|Monthly Tech-Tip |
They need ten days to dry in our climate! This 14" plate has been thrown with a 1" thick base. The rim is a quarter of that. During the first few hours, the rim would dry quickly, leaving the base far behind. So as soon as it will support the weight of a cover-cloth I put it under plastic for several days. After that it does not detach from the plaster, it has to be cut off with a wire (there is a lot of clay here, it waterlogs the bat). The rim is stiff enough to support it for trimming but the base will still be quite soft. Thus, it is doubly important to trim it deep enough to create a cross-section of even thickness. Then, to try to even out the water content between base and rim I place it under layers of cloth and under plastic for several more days. Finally, out of the bag, it dries, with cloth still covering. Hopefully, the base will not bow upwards or crack. These are difficult, there is no getting around it!
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.
The difficulty of crying large plates increases dramatically by size and thickness. The better the mixing and wedging, the smaller the piece, the thinner the walls, the more even the wall thickness, the less porcelaneous the clay, the lower the drying shrinkage, the higher the dry strength and the more even the drying the fewer cracks there will be. The poorer the mixing and wedging, the larger the piece, the thicker the walls, the more uneven the cross-section, the more porcelaneous and smooth the clay, the higher its drying shrinkage, the lower its dry strength and the more uneven the drying the more cracks there will be. Do not dismiss the possibility that it could take a month or more to dry this type of piece.
This 14" plate was made using Plainsman Snow, a dolomite body. It was fired to cone 04 using Spectrum glazes, they were watered down and applied inside and out using a gravity feed sprayer. The high porosity of the body makes the use of these watery gummed glazes possible (older talc bodies would not have worked). Difficult drying and firing make these extremely difficult at stoneware temperatures. But not with this body, it is super plastic yet dries exceptionally well. And it has zero firing shrinkage and can be fired in a fraction of the time! These factors, coupled with the brilliant colors possible at low temperatures, make such bodies an attractive option for potters to make extra money with much less effort. What about fired strength? Large porcelain and stoneware plates are harder but also much more brittle. And they host residual stresses from the drying and firing processes. Pieces made from this body are surprisingly strong. This plate was made by Karel Peeters, Aug 2022.
Clay Cracking During Drying
The best way to avoid drying cracks when making ceramics or pottery is to avoid doing the things that cause it. Do not just blame the clay, anything can technically be dried.
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.
In ceramics, drying performance is very important to optimizing production. More plastic clays shrink more and crack more, but they are also better to work with.