tech-Tip |
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.
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.
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.
Drying Ceramics Without Cracks
Anything ceramic ware can be dried if it is done slowly and evenly enough. To dry faster optimize the body recipe, ware cross section, drying process and develop a good test to rate drying performance.