It’s unusual for a porcelain item to dry uniformly from the top to the bottom. Thinner edges will usually dry faster since there is less clay and more surface area, while thicker sections and areas where pieces have been joined will tend to stay wetter. If the difference between these sections becomes too large, then uneven shrinkage can occur and the piece can become distorted before it even gets into the kiln.
The drying process should be controlled as soon as possible after forming. Greenware is typically shielded from excessive air movement, direct heat, and rapid moisture loss. Draping with loose plastic will reduce moisture loss, which allows the difference between thicker and thinner areas of a piece to diminish as they dry. This does not mean keeping the porcelain wet, but simply preventing one area from drying much faster than another.
Edges require special care since they are usually thin and more exposed. For example, if a bowl is left uncovered, the edge may become leather hard while the base is still soft. At some point, usually when the bowl is sturdy enough to turn over, it should be turned upside down to allow the base to dry more evenly. Just make sure the piece has dried sufficiently before turning over so that it won’t deform under its own weight.
An excellent way to see the effects of different drying conditions is to dry several similar pieces of porcelain using different methods. You might cover one piece completely, leave another partially covered, and leave a third piece with no cover. Make sure to use the same type of porcelain, similar wall thicknesses, and roughly the same size and shape. As they dry from soft greenware to leather hard to bone dry, examine the differences in the rim, base, walls, surface color, and cracks. Since you’ve changed only the drying method, the resulting differences will be clearer.
Particular care should be taken when joining porcelain. Ideally, the two parts being joined should be similar in moisture content. If one part is significantly wetter than the other, they will shrink at different rates and this can lead to uneven drying. Scoring and slipping the joint can aid the bonding process, but it can’t overcome the effect of a very large difference in moisture content. It may be beneficial to cover the joint longer so the moisture levels can become more equalized.
Don’t be tempted to speed up the drying process just because the piece appears to be dry. Sometimes the surface of the porcelain may look dry and feel firm, but there may still be significant moisture present inside a thick base. If a piece that is not fully dry is put into the kiln for bisque firing, there could be problems. Always follow the manufacturer’s recommendations for the kiln and the clay body. Drying properly takes time; it’s part of the process and not a waste of time.
When determining whether a piece is bone dry, don’t rely on the color alone. Check the rim, walls, base, and joints to ensure they all feel equally firm and dry, and look for any signs of distortion or warping. If you’ve used a consistent drying method and the piece hasn’t changed shape significantly, then you know your method is effective. You can use that method the next time you make the same form rather than having to guess how much coverage to use for the porcelain.