Glazing and Firing Pottery
Once a pot is built and comes off the wheel it needs to sit and dry until it's leather hard, meaning it's still a little damp, but can be handled without changing its form. At this point, you can trim off any excess clay and carve details into the piece. Pottery at this stage, called greenware, is very fragile and needs to be handled with care.
The next step is to put the piece into the kiln for the first round of firing, called a bisque firing. The purpose of this initial firing is to turn your pottery into ceramic material. The firing process is measured in cones, a standard unit of measurement that accounts for time and temperature. It's important that the temperature rises slowly and cools slowly. Failure to do so could cause the piece to burst, putting you back at square one. Many kilns have programmable cone settings to help prevent this.
After the bisque firing, you want the piece to be strong enough that it doesn't fall apart during glazing, but porous enough to accept the glaze. This allows it to bake without completely drying out. The temperature of a bisque firing typically ranges between 1700 and 1900 degrees Fahrenheit (around 926 to 1038 Celsius). This is the equivalent of 05 to 04 cones. If you went to a craft store where you can paint your own pottery, you'd be decorating the product of a bisque firing, or bisqueware.
Now comes the fun part -- you get to decorate or color your work of art by painting or glazing. Painting is pretty straightforward -- all you need is acrylic paint and your imagination. Glazing is more complex, but we can offer an easy-to-understand overview. Glazes consist of silica, fluxes and aluminum oxide. Silica is the structural material for the glaze and if you heat it high enough it can turn to glass. Its melting temperature is too high for ceramic kilns, so silica is combined with fluxes, substances that prevent oxidation, to lower the melting point. Aluminum oxide is used as a stiffening agent, allowing the glaze to adhere to the surface of a bowl or vase without run off. Glazes get their colors from a wide variety of mineral oxides.
Using glazes requires a lot of experimentation and practice. Many factors, like the kind of kiln or the kind of clay you use, impact the final result. Glazes can be applied with a brush or the entire piece can be carefully dipped into a glaze bath. Glazes often require multiple coats and a lot of patience to get them just right. When that time comes and the piece is dry, you're ready for the glaze firing, where the pottery is heated to maturity. Next, we'll talk about different types of kilns.