At the studio we see objects warp and crack even before they have gone into the kiln. After hours of hard work making ceramic objects, members are disappointed when handles come off or cracks appear in the base of their bowls. Most of the time the problem is simply to do with how they dried their ceramic object. To solve problems of warping or cracking, consider two features of the drying stage, the shrinkage and rate of drying.
When pottery is drying on the shelf, the evaporating water will cause the object to shrink in size. The leather hard stage simply means that the water has evaporated from between the clay particles and the remaining clay particles are in contact. Shrinking and drying go hand in hand and how fast a clay object dries? and how much the clay will shrink? will affect how the objects survives in the kiln.
There are two types of water in your clay, physical and chemical water. Letting the pot dry on a shelf will remove physical water. Chemical water starts to evaporate from your ceramics at about 310C and continues through about 510C. Chemical water is formed as the heat of the kiln begins transforming the clay and molecules start to rearrange themselves also rearranging some of the water. At the leather hard stage, the particles themselves are still damp and have chemical water in them, but at 510C there is not much additional shrinkage happening.
Troubleshooting for cracking and warping.
First, good throwing and trimming is required. Pottery must be evenly thick throughout, and well-compressed as it is thrown. A thin and think areas will crack, because these cause uneven drying and shrinking, which the clay cannot stand.
When one part dries faster than the other part, the dryer part will shrink more than the damper part which will create stresses on the clay which causes cracking.
Slabs and tiles are especially prone to warping and cracking because they usually have only one surface exposed to the air causing the slab to dry unevenly. Roll slabs or tiles on sheets of newspaper. Once rolled, transfer the slab or tiles to the first piece of drywall. Cover with another sheet of newspaper, then place the second piece of drywall on top.
Edges are thinner and completely surrounded by air making them vulnerable to fast or uneven drying that causes warping and cracking. Protect thin edges by tearing up plastic strips and place them on the rims of still-damp pots to slow the drying process. Another way to slow the drying of rims and edges is to temporarily put strips of clay on the edges and removing them after drying is complete.
It’s worth your time and trouble to protect your work while it dries to insure your
pottery survives the ceramics process
Guidelines for successful drying
An easy way to test for dryness is to hold the clay piece against your cheek, if it feels cool it is not dry.
Slow and even drying is best way to dry all ceramics.
Air must freely circulate around each object.
Avoid using heaters or hair driers that can cause severe cracking
Cover objects loosely with plastic to avoid quick drying.
Dry pots upside down to even out the drying process.
Wrap handles and rims with plastic to help equalize drying.
If there is high humidity cover the piece with newspaper first, to allow the newspaper to absorb any condensation, then cover with plastic.
remove any inflexible frames before drying beyond the leather-hard stage.