Firing clay is the most critical part of the ceramics process because it is the one thing that makes clay durable, hence ceramic. Electric kiln firing is one of the most common methods for firing clay because electric pottery kilns are readily available and simple to install.
In this post, we are sharing some of the principles of firing clay and getting the best results with electric kilns. –Jennifer Poellot Harnetty, editor.
Firing Clay – From Mud to Ceramic
Firing converts ceramic work from weak clay into a strong, durable, crystalline glasslike form. Ceramic work is typically fired twice: it is bisque fired and then glaze fired. The goal of bisque firing is to convert greenware to a durable, semi-vitrified porous stage where it can be safely handled during the glazing and decorating process. It also burns out carbonaceous materials (organic materials in the clay, paper, etc.). As the temperature in a kiln rises, many changes take place in the clay.
Firing Clay – More Science
Heat in an electric kiln is transferred in three ways:
1.) conduction heat transferred through physical contact
2.) convection heat rising through the air
3.) radiation heat emanating from all the kiln elements
Electricity passing through coiled heating elements (made especially for high temperatures) generates radiant heat, which rises and is absorbed by everything in the kiln.
1Firing Clay – How Hot
All clays and glazes are formulated to mature at certain temperatures. Firing clay too high can cause it to deform or even melt, too low and it will not be durable. Firing glazes too high can cause run-off on the pot, too low and they will be dry and rough. To fire to the right temperature, pyrometric cones are used. Cones are made from various oxide mixtures and bend at known temperatures. In general, the following cones are used in the pottery studio: bisque fire (cone 08-05), low fire (cone 06-04), mid-range (cone 4-7) and high fire (cone 8-10).
Firing Clay – Using Pyrometric Cones
Cones are used in every firing. Typically, a three-cone system (either large or self-supporting), consisting of a guide cone that is one cone below the target temperature, the firing cone and a guard cone provides the best information about the firing. Bar cones and small cones are used in a properly adjusted Kiln-Sitter(r), an automatic shut-off device. While the three large cones are not required for kilns equipped with a KilnSitter or an automatic controller, they do provide a second point of reference for how a kiln is operating.
Get Ready to Fire your Kiln
Before firing any kiln, vacuum it out if necessary-bottom, sides, element channels and lid. Check the elements for breaks, and chisel off any glaze drips on the shelves. Visually check the electrical cords and connections. Make any repairs required (see owners manual or call your local supplier for service).
An assortment of kiln furniture is needed to hold and support ware during a firing. Furniture consists of shelves, posts, stilts and tile setters made from refractory materials. Kiln furniture is designed to withstand the repeated heating and cooling to high temperatures without deforming.
The Bisque Load
Loading a bisque kiln is a fairly simple task, but there are some basic rules. Fire full loads to take advantage of conduction heating and also save electricity. All work should be bone dry . If the work is cool or cold to the touch, it is not bone dry. Handle all work very carefully because it is extremely fragile at this stage. Place the bottom shelf on 1-inch stilts to aid circulation, and keep ware 1 inch away from elements, walls, thermocouple and KilnSitter (figure 6). Unglazed pieces may touch each other. Place a small cone in the KilnSitter and/or a cone pad on the middle shelf. Fire to cone 08-05, depending on the type of clay and amount of porosity you want for glazing.
Firing is a potentially hazardous activity and all students must obey safety rules to avoid injury. Instructors must read and understand all the safety information that came with the kiln, and assure that the kiln is properly installed and maintained. If a manual is not available, many companies post them online or you can request a replacement copy from the manufacturer. For operating the kiln, students must:
- Turn off kiln prior to loading or unloading. Disconnect the kiln for any servicing or when kiln is not in use.
- Do not touch heating elements with anything since they carry high voltage.
- Do not place any combustibles within 12 inches of any surface of the kiln.
- Do not leave kiln unattended while firing.
- Never look into a hot kiln without properly tinted safety glasses (e.g., welder’s glasses). Sunglasses only block ultraviolet light.
- Make sure the ventilation system is working properly
- Never add extra insulation around a kiln to conserve energy. Extra insulation can cause the wiring and the steel case to overheat.
- Remove all tripping hazards. Keep the power cord out of the way.
- Do not fire with cracked shelves. They can break during firing, which could damage the ware inside the kiln. Store kiln shelves in a dry area.
- If you smell burning plastic, turn the kiln off. Examine the wall outlet and power cord for signs of burning.
- Never wear loose-fitting clothing around a hot kiln.
- Do not open a kiln until it has cooled to room temperature. Pots may break from thermal shock.
- Keep the kiln closed when not in use, and never place anything on the kiln lid, even when the kiln is idle-you may forget.
- Always keep unsupervised children away from the kiln.
- Do not place any objects under or around the kiln stand.
- Blocking airflow changes the kiln’s heating characteristics.
- Remove all flammable materials from the kiln room.