Almost every finished pot goes through the kiln twice, and the two trips do completely different jobs. The bisque firing turns soft, fragile, dried clay into a hard, porous ceramic that can be handled and glazed. The glaze firing melts a coating of ground minerals into a layer of glass fused to the surface. Confusing the two, or getting the temperatures the wrong way round, is one of the most common ways beginners lose work. Bisque firing is the first firing, and it is a chemical one-way door. Dried greenware is still just clay: leave it in a bucket of water and it will slake back into mud. Heat changes that permanently. As the kiln climbs, the last atmospheric water leaves, then between roughly 900 and 1100 degrees Fahrenheit the chemically bound water is driven out of the clay molecules themselves. This is dehydroxylation, and once it happens the material is ceramic and can never be reclaimed. Along the way, organic material in the clay burns out, carbonates and sulfates decompose, and at 1063 degrees the quartz in the body flips to a different crystal form, expanding slightly as it does. Most studios bisque somewhere between cone 08 and cone 04, roughly 1730 to 1945 degrees, with cone 06 and cone 04 being the most common choices. So why bisque at all? Three reasons. First, handling. Greenware is brittle and will snap if you grip it wrong, and glazing means dunking, holding, wiping, and turning a piece repeatedly. Bisqueware survives that. Second, and most important, absorption. Bisqued clay is porous, so when you dip a pot in glaze the body pulls the water out of the slurry and leaves an even, controlled layer of glaze particles on the surface. Raw dry greenware would dissolve on contact with a wet glaze. Third, burnout. Organics, carbon, and sulfur need oxygen and time to leave the body. Doing that in an open, unglazed bisque firing means they escape cleanly. If they are still trapped when a glaze melts over the top and seals the surface, the escaping gas has to punch through the molten glaze, and you get pinholes, craters, blisters, or bloating in the wall. Glaze firing is the second trip, and its temperature is set by the clay body, not by the glaze alone. A cone 6 stoneware needs to reach cone 6, around 2232 degrees, so the body vitrifies. A cone 10 body needs around 2345. The glaze must be formulated for that same range. Putting a cone 06 glaze on a pot fired to cone 6 will boil it off the surface. Putting a cone 10 glaze on a cone 6 firing leaves a dry, unmelted, scratchy crust. Glaze and body are a matched pair, and they also need to shrink at similar rates on cooling, which is what glaze fit means and why crazing happens when the fit is wrong. For stoneware, the bisque is lower than the glaze firing, and that is deliberate. You want the body still thirsty when the glaze goes on. Bisque a stoneware pot too high, say to cone 02 or above, and the body starts closing up. The glaze will not soak in, coats go thin and patchy, and you end up preheating pots or applying three coats to get anywhere. The order reverses for some work, and this catches people out. In low-fire earthenware traditions, including majolica, potters often bisque higher than they glaze. The bisque might go to cone 04 or cone 02 to make the fragile low-fire body as strong as it will get, and the glaze firing then goes lower, to cone 05 or 06, where the bright color palette survives. The result is a stronger pot with more vivid decoration. The principle to hold onto is that the bisque sets the strength and porosity of the body while the glaze firing sets the surface, and either can be the hotter of the two depending on what you need. Ramp speed matters more in the bisque than most beginners expect. Bisque wants to be slow, especially early. Candling means holding the kiln at a low temperature, around 180 to 200 degrees, for several hours with the lid propped or the vent running, to drive off remaining atmospheric water before it can turn to steam. Thick pieces, sculptural work, or anything not truly bone dry needs a long candle. After that, a slow climb of around 100 degrees per hour up through 1000 degrees gives organics time to burn out and gets you gently through quartz inversion. Above that the kiln can move faster. Glaze firings can generally climb quicker, since the water and organics are already gone, but the last stretch benefits from control, and many potters add a soak at top temperature to let the glaze smooth out and heal small defects. Controlled cooling matters too: matte and crystalline glazes develop their surface on the way down, not on the way up. The failures are predictable. Damp ware plus a fast start equals a pot that explodes and sprays shards across the load. Rushing the bisque leaves carbon trapped in the middle of the wall, which shows up as a grey black core when a piece breaks, and as bloating blisters at glaze temperature. Cooling too fast through quartz inversion cracks pots and can crack kiln shelves. Dusty or oily bisqueware makes glaze crawl away from the surface in bare patches. Glaze applied too thick runs onto the shelf and welds the pot down. Glaze left on the foot does the same. Wipe every foot before loading, and use kiln wash on shelves as insurance. A safe default for most functional work is a slow bisque to cone 04 with a candle at the start, then a glaze firing to whatever cone the clay body calls for. When our studio opens, our classes will cover reading pyrometric cones, writing a firing schedule, and diagnosing the difference between a glaze problem and a firing problem, since they look alike and are fixed very differently.
The Key Differences
Understanding what each firing does, why you bisque at all, and the temperatures that matter. helps you make informed decisions about your pottery journey. Whether you are researching before your first class or looking to deepen your knowledge, this comparison covers what matters.
What This Means at OKC Pottery Studio
At our Oklahoma City studio, we offer the equipment and instruction to explore both sides of this comparison. 18 Shimpo wheels, 4 kilns (electric and gas), 45+ glazes. Our instructors help you understand the nuances.
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