The signs of trouble often show up before there is actually a crack visible. Take a small handbuilt bowl, for instance, that may look fine while it is still soft. Yet, the clay could already be unhappy with it because of areas of uneven thickness, poor joins, air rushing over one side of the bowl but not the other. This isn’t a downtime period. It’s an essential part of the creation process that requires the same degree of attention as when you are building, smoothing and/or glazing.
As clay dries, it loses water and shrinks. This shrinking causes clay that’s drying at a different rate than another area to pull away, creating stress in the clay. When this stress is great enough, a crack will happen, often at the base, near a handle, around a foot, or where thin rim meets the body. The most problematic areas are thick bases and thin walls because they don’t dry at the same speed, resulting in a crack that starts at the thick portion of the bowl. A bowl built with uneven walls and a thick base may make it through to the greenware stage, but it will be a challenge to keep the cracks from opening.
Before placing the ceramic piece to dry, examine the areas where the thickness of clay changes or where it’s joined with other clay sections. Look closely at corners, handles, foot, embellishments and coil joins. These are all places that will stress during drying. If an area of clay has not been adequately scored and slip added, it might separate as drying causes the clay to shrink in on itself. Just because a seam looks well-blended doesn’t mean that it won’t crack. Check any seam, joint or added piece of clay for weakness. Run a fingernail gently over seams and joints to look for weak spots or lifting areas and check to see that nothing will lift from the main body.
Don’t forget to look at the bottom of a bowl. Carefully lift and tilt the bowl to check if the bottom is excessively heavy. A thick bottom will hold water and stay wet longer than thin walls or base of the same clay body. If the piece is in the early to mid stages of drying, smooth out the weight of the bottom of the form with gentle finger pressure. Once the clay reaches the leather-hard state, be careful about removing clay in the bottom because you don’t want to remove so much that you make the wall thickness too thin.
Covering work to dry is more than a dust protection step. Covering a piece allows for a more even drying rate by allowing air and moisture to escape. Cover the clay loosely with plastic, damp cloths, or a plastic bag or place inside a large trash bag with a few holes punched in it. You want the whole piece to change more evenly so you don’t seal the piece completely from all movement of moisture. This is especially important for slab work, coil-built forms, tall pieces, or pieces that have added clay embellishments. Don’t expose the work to the direct sunlight, heat, or a draft from the fan or AC vent. Slow, more even drying helps keep a piece together.
Check a piece two or more times. Immediately after forming and right after the first drying stage when the clay has lost some of its free moisture. In the beginning, check to make sure walls are even and there are no sharp corners. After drying stage one, look for any curling or dry spots or rim that seems drier than the bottom. At this point, you still have enough work to do with the piece to change it up before you’re left with an unsalvageable bowl. You will probably have to wait and let the clay fully dry to see if your corrections will solve any future cracks. This early drying phase is a good time to make note of areas you may need to add clay to, work on more carefully, or slow down your drying time.
If you find a crack, learn from what you discover about the clay body and what you were doing wrong when it happened. A crack near or through the handle of a pot indicates a poorly attached piece of clay that separated. A crack across the bottom may point to heavy thickness. A split along a slab corner may show that the edge dried too fast or was compressed poorly. Your next bowl will benefit from knowing what thickness and how long to let the clay air dry so your handbuilt bowl won’t crack at the greenware stage.