|Monthly Tech-Tip |
Two mugs have dried. The local terra cotta native clay on the left shrinks 7.5% on drying, the porcelain one on the right only 6% (it is made using Kentucky ball clay). Yet few pieces of the terra cotta are ever lost due to drying cracks, even if it is uneven! For example, in a batch of a dozen mugs none of these will be lost whereas one or two of the white ones will always crack. Why? Dry strength. The clay on the left is very strong in the dry state, likely double or triple the white clay (the strength is a by product of its high plasticity and particle size distribution profile). That strength is enough to more than counter the extra shrinkage.
These are DFAC drying performance disks of a large-particle kaolin (Opticast) and a ball clay (Plainsman A2). The DFAC test displays a plastic clay's response to very uneven drying (these disks are dried with the center portion covered to set up a water content differential). These materials both feel super-smooth, in fact, the kaolin feels smoother. But the ultimate particles are ten to one hundred times smaller than kaolin, thus it shrinks much more. The ball clay also has much lower water permeability, being unable to channel water from the center protected portion fast enough. When the inner section finally dried the outer was already rigid so it split the disk in two and pulled all the edge cracks. Most ball clays shrink more and crack worse than this (cracks concentric to the center also appear). So why use ball clay? This kaolin is so lacking in plasticity it was barely possible to even make this disk. And it is so weak that it can easily break just by handling it. Still, it is useful to make casting bodies. But the ball clay, when used as a percentage of a body mix, can produce highly plastic bodies than can be dried without trouble if done evenly.
Fewer drying cracks! These were all made at the same time and dried in the same way. Left: Three out of five porcelain mugs have cracked on the bottoms! Right: None of the stoneware ones have. Sometimes I feel as if those are "crying cracks" instead of "drying cracks". Of course, I could have been more careful with the porcelain. But I keep forgetting items on the "drying success checklist"! So, for production, it makes sense to use a stoneware if at all possible. I can make hundreds of mugs using the body on right, M340, and will not lose a single one!
Drying cracks are opportunistic, especially in highly plastic or fine particled clays. They like to initiate inside sharp acute angles. The sharper the angle the greater the chance of crack. By doing this procedure before the clay gets too stiff (in the leather hard stage) you will deny a crack a place to start. Of course even drying is still important, the water content of a handle should now be allowed to get to far ahead of that of the main body of the mug at any time. In the pictures on the right, two tools are being used to compress and round the angle at which the handle meets the wall of the mug.
The foot ring on these leather-hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewets the bodies of the mugs, almost to the same point as freshly-thrown. But the handles did not get rewetted. To re-dry these mugs to the point of being able to turn them over will take 4-6 more hours. But by that time the handles will be too dry. To prevent that I waxed them after trimming (leaving a just the inside handle-curves bare). That slows their drying down enough to keep them even with the body of the mug. Keeping all parts of a piece at the same water content throughout the process, that is a key to successful drying.
The green strength of clay bodies is an important property, it makes them resistant to breakage or damage during handling in production.
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.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.
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.