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If you've ever open the kiln and found a big crack on your favorite piece, you know how frustrating it is. And you've probably wondered what you did wrong. There are many reasons why pots crack, and this should give you some ideas of how to prevent such cracks.
In general, cracks result from stresses in the clay. There is always some stress in clay because of the fact that it shrinks as it dries and when it is fired, and it also expands and contracts during firing. Sometimes the stress is too much for the clay to handle and it cracks.
The way the piece is made can either minimize cracking, or contribute to cracking. For example, if you sit an unfired pot inside down (on its rim), you will put stress on the rim. A crack may not appear right away, but it could show up later as the pot dries more or when it is fired. So it's a good idea when turning pots upside down to place the rim on a piece of soft foam.
Different clays can handle different amounts of stress without cracking.
The form of the pot may encourage cracking. For example, sharp corners concentrate stress and are more prone to cracking.
Fast drying will tend to cause more stress than slow drying.
Overfiring a pot, or firing it multiple times will tend to make it more subject to cracking.
Variations in thickness will set up stresses, since the thin areas will dry faster than the thick, and stresses won't be evenly distributed. This is very common when the base of a pot is thicker or thinner than the walls. Sometimes this will show up as a circular crack around the bottom edge of a pot.
A thick layer of glaze on the inside of a pot, and a thin or no layer on the outside will cause stress. The result is often a spiral crack up the sides.
If glaze pools on the inside of a pot, tension is created and the pot may crack or split across the base.
Sometimes a piece of glaze will crack off, normally near a rim or at edges. Some clay may be attached to the glaze piece that cracks off. This occurs because stress has built up between the clay and glaze that can't be absorbed. It is often caused by over-sponging which takes away the fine clay particles and leaves behind the groggier clay particles which are not elastic enough to absorb the stress.
A network of very fine cracks in the glaze is called crazing. It is caused by a mis-match between the clay and glaze. It often will not show up until the pot is cooled, or sometimes even until it has been heated and cooled a few times. Some people believe slow cooling will prevent crazing, but the stresses still exist and eventually the crazing would occur.
Dunting is a special type of crack which occurs from stresses caused during firing and cooling. These stresses primarily occur during two critical points of firing called silica inversions which occur at 1063 degrees F (573 degrees C), and 439 degrees F (226 degrees C). At these inversion points, the structure of the silica molecules rearranges. It is important to fire slowly through these two temperatures, and electronic kiln profiles often do this for you automatically while they are heating.
Most dunting however is caused in cooling. These cracks appear as long, clean, body cracks with sharp edges. If the ware is glazed, the glaze edges are sharp. They may be vertical, horizontal, or spiral.
There are 3 main reasons why cooling dunts occur.
The first occurs as you cool through the first silica inversion at 1063 degrees F. At this inversion the body contracts suddenly. The more silica (quartz) in the body, the more contraction. Since different parts of the pot reach this temperature at different times, it doesn't all contract together, and that causes stresses which can crack. Take for example a tall pot. The top will cool much faster than the bottom, because the bottom has the whole temperature of the kiln shelf keeping it warm. So the top will cool faster than the bottom, causing a crack around the bottom wall.
The second occurs as you cool through the 439 degree F inversion. A similar thing happens as above. However, potters sometimes like to open their kilns at about this temperature to see their pots, and they can make this much worse.
The third type of cooling dunt occurs months or even years after firing. For example, a pot might split right in half after 3 months. This is likely the result of thermal shock. In this case the clay and glaze expand at different rates when exposed to temperature variation, and this change causes the object to crack. To be more specific, the body has contracts more than the glaze. If the glaze is weaker it will shiver (see above). If the clay is weaker the object will crack.
One of the most common cracks found in pots is the "s" crack, which occurs at the bottom of a pot, in the shape of an s, usually on thrown pieces. I have heard so many theories about this and how to prevent it, that I am not even getting into it now. Perhaps later I will do a whole tip on the different theories so you can decide for yourself! The one thing everyone agrees on is that you should keep the bottom of the pot as dry as possible while throwing, and compress the bottom during throwing and trimming.
To troubleshoot a Crack
If you have a crack, find the point where it is largest (widest). This will be the point where the crack started, and can help you understand what happened. Cracks in the rim usually were caused by stresses in the raw stage. Cracks in the base usually occur in the firing.
Another way to determine the cause of a crack is to look at the surrounding glaze. If the glaze at the end of the crack is sharp, it cracked in the later stages of firing, probably during cooling. If the glaze is round at the edge of the crack, the crack probably occurred early in firing and the glaze had time to heal over.
This just touches on some of the reasons ware can crack. But obviously most clay is pretty tolerant, or you would get a lot more cracks and it would be very annoying! Remember that most cracks occur from stresses that occur during drying, even if they don't show up until later. But some cracks occur because of firing and cooling, so it is usually best to fire and cool slowly.