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Art Savvy Questions & Answers CERAMICS 
Q: Do you have to fire clay once (bisque firing) before applying glaze? A: Yes. Gases, carbon and water molecules are released from the greenware (bone-dry clay) when it is fired properly. The release of these gases and other substances could cause problems with the glaze such as bubbling or an undesirable color. First or bisque firings are generally done at one cone higher than the glaze firing will be. The bisque firing should be done slowly to allow for release of the gases, water and other changes in the clay. If the glaze you are using is a Cone 06 glaze, bisque fire at Cone 04 (see chart below.) Q: How do you fire a piece that you wish to underglaze? A: Underglaze is basically colored clay in slip form. It, unlike glaze, may be applied to greenware or bisqueware (clay that has been fired once). Paint (or draw, depending on the type of underglaze you are using) the design you wish using underglaze. You must apply three coats. Each coat should be applied with the strokes at right angles to the last coat. If the clay is greenware, fire it as a bisque firing. Remember that bisque firings should be two cones hotter than the glaze firing. See cone chart below. If you want a gloss finish and would like the underglaze colors to appear richer, apply transparent glaze as follows: coat the underglazed, bisque-fired piece with three layers of clear transparent glaze. Fire again to the cone temperature of the glaze. Be sure to use clays and glazes of compatible cone temperatures. Note: The colors of underglazes, unlike glazes, are basically the same in unfired form as they are when fired. Q: My new ceramic clay has mold all over it. Should I throw it away? A: No. Mold on ceramic clay occurs naturally and is actually desirable. It improves the plasticity or ability to be formed. If you notice mold on the outside of your clay, wedge it thoroughly to obtain maximum uniformity. Q: I have bags of ceramic clay that are totally dried out and rock-hard. Should I throw them away? A: No! Dried ceramic clay can be easily rehydrated. One of the best investments you can make, if you work with ceramic clay, is a tightly covered plastic container. We sell several by Rubbermaid in the Ceramics Section of our Full-line catalog. Buy the size that will best meet your needs. If you do a lot of ceramics, get a larger container. To rehydrate the clay, break it into small chunks, put it in the container and add some water. Cover it and let it sit for a day or two. Check it and add more water if it's not soft enough to work with or wedge on an absorbent surface it if it is too moist. If you are recycling a lot of clay on a regular basis, have a 2" x 4" x approximately 36" piece of wood to tamp the clay down in the bucket and a 1" dowel to make holes in the clay at varying depths to catch the water that you pour in. Let the clay sit for a day or two and wedge it. If the clay becomes overly wet, lay the moistened clay on a plaster wedging board for several hours. Excess water will be absorbed and the clay will be ready for wedging. By using this method you can recycle a huge quantity of clay at one time. Note: It is advisable to have a separate bucket for each type of clay you use, e.g. white earthenware and terra cotta earthenware. Do NOT mix earthenware clays with stoneware clay. Q: How many pounds of ceramic clay or earthen air-dry clay do I need for a class of 25 students? A: The volume of these clays is roughly the same as that of butter. One pound of wet clay is about two cups. A medium-sized project such as a pinch pot or small box would require about ½ pound. A larger project such as a mask or a vase would require a pound. So you would need about ½ to 1 pound of clay for each student depending on the size of the project. Q: What temperatures do the numbers on pyrometric cones used in kiln firing ceramics stand for? A: Each cone number represents a different temperature. As the kiln temperature reaches the cone's "maturing temperature", the cone will bend. Large cones are generally used for witness cones. Small cones are inserted into a Kiln Sitter and require a slightly higher temperature to mature. Generally, bisque firings should be done two cones hotter than the glaze firing. See Chart Below. Cone Firing Ranges 
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