Vocabulary
- Bisque: Refers to pottery that has been fired once but has not yet been glazed. This first firing removes all moisture, chemical water, and carbon, making the pottery porous and ready for glazing.
- Cone: Pyrometric cones are small pyramids made from ceramic materials designed to melt at specific temperatures. They are used to gauge kiln temperature during a firing.
- Glaze: A glassy coating applied to ceramics before firing forms a vitreous surface. Glazes can enhance the piece’s aesthetic and make it waterproof.
- Greenware: Unfired pottery or bone-dry sculpture, in which clay forms are the most fragile.
- Kiln: A furnace or oven built specifically for firing ceramics and glass. It can be electric, gas, wood, or coal-fired.
- Vitrification: The process by which a ceramic material is fired to the point where it becomes glass-like and impermeable to water. This is generally achieved at high temperatures.
- Sintering: The process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.
- Crazing: The network of fine cracks that can appear in a glaze layer of ceramic pieces, often due to a mismatch in the thermal expansion of glaze and the clay body.
- Earthenware: A type of clay that is fired at relatively low temperatures and is porous and not vitrified, often glazed for waterproofing.
- Stoneware: Clay that, when fired, becomes vitrified and non-porous and is usually fired at mid to high temperatures.
- Porcelain: A high-firing clay body typically vitrified, translucent, and white, known for its strength and glassy surface.
What is happening to the clay during a Bisque Fire?
- Between Ambient – 200°F: Physical water evaporation.
- From 400°F – 1500°F: Organic material, such as paint, wax, and oils, is burned off. This process often produces smoke, hence why ventilation is important.
- About 660°F: Chemical water evaporation. Chemical water is part of the clay’s molecular structure. Losing this chemical water marks a permanent change in the clay; it moves from clay to ceramic. Once this change occurs, the object can no longer be recycled by simply adding water.
- Above 1500°F: Sintering. Sintering is the fusing of particles under extreme heat (and/or pressure). The particles do not melt but fuse, making the ceramic harder and more compact. Your object will shrink during this process, and the shrinkage depends on the clay.
- Between 1500°F – 1971°F (Cone 04): Porosity development. Before complete vitrification, there is a sweet spot where the ceramic has sintered but is still porous. This allows the glaze to penetrate the surface and adhere well during subsequent glaze firing.
What is happening to the clay during a Glaze Fire?
- Between 500°F and 1500°F: Gasses escape from the glaze.
- Between 1800°F and 2300°F:
- The glaze melts, transforming from a powdery coating to a glass-like surface. The exact temperature depends on the Glaze.
- Chemical Reactions: Various reactions within the glaze take place. This includes the melting of silica, metal oxides, and fluxes.
- Interface Reactions: The glaze interacts chemically with the ceramic body, significantly affecting the glaze color.
- 1940°F to 2269°F: Vitrification of the Clay Body. The clay body undergoes its final vitrification. This turns its molecules from a porous structure to a glass-like structure. Higher temperatures make for a more complete vitrification. (We allow our kilns to be fired to Cone 6!)
- Cooling:
- Molten glaze solidifies: Crystallization may occur, affecting the appearance of some glazes. Controlled cooling can impact the crystal formation and, therefore, the appearance of the glaze.
- Shrinkage: Both the glaze and the ceramic shrink as they cool. The shrinkage is (almost always) at a different rate, which can cause cracks in the glaze (called crazing).
Before Firing: Maintenance and Troubleshooting
- Inspect Elements: Check the heating elements for wear, damage, or distortion. Worn or damaged elements can affect the firing quality and may need to be replaced.
- Check the Kiln Lid or Door Seal: Inspect the kiln lid or door for cracks or defects that could lead to heat loss. Ensure that the seal is intact and repair or replace it if necessary.
- Vacuum the Interior: Regularly clean the inside of the kiln to remove kiln-wash flakes, debris, and dust that could contaminate firings or obstruct the elements. Ensure the kiln is cool before vacuuming.
- Inspect the Thermocouples: Check their condition and ensure they are correctly positioned in the kiln. Replace them if they show signs of corrosion or other damage.
- Test the Electrical System: Regularly inspect the wiring and connections for signs of wear or damage.
- Monitor the Kiln Furniture: Inspect kiln posts, shelves, and other furniture for cracks or warping. Replace any damaged furniture to ensure the safe and even firing of pieces.
- Check the Digital Controller: Ensure the controller is functioning correctly.
- Check for Loose Connections: Inspect all mechanical connections, including the lid or door hinges and locks, to ensure they are secure and operate smoothly.
- Calibrate the Kiln: Periodically, a calibration firing with witness cones might be necessary to verify that the temperature inside the kiln matches the controller settings.
- Review Safety Equipment: Ensure that all safety equipment, such as fire extinguishers and first-aid kits, is accessible and in good condition.
Unloading the Kiln and Post-Firing Cleanup
Before You Begin
- Ensure the Kiln is Cool: Always wait until the kiln has fully cooled to below 200F before opening it. This can take anywhere from several hours to over a day, depending on the size of the kiln and the temperature reached during firing.
- Prepare Your Workspace: Clear a space around the kiln for easy access and prepare a surface to place items as you unload them. Ensure this area is clean to avoid contaminating your work.
Unloading the Kiln
Open the Kiln Gradually: Slowly open the kiln lid or door to avoid sudden temperature changes. Thermal stress can cause pieces to crack!
Inspect the Interior: Before unloading, visually inspect the load. Look for signs of melting, glaze runs, or other anomalies. Take a photo!
Remove the Shelves and Posts: Carefully remove any kiln furniture, such as shelves and posts. Check them for damage and set them aside on a stable, heat-resistant surface.
Unloading Ceramics: Using heat-resistant gloves, gently remove each piece, starting from the top shelf and working your way down. Handle each piece with care to avoid damage.
Check Each Piece: As you unload, inspect each ceramic item for defects such as cracks, crazing, or unexpected glaze results.
Cleaning Up
Vacuum the Kiln: Once emptied, vacuum the kiln’s interior to remove debris and dust. Pay special attention to the grooves of the element where debris can accumulate.
Clean the Kiln Shelves: Remove kiln wash flakes or glaze drips from the shelves. If necessary, scrape them gently with a putty knife and apply a fresh coat of kiln wash if you’re preparing for another firing.
Inspect and Store Kiln Furniture: Examine your kiln posts and shelves for wear or damage. Then, neatly return them to their shelves.