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source: BigCeramicStore.com
| Clay, the Drying and Firing process Many potters, especially beginners, use prepared clays without much thought. However, there comes a point when it is useful to understand the process that clay goes through during drying and firing. This can help troubleshoot many of the problems you experience with your pieces. Plus it is darn interesting! 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. This has been discussed in several previous tips. 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. Initial Kiln Drying Complete drying doesn't take place until the piece is in the kiln. This happens when the boiling point of water has been reached (100 degrees C, or 212 degrees F.) This must happen slowly, or the formation of steam within the body of the clay may cause it to burst. For this reason, the early stages of firing are done slowly, and with a peephole or lid open for steam to escape. Dehydration The next change which occurs is at about 350 degrees C (662 degrees F), the point where the chemically combined water of the clay is driven off. This is water that is part of the molecular structure of the clay, not the previously described water that is between the particles of the clay. This drying is completed by about 500 degrees C (932 degrees F). After this point you could no longer mix the dried clay with water to make new wet clay. An irreversible chemical change has taken place, known as dehydration. No shrinkage is observed during this stage. Burn-off Another thing which happens up to about 900 degrees C (1652 degrees F) is the burning off of organic and inorganic materials, such as carbon and sulphates. These are the fumes that it is important not to breathe, and the reason a kiln should be well ventilated even during bisque firing. Quartz Inversion After dehydration, the next change that happens is Quartz Inversion, which happens at 573 degrees C (1064 degrees F). At this point, quartz crystals rearrange themselves into a slightly different order. A slight and temporary increase in volume occurs at this point. This is why you always need some space around pieces during firing, as they will expand somewhat. Firing should proceed slowly during this Quartz inversion. A large percentage of ware that is cracked during firing happens from fast firing through this stage. The factory set program on electronic kilns usually slow down the firing at this stage for you.
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