Stoneware Pottery
The making of stoneware pottery seems to be a growing activity in the north, perhaps as many as a hundred Alaskans are spending considerable time at it. Some use imported clay, but many dig their own at Healy, Alaska, or other localities.
Plasticity is the essential feature of clay that makes pottery production possible. It derives from the plate-like shape of clay particles, very thin but elongated in two dimensions. This shape gives extensive surface area for adsorbing of thin layers of water molecules which bond together the clay particles but allow them to slip past each other when clay is thrown on a potter's wheel or otherwise fabricated into particular shapes. Very tiny, of the order of one-thousandth of a millimeter in length, clay particles result from weathering, grinding up and disintegration of rock materials. Two general types of clays exist: primary clays are those formed in place, and secondary clays are those transported by water and later laid down in beds. The secondary clays usually have greater plasticity than the primary clays. The clay layers intermixed with the Healy coal beds are highly plastic secondary clays.
The chemical composition of clay is similar to the average composition of rocks found at the earth's surface. Silicon, aluminum and iron oxides make up more than 80% of this average composition. A general formula for clay, considering it to be a mineral, is Al 2 O 3. 2SiO 2. 2H 2 O. Here the water contained is actually within the clay particles, other water molecules adsorb to the particle surfaces when clay is wetted. Clay takes up so much adsorbed water that when it is plastic enough to shape, it will be about one- fourth water. But all that added water only increases the volume by about 5%.
Once an object has been molded or formed on a potter's wheel from the plastic clay, it must be dried to a leather-hard state at room temperature to let most of the adsorbed water escape. The drying must proceed slowly and evenly to avoid warping and cracking during the shrinkage that occurs as water leaves.
Further drying occurs during the early stages of kiln firing. That process also must proceed slowly to avoid explosions due to steam trapped in the clay pore spaces. Starting at 350°C (660°F), the water chemically bound in the clay mineral emerges. When the temperature reaches 500°C (930°F), the water molecules are completely gone, leaving only the Al 2 O 3 and SiO 2 , and whatever impurities existed in the original clay body. Though the clay object was fragile when wet or leather-hard, it is even more fragile at this stage. Interestingly enough, the driving off of the chemically bound water molecules causes no additional shrinkage.
A critical event occurs as the temperature reaches 573°C (1063°F). Quartz (SiO 2 ) in the clay rearranges its molecular form at this temperature and a slight 2% volume increase occurs. If the temperature goes through 573°C too fast, cracking may occur.
After some hours of firing the temperature reaches approximately 1000°C (1800°F) and the clay object becomes red hot. Then the process of vitrification begins. Vitrification is a process wherein the chalklike clay material fuses into a dense glassy material, and long needle shaped aluminum silicate crystals grow to further bind the matrix together into a substance of great strength and cohesion. The object shrinks even more and becomes more dense.
Slow cooling, being especially careful of the critical temperature 573°C where a sudden 2% volume shrinkage occurs, brings the object back to room temperature. The object is now sturdy but does not hold water nor usually has it yet been decorated. After application of liquid glaze material the object is fired again, this time to even higher temperature, perhaps 1400°C (2500°F). The glaze vitrifies and bonds to the surface of the clay object and makes it waterproof. Both the final temperature and the glaze composition critically determine the final appearance.
The end result is only semi-predictable. The potter usually tries for a particular effect, but it is always an exciting moment when the kiln is opened after the final firing. The first sigh of relief accompanies the finding that nothing has exploded or melted down. Moans indicate bad glazing resulted, and smiles indicate satisfaction. Gleeful remarks mean that something better than expected happened--for reasons rarely understood.
