|Monthly Tech-Tip |
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.
Then they may need a week to dry! This plate had a one-inch-thick base (while the rim is a quarter of that). During the first few hours a thin rim like this will dry quickly, leaving the base far behind. But as soon as it would support the weight of a cover-cloth I put it into a garbage bag and sealed and left it for several days. Even after that it did not detach easily from the plaster, even though the bat had been dry. When I did get it off the base was still quite soft but the rim was stiff enough to enable turning it over and trimming it (I endeavoured to create a cross section of even thickness). Then I dried it under layers of cloth for several more days. It took at least a week. Had I allowed the rim to dry out during the first few hours it would likely have cracked later on.
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.
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.
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