![]() This drying is completed by about 500 degrees C (932 degrees F). 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. The next change which occurs during the firing process is at about 350 degrees C (662 degrees F), the point where the chemically combined water of the clay is driven off. For this reason, the early stages of firing clay are done slowly, and with a peephole or lid open for steam to escape. This must happen slowly, or the formation of steam within the body of the clay may cause it to burst. ![]() This happens when the boiling point of water has been reached (100 degrees C, or 212 degrees F). The particles themselves are still damp, but their drying will not cause any additional shrinkage.Ĭomplete drying doesn't take place until the piece is in the kiln. When the water has evaporated from between the clay particles, and all the remaining clay particles are in contact, drying shrinkage is complete. 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. Grog is clay which as already been fired and then ground to various particle sizes. Groggy clays such as sculpture bodies shrink the least. These bodies have the most strength in the dry state. Porcelain clay has very fine particle sizes which makes it very plastic and also shrinks the most. This has been discussed in several previous clay tips.Ĭlays which have very fine particle sizes will shrink more than clays with larger particle sizes. This is done by ensuring uniform thicknesses throughout the piece, drying slowly, and even slowing down the drying of certain parts. So it is important to ensure drying is even. Sometimes these stress show up right away as cracks or warpage, other times not until during or even after firing clay. ![]() Many problems with clay are formed by uneven rates of drying, which create stresses in the clay. As this happens, the particles of clay are drawn closer together resulting in shrinkage. When clay starts to dry, water evaporates from it. Wet clay contains a large amount of water, a minimum of 25%, actually. This can help troubleshoot many of the problems you experience with your pieces. However, there comes a point when it is useful to understand the process that clay goes through during drying and firing. Many potters, especially beginners, use prepared clays without much thought.
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