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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 Dry Clay
Wet clay contains a large amount of water, a minimum of 25%, actually. 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 clay. 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 clay 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 from 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 clay are done slowly, and with a peephole or lid open for steam to escape.
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. 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.
Another thing which happens up to about 900 degrees C (1652 degrees F) is the burning off of organic and inorganic materials, such as sulphates and carbon. It is important not to breathe these fumes, and that's the reason a kiln should be well ventilated even during bisque clay firing.
After dehydration, the next change that happens during the firing process 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 the clay firing process happens from fast firing through this stage. The factory set program on electronic kilns usually slow down the firing at this stage for you.
Shrinkage happens at the vitrification stage. This is due to diminished size of the particles as they approach fusion and to the closer arrangement of particles in their glassy matrix. The firing shrinkage of a clay is usually about the same as the drying shrinkage. Total shrinkage will usually be about 8-12%.
The next stage that happens during the firing process is vitrification. This is the hardening, tightening and finally the partial glassification of the clay. Vitrification results from fusions or melting of the various components of the clay. The strength of fired clay is increased by the formation of new crystalline growth within the clay body, particularly the growth of mullite crystals. Mullite is an aluminum silicate characterized by a long needlelike crystal. These lace the structure together, giving it cohesion and strength.
Clays vitrify at various temperatures depending upon their composition. A red clay high in iron and other impurities might fire to hardness at about 1000 degrees C (1832 degrees F) and melt to liquid at 1250 degrees C (2282 degrees F). A kaolin body which is very free from impurities might not melt until over 1800 degrees C (3272 degrees F)! By mixing the ratios of different types of clays that melt at different temperatures, clay bodies are developed for different firing temperatures.
If you fired high enough, the clay would first swell up (bloat) then fuse into a liquid which would cool as a glass. Of course in ceramics we don't fire that hot; we stop at the point where we have just enough fusion and hardness for durability, but not too much so we cause deformation or melting of the ware. This point is called the maturing of the clay.
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