S-Cracks and How to Avoid Them

-cracks are something almost anyone working on a potter's wheel will have experienced at some stage or another.

What is an s-crack?
An s-crack is a crack that appears in the maturation firing. It has the shape of an 'S'. It can happen with stoneware, but also with lower fired earthenware clays. Porcelains can especially be a problem, due to their delicate nature. Typically s-cracks appear at the bottom center of a vessel form, e.g. a bowl, vase, mug and sometimes even in saucers, plates and platters. S-cracks appear as an 'S' because of the way we compress the clay on the wheel in a circular fashion. The base is often not as compressed as the rest of the vessel. Tension in the clay then causes it to split in the firing, generating the crack. This problem is normally not visible in the bisque firing, although s-cracks may sometimes appear even in the drying process. Uneven drying may also contribute to the problem.

What causes an s-crack?
There are several causes that can contribute to s-cracking. They are as follows:

• Insufficient wedging (to a lesser degree)
• Uneven compression in the throwing
• Water accumulating at left too long in the bottom of the vessel

How can s-cracking be avoided?
There are a few simple things we can do to avoid this unnecessary problem altogether. I don't recommend wedging the clay a lot. Actually I wedge very little - - just enough to soften up the clay and get a good enough, even consistency. The trick lies on what we do with the clay on the wheel. There are two methods to beat the s-crack. The first, which I have not tested myself, but which is supposed to work, is to center your clay with the wedging sideways on the wheel, i.e. the wedging spiral will be lying down on its side. This will help to even out the compression in the base. My own tried and test method (believe me, I used to have s-cracks in my porcelain, but not once since I use this method!) is to center the clay on the wheel. Bring it up and compress it nicely at least once. Then cut the lump off the wheel, turn it around on it's head and recenter it. The bottom portion of the clay is now as compressed as the top will ever be.

The other important thing to remember is to remove excess water from the bottom of your vessel. Using the above methods are fail-safe in avoiding s-cracks, but not if you let water sit in the bottom for a long time. Get a piece of sponge and tie it to a chopstick with some cotton. This is a handy tool for getting down into long-necked vessels, where you can't reach in with a larger sponge in your hand. Make it a habit to remove water or slip that has collected immediately. S-cracking should never be a problem again!

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