Pottery (nothing to do with Harry)

I majored in journalism in university but I minored in art. (Well, actually I minored in Dungeons and Dragons, but the university refused to give me credit for it. Go figure.)

Of all my art classes, my favorite was pottery. I loved making pots, but as a certain hideously ugly four-kilogram cream pitcher can attest, I was never very good at it.

It’s been a while since I made any pots. However, I now have a potter in the family: my sister-in-law, Cindy Clarke, is a professional potter, and my oldest brother, Jim, is perforce also involved in the throwing, glazing and firing of clay. They own a gallery, Out of the Fire Studio , on Jasper Avenue in Edmonton.

Fortunately for their business, they’re both far better at the shaping of clay than I.

Pottery, one of the oldest human crafts, is also known as ceramics, which comes from the Greek word “keramos,” or clay–and indeed, pottery begins with clay.

Clay is made up of fine, plate-like crystals of hydrated aluminosilicates, about one to 10 microns (a micron is 1/1000th of a millimetre) long. A thin film of water binds these crystals together, and the lubrication of the water, plus their plate-like shape, enables them to slide easily over one another.

Because of that, clay can be manipulated in many ways: it can be flattened into a or liquefied and poured into one; rolled into coils; pinched; carved; or, as some unsung genius discovered in the fourth millennium B.C., spun on a wheel and there shaped into beautifully symmetrical vessels.

Wet clay falls apart under pressure, but sometime or other some other genius decided to try to harden a bowl by sticking it in the fire.

If enough heat is applied to clay, its crystals begin to melt together to form a single, coherent mass–a process called sintering. At low temperatures (900 to 1200 degrees Celsius) relatively large, continuous openings remain in this material. That produces the porous pottery called earthenware.

At temperatures between 1200 and 1280 degrees, clay becomes stoneware: much more durable and, because the continued heating seals the pores, waterproof. The ancient Chinese made stoneware, but it didn’t reach northern Europe until after the Renaissance.

Finally, there’s porcelain, fired at 1280 to 1400 degrees. The Chinese were also the first to make this hard, glassy, white form of pottery, which is why it’s also called “China.” Porcelain begins with a mixture of a pure, white clay called “kaolin” (which is actually decomposed granite), feldspar and silica sand. During firing, the feldspar melts and coats the clay crystals, which turn into fine, needle-shaped crystals called mullite. Meanwhile, the grains of sand partially dissolve in the feldspar. When all this cools, you get the stuff from which teacups are made.

Porcelain is glassy and waterproof on its own, but earthenware has to be glazed if it’s going to hold liquid. Glazing was always my favorite part of pottery class, probably because it was more like chemistry than art: you mixed a variety of exotic ingredients in an attempt to create a particular effect.

Glazes basically consist of powdered glass, with materials such as lead, soda or tin added to lower the glass’s melting point, and metal oxides added for color. Raw glazes look like thick soup, but after firing they make a glassy coating.

The color depends on the ingredients used and how the glaze is fired. In fact, variations in firing can produce so many variations in the glaze that it’s very difficult to create a glaze that does just what you want–although fortunately, one of the great things about making pottery is that unforeseen effects can look even better than what you had in mind.

Ceramics continue to be used for plates and bowls and pots, but they’re also used for everything from protecting the space shuttle from the heat of re-entry to replacing parts of our bones and teeth. Someday you may be able to buy an all-ceramic automobile engine.

Considering my four-kilogram cream pitcher, though, it might be best if I’m not the one asked to build it.

Maybe Cindy and Jim would give it a try.

Permanent link to this article: https://edwardwillett.com/2007/06/pottery-nothing-to-do-with-harry/

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Easy AdSense Pro by Unreal