A kiln cookie is a protective slab of clay placed directly underneath a glazed ceramic piece during a high-temperature firing. It creates a sacrificial barrier between the glazed ware and the kiln shelf. When glazes melt and flow excessively, the cookie catches any drips or runs, preventing the molten glass from fusing the pottery permanently to the kiln shelf. Using this preventative measure safeguards equipment from damage and avoids the time-consuming process of grinding away fused glaze.
Essential Materials and Preparation
The selection of clay body determines the longevity and effectiveness of the finished cookie. Use a high-fire stoneware clay, such as a Cone 10 formulation, even if firing at lower temperatures like Cone 6. Using clay rated for a higher temperature ensures the cookie remains structurally sound and withstands multiple glaze cycles without warping or melting.
Before construction, the clay must be thoroughly prepared to eliminate air pockets and achieve a uniform moisture level, a process known as wedging. This preparation prevents trapped air or uneven consistency from causing the slab to crack or warp during drying or firing. Basic tools needed include a rolling pin, a cutting wire or knife, and two wooden guide sticks to ensure even thickness. Preparing the workspace with a non-stick surface, like canvas or drywall, aids in a smooth construction process.
Forming and Finishing the Cookie
Forming the cookie begins with rolling the clay to a uniform thickness. The ideal thickness ranges from one-quarter to one-half inch. Using wooden guide sticks placed on either side of the slab helps the rolling pin maintain an even surface. This uniformity prevents internal stress points that could lead to cracking during drying or firing.
Once the slab is rolled out evenly, the cookies are cut into shapes slightly larger than the base of the ceramic piece they will protect. Simple round or square shapes are common, though interlocking shapes like hexagons maximize shelf space. Some potters gently curve the edges upward, creating a subtle lip to better contain runny glazes. After cutting, the cookies must dry slowly and completely. They should be sandwiched between flat surfaces like masonry boards to ensure they remain perfectly level and do not warp.
The final step before use is a bisque firing, which converts the fragile, dry clay into a durable, porous ceramic. This initial low-temperature firing removes chemically bonded water and solidifies the shape. This makes the cookie ready for its protective role in subsequent high-temperature glaze firings. For added protection, applying a coat of kiln wash—a mixture of refractory materials like alumina hydrate—to the top surface of the bisqued cookie prevents molten glaze from fusing to the cookie itself.
Using and Maintaining Kiln Cookies
Placement and Firing
During the glaze firing, the cookie is placed directly onto the kiln shelf, centered beneath the ceramic piece that has been glazed near its foot. The cookie must be large enough to catch any glaze that might flow past the foot of the pottery, but not so large that it interferes with surrounding pieces or the kiln wall. This deliberate placement ensures that if the glaze runs, the molten material fuses to the expendable clay cookie instead of the permanent kiln shelf.
Separation and Maintenance
After the kiln cools, the cookie may be stuck to the pottery base if the glaze flowed significantly. To separate them, the cookie is gently tapped or pried off using a flathead screwdriver or chisel, aiming to break the cookie rather than the ceramic piece. Since the glaze is essentially glass, wear eye protection during this process.
Cookies with only minor glaze drips can often be reused after scraping away the residue. Heavily contaminated cookies, or those that have cracked during firing, should be discarded, as their integrity is compromised. Using a high-fire clay and applying kiln wash extends the useful life of a cookie to dozens of firing cycles before replacement is needed.