When I was a kid, the street in front of me was torn up for reconstruction, and we neighborhood hoodlums dug in the exposed clay, a novelty for Iowans, and used it to make pots. We were engaging in some of the most basic chemistry, that is, allowing something to lose water and change its chemical structure. Most of chemistry is simply rearranging things and making pottery is no exception.
Clay begins as rocks which are dissolved by rain and water. The elements within are aluminum and silicon oxides that are held together with water. When the water is removed, the rocks reform as pottery. The most useful form of clay is kaolinite in which the aluminum and silicon oxides are in equal proportions.
Handling clay by throwing it on a wheel or pressing it together removes some of the water. With the water gone, the silicon and aluminum bond through the oxygens and not through wet hydrogen bonds. The new bonds are stronger. Firing the pot will drive off more water and change the chemical structure of the kaolinite from sheets to an amorphous glass.This process removes and locks out any water that might permeate the surface. Stoneware is fired at a higher temperature and contains less (no) water and is stronger than earthenware. Containing no water, it doesn’t heat up in the microwave.
Above: an earthenware bowl from Sunflower Pottery. http://www.sunflowerpottery.com/home.html
Glazes are made from quartz and corundum, colorless forms of silica and aluminum oxides. Transition metals which take on various colors depending on their oxidation state and bonding, are added for color, and once again, firing the glaze allows bonds to form.
Of course, none of these technicalities can describe the art that goes into making beautiful pottery. Most recently, I visited Pewabic Pottery in Detroit.
One thing I love about pottery is that you can use it. It appeals to the practical side of me and the artistic side. Even the roughest of mornings can be brightened with the right tableware. And now, it’s time for breakfast.
Thank you to the Royal Society of Chemistry for this article