Once the pot is formed, it is put into a kiln for a bisque firing. This initial firing of the pot is to hold the shape and make it easier to decorate, and is usually at 1800 degrees Fahrenheit or higher. Some pieces are finished at this point, but many are glazed. Glazes come in a variety of colors, and each needs a specific temperature for the chemical reaction to occur that binds the glaze to the clay and turns it to glass.
The many ways of covering a pot with glaze is a big part of the artistic aspect to creating a pot. If applied thinly it lets the color of the clay show through, or thickly makes the finish smooth, but doesn't allow as many fine details to show through. Glaze can be painted onto a pot, a pot can be dipped in glaze, and creative artists will find a variety of interesting ways to apply the glaze that give different effects to the final piece.
A piece can be fired many times to achieve different effects. Some artists like to paint only in certain places, fire the piece so the glaze melts into glass, and then once it cools paint more over that and fire it again, repeating until they are satisfied. This can create different textures and very unique designs.
Defects in Pottery and Glaze
With such an involved and complex process to making each individual pot, it's not uncommon for there to be defects in the finished piece. There can be too much air in the clay, an incorrect mixture for the glaze or clay, too hot or too cool of a kiln, opening the kiln too soon or too late after firing is complete, using incompatible glazes, the amount of glaze used or even simple human error can all have a massive impact on how a ceramic or pottery piece looks once it is complete.
When clay is fired in a kiln it shrinks by a significant amount because of the loss of water. Glaze is typically engineered to shrink or expand as well, but sometimes the shrinkage of the clay doesn't match that of the glaze. This can result in crazing or peeling of the glaze. Crazing results when the glaze shrinks more than the pot, and results in spider-like cracks in the glaze, but not the pot.
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Other Methods and Pottery Safety
There are many different ways to achieve a finish a pot aside from using a powder or liquid glaze, and one such now out of use process is called Salt Glazing. Salt glazing is when common salt is thrown into a kiln at high temperature, which causes the salt and the silica in the clay to react to each other. Salt glazing is no longer in use because, while it was popular for some time, environmental cleanliness initiatives stopped most production with this method.
The selection of glaze determines how the pot can be used. Some glazes, while nice for decoration and beautiful to look at, would be toxic to eat from because of chemicals that would leech into food and be harmful once ingested. These chemicals are safe to be around and, with precautions, safe for the artist to work with.
The highest regulated hazardous materials in glaze are lead and cadmium. When buying pottery or ceramics, try to keep in mind what you intend to use it for. If you are looking for a dish to eat or serve from, be sure that the piece is considered safe to eat from, and if you have a ceramic dish, never clean it in a dishwasher. Many glazed pieces will have spots with no glaze, where they were rested inside the kiln, and these spots will let water into the clay, which will crack or crumble.
If you are interested in finding out more about ceramics and pottery or about artists who work in clay, do research! Contact your local pottery guild, or here are a list of artists to get you started!
Beate Kuhn - German
Heo Jin Kyu - Korean
Janis Mars Wunderlich - American
Halima Cassell - British
Maria Geszler-Garzuly - Hungarian
Shao Ting-Ju - Taiwanese
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