|Monthly Tech-Tip |
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.
Failure is often not something specific. The greater the wall thickness variation the more difficult it is to dry and fire it evenly. The stronger the rim the better it will withstand stresses. The finer particled and more porcelaneous the body the more difficult drying and firing will be. After throwing, the longer a piece is left rim up in air the greater the moisture content difference will be between it and the base. The earlier a bowl can be trimmed the better the success. The less a soft or leather hard bowl is handled such that it goes out-of-shape, the better. The slower and more even the drying the better the success. The better the air flow and slower the heat-up through water-smoking in the kiln, the better. The more that heat can reach all sides of the piece the more evenly it will heat up during firing. The less the point of contact between the base of the bowl and the kiln shelf, the less the shelf will heat-sink it and increase temperature gradients within it.
The cracks happened on heat up (since they have opened up wide). A combination of issues contribute. The kiln shelves heat-sink the wide flat bottoms, vessel walls are thick, there is some unevenness of wall thickness and only a 30-minute hold at 220F to remove glaze water from the bisque (that could have left dampness in thicker sections). Factors like these combine to produce temperature gradients within the piece. The firing schedule rose rapidly from 250-2100F (400F/hr) amplifying these gradients as it climbed. At quartz inversion these gradients produced a wave of volumetric change moving through the bisqued piece and this likely initiated a crack where stresses met at a combination sharp-contour and thickness-change, the bottom corner.
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!
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.
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.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.