Question:

How do I avoid S cracks in wheel thrown ware

Avoiding s cracks

Have you ever had trouble with cracks developing in the base of your wheel thrown work? This problem can be avoided by following a few simple guidelines.
Cracks develop for two main reasons: uneven drying and uneven compression in the throwing process.
 Avoid uneven drying
OK, so you put your wheel thrown work in a drying cupboard. You even additionally cover the work with plastic sheeting, trying to ensure even drying. But if you are throwing on a bat, no air reaches the bottom of a platter, vessel or other thrown object. What happens? The top dries a bit slower. As the top dries, it shrinks and pulls together, cracking the slightly wetter and therefor larger surface at the bottom. Once a crack has developed, it is near to impossible to get rid of, even by turning (trimming). One way to avoid this might be to turn over the work halfway in between the drying process, but this is not usually feasible. So what to do? How to get that bottom drying at a similar rate? The answer: throw your work on plaster bats. (How to make plaster bats will be the topic of a feature soon to come.)
The bat will need to be moistened, otherwise the clay will pop off too soon and throwing will become an impossibility. After throwing your work, the piece can be lifted off while still on the bat, and the bat, work and all, can be placed in your normal drying area. After a few hours, depending on moisture content of the bat, the work will pop off all by itself, and may even be ready for turning that same day.
 Avoid uneven compression when wheel throwing
Cracks may not appear until in the firing itself, often even until a second high temperature firing, especially in fine clays like porcelain or porcelaineous stoneware. Rougher clays like raku are generally not so prone to cracking, unless you really treat them badly, as the grog tends to absorb a lot of tension. For finer clays, the best technique to get a consistently compressed clay throughout is to center the clay on the wheelhead, then cut off the clay with a cutting wire. Invert the clay and recenter 'upside down'. This ensures that the clay will be quite compressed at the bottom as well as at the top.
Combined, these two techniques should be able to cure most cracking problems with wheel thrown work

Introduction

Wheel throwing with standard clay. Again, a kiln is required. And once you are an experienced thrower you will undoubtedly want a large kiln, since you will produce pieces much more quickly than in hand-building. However, at the beginning I would recommend joining a class if at all possible. You can learn to throw from books or even better, videotapes, and some people do. But it is difficult. It takes a while to get the knack for throwing, and can therefore get frustrating unless you have interaction with an instructor and see other students struggling as you are. I think most people with no experience, buying a wheel and trying it out on their own, would get frustrated and give up. A class will offer you instruction, a kiln, and glazes so you don't have to worry about that quite yet. Your only investment will be the cost of the class, and perhaps some materials costs such as clay and a basic tool set for under $20. At some point you may find that you don't like to be constrained by the hours of the class, that you want complete control over your work (so someone else doesn't smudge the glaze, break, damage or steal your pieces), or that you want to do things beyond the capability of our class studio. At this point I would recommend getting your own wheel, and kiln unless you have access to someone else's kiln. However, if you are patient, you could continue to build your throwing skills, never firing anything but instead recycling the clay to be thrown again. It is tempting to want to finish each piece at the beginning, because you are very proud of it, but soon you will hate the look of those early pieces! so not even firing them at all would be a fine strategy. While you are in the intermediate stage, assuming you are firing your work, I would recommend an extruder. The reason is that you can make use of the hundreds of small cylinders you will throw, by putting nice handles on them and making mugs. Otherwise practice making hand pulled handles (shown in most pottery technique books.)

avoiding cracks through uneven drying

Have you ever had trouble with cracks developing in the base of your wheel thrown work? This problem can be avoided by following a few simple guidelines.

Cracks develop for two main reasons: uneven drying and uneven compression in the throwing process.

Avoid uneven drying
OK, so you put your wheel thrown work in a drying cupboard. You even additionally cover the work with plastic sheeting, trying to ensure even drying. But if you are throwing on a bat, no air reaches the bottom of a platter, vessel or other thrown object. What happens? The top dries a bit slower. As the top dries, it shrinks and pulls together, cracking the slightly wetter and therefor larger surface at the bottom. Once a crack has developed, it is near to impossible to get rid of, even by turning (trimming). One way to avoid this might be to turn over the work halfway in between the drying process, but this is not usually feasible. So what to do? How to get that bottom drying at a similar rate? The answer: throw your work on plaster bats. (How to make plaster bats will be the topic of a feature soon to come.)

The bat will need to be moistened, otherwise the clay will pop off too soon and throwing will become an impossibility. After throwing your work, the piece can be lifted off while still on the bat, and the bat, work and all, can be placed in your normal drying area. After a few hours, depending on moisture content of the bat, the work will pop off all by itself, and may even be ready for turning that same day.

Avoid uneven compression when wheel throwing
Cracks may not appear until in the firing itself, often even until a second high temperature firing, especially in fine clays like porcelain or porcelaineous stoneware. Rougher clays like raku are generally not so prone to cracking, unless you really treat them badly, as the grog tends to absorb a lot of tension. For finer clays, the best technique to get a consistently compressed clay throughout is to center the clay on the wheelhead, then cut off the clay with a cutting wire. Invert the clay and recenter 'upside down'. This ensures that the clay will be quite compressed at the bottom as well as at the top.

Combined, these two techniques should be able to cure most cracking problems with wheel thrown work.