It is important to have in depth understanding of the physical and chemical process clay goes through during drying and kiln firing. Having this understanding will help in avoiding clay and firing defects with your project and can help troubleshoot many of the problems you experience with your pieces such as cracking, breaking, exploding and glaze defects.
Shrinking
Clay shrinks both in drying and in firing. Different clay bodies shrink at different rates which can be as little as 4%, or as much as 15% for some clay bodies. Even one percentage point can make a difference in the final product depending on your need for precision. Your design, therefore, needs to account for the shrinking for the temperature to which you fire. For example of the clays we use at Lakeside Pottery is Standard’s Baldwin 192 and it is specified to shrink 11% for cone 6 firing temperature. Our testing showed that about 5% shrink during the drying process, 1/2% during bisque firing (cone 06) and 5.5% during glaze firing (cone 6). The total of 11% is shown in the lantern picture below (before and after).
We also found throughout after years of firing that even with the same clay body, shrinkage can vary 1-2% from batch to batch. Where accuracy is important, we run a batch test before proceeding with a project or tell our commission customers to expect +/- 3% variance.
Why is it important to let clay work dry slow?
Be aware that the larger to piece is, the more movement of shrinkage will take place. For a example, if clay shrinks 5% during drying, a piece of clay that is 5″ long, will shrink 1/4″ and a piece that is 20″ long, will shrink 1″ during the drying. This is why it is beneficial to know that the larger the piece is, the more important it becomes to let it dry evenly (slow drying will yield even drying). See next chapter about Air Drying. If one part drys faster than the other part, the dryer part will shrink more on wetter part which will create stresses on the clay, thus, cracking could occur.
Air Drying
Wet clay contains a large amount of water, a minimum of 25% water. When clay starts to dry, water evaporates from it. As this happens, the particles of clay are drawn closer together resulting in shrinkage. Many problems with clay are formed by uneven rates of drying, which create stresses in the clay. Sometimes these stress show up right away as cracks or warpage, other times not until during or even after firing. So it is important to ensure drying is even. This is done by ensuring uniform thicknesses throughout the piece, drying slowly, and even slowing down the drying of certain parts.
Clays which have very fine particle sizes will shrink more than clays with larger particle sizes. Porcelain clay has very fine particle sizes which makes it very plastic and also shrinks the most. These bodies have the most strength in the dry state. Groggy clays such as sculpture bodies shrink the least. (Grog is clay which as already been fired and then ground to various particle sizes.) These bodies shrink less because they have lower water content to start with, and also provide channels through which moisture can escape toward the surface. These are called “open bodies”.
When the water has evaporated form between the clay particles, and all the remaining clay particles are in contact, drying shrinkage is complete. This is called the leather hard stage. The particles themselves are still damp, but their drying will not cause any additional shrinkage.
All information on this post is a direct copy from the the website stated below:
http://www.lakesidepottery.com/HTML%20Text/Tips/Clay%20drying%20and%20firing%20process.htm
Technical Research 