What is the cause of the same lump of clay to behave like a friend some of the time and then like an enemy at others? Moisture is usually the reason. Clay is not a static material when it is being worked and manipulated; it changes while we handle it in our hand, it changes while it is resting on our board, it changes when its rims are exposed to air, and it changes when it is wiped with a damp sponge. These subtle changes affect the outcome of a handmade plate or bowl, sometimes in a beneficial way, more frequently in a detrimental one.
Clay that is very wet is easier to manipulate but it doesn’t hold its form very well. This can be an advantage when throwing an open pinch bowl or pulling an almost transparent thin wall, but it can be a disadvantage when throwing a slab plate or a flat plate. A very wet slab, for example, might sag or bend when picked up or lifted from the board, while a rim that is overworked will not be able to hold a good curve or shape when a shallow bowl is thrown. The very wet clay will also react and change too quickly; it will show too many fingerprints and too many dents, and will stretch and weaken too quickly when being handled.
A firmer or less plastic clay has a different behavior as well. It allows a very clean sharp rim, it can be trimmed, tucked, cut, carved, or scored to add texture to the surface. It also provides more resistance when you attempt to flatten a wall, to straighten a rim that is bent out of alignment. While this is helpful for a skilled potter, it can be a hindrance to a beginner who is used to wetter clay; when a beginning student has been working a rim of a plate and it has become stiff to the point where it cracks when pushed in, they may not know how to handle that situation. If they add more water to dry leather-hard clay, it will just smoothen and smear the rim rather than repair it.
Moisture changes more quickly on thinner areas like the rim of a small plate than it does on the thicker base. It also changes much faster around a thin rim of a small plate than it does around the thick base of a foot ring. This means that the rim might dry faster and crack, or it might warp and twist, while the base of the plate remains wet or still plastic. A decorative detail on an otherwise flat plate can sometimes warp when it is drying more slowly than the flat area around it. This is a significant cause for cracking and warping in handmade plates. The pot is affected not only by its shape but also by the moisture level that is drying or evaporating off the wet clay form.
To help understand the effects of drying clay, make a couple small flat tile and a shallow pinch bowl or small plate, and pause and check on the form at several different points and then resume working after a short period of time has passed. At the first checkpoint, check the softness and plasticity of the clay, note how a finger presses in, check how the wall holds when a form is being pulled or lifted, and check how it is able to maintain its shape as the clay begins to shrink and settle. At the second stage, when the form is somewhat more rigid but not hard or stiff, check the firmness of the form; can the plate rim still be smoothed and tightened without buckling the wall or cracking?
A very wet sponge that is squeezed with the thumb or fingers and applied with repeated pressure on the form will not only change the behavior of the clay, it can weaken the area and cause it to buckle or crack in ways that may be difficult to repair. Try this with your fingers; press a very wet sponge with your thumb against the clay. If the surface becomes sticky or smeary, the clay is too wet to be smoothed or manipulated in the current way. Try using a damp sponge and pressing with your thumb and fingers to smooth or tighten and then press firmly with your thumb or finger to flatten the wall; this may result in buckling, warping, or cracking if the sponge or water was not damp enough or too wet to smooth the wall.
When you are having problems with your forms, check for problems with moisture. Does the wall crack when pressed or bent? Does a plate warp when it is left to dry? Is the rim cracking or buckling? Does the surface look smearing or sticky, or does it shrink or pull away from the rim? Are the walls too soft? Does the clay feel dry or brittle? Does the clay feel too stiff or unworkable? Can it be smoothed or tightened without cracking, buckling, or warping? These are the questions you want to ask yourself when the plate or bowl is not performing as you want. Once the problem is identified, then you can determine how to respond with better moisture control and timing; then the plate or bowl will behave in a much more consistent and predictable way. The better the timing, the better the rest of the process will be as well; from trimming to texturing to glazing and firing.