How To Fire Clay:
Decorative and Finishing Techniques
Over the next few weeks we will focus on the basics of glazes, including choosing, mixing, applying, and firing them… basically we will learn how to fire clay. Many books focus on the topic of glaze mixing and others focus on the decorative applications of glaze. These will be a lot of fun to read once you become comfortable with simple glaze preparation.
Glaze is a glass-like coating that is applied to ceramic ware and fired to cover the surface of the clay. Practically, it makes a pot nonporous, but it has many decorative applications, too . The glaze mi”xing and firing aspects of the ceramic cycle require a scientific approach that will aid your understanding of h ow to work with th e materials. Fear not, the scie nce can be fairly basic, but it can evolve into a very precise chemical process if you choose to i nvestigate glaze chemistry at a deeper level.
A very important thing to consider when choosing a glaze is the maturing temperature of the clay being used. This is important for several reasons. First, a clay body should be fired to its maturing temperature to achieve the strongest density in the body for durability. Second, the glaze should have a good fit on the fired clay. When melted, glaze should expand an d co ntract with the clay body and not craze, crawl, or run off the pot during the firing.
Sometimes a glaze will shrink more than the clay and create an interrupted surface. (For functional ware this is undesirable yet for aesthetic reasons some potters choose to work with this discrepancy.) And third, if you put a high-fire glaze on low-fire clay and high fire it, the clay will melt into a puddle dur ing the glaze firing. Conversely if you put a low fire glaze on a high-fire pot and over fire the glaze it will run off the pot onto the kiln shelf and create a puddle of glaze on the shelf. A good way to avoid some of these problems is to think about the type of work you want to make, talk to the ceramic supplier, and begin with the appro priate clay for a particular temperature range of glazes.