China Clay or clay is called ceramic, bisqueware, earthenware, stoneware and glazeware after firing. All terms used to describe fired clay particles. Clay is fired twice. Bisque firing is the initial firing. Ball clay is transformed from bone dry clay to bisqueware. After bisque firing, clay objects are changed forever and are now a ceramic material. Once fired, unglazed clay is called bisqueware. Clay is called ceramic after the first firing, or bisque firing.
When talking about clay, most folks are referring to what we call China Clay, a synonym for kaolin. And kaolin is the primary ball clay used for producing porcelain. In ceramics, kaolin is the major component of porcelain.
The main concept to understand regarding kaolin or new clay is that it contains water and clay shrinks during firing. Also, there are a wide variety of clay on the market today used for hand building and making pottery on a potters wheel. I am talking in very general terms and not referring to any specific type of clay in this article.
Both drying and firing cause new clay to shrink. It’s important to also understand clay bodies or clay objects shrink at different rates. Before firing clay should be bone dry or what’s called “Bone Dry Stage”.
Most clays shrink on an average of about five percent during drying. And then shrinking a little more during bisque firing (at cone 06). And again during glaze firing (at cone 6). Here I am referring to Pyrometric cones.
Pyrometric cones are pyrometric instruments that are used to measure heatwork in ceramic materials during burning.
The cones, which are often used in sets of three, are placed in a kiln with the wares to be fired and serve as a visual indicator of when the wares have attained the appropriate stage of maturity, which is determined by a combination of time and temperature.
What Happens To Clay After It Is Fired?
When the water content of clay is diminished during firing, the clay body loses a bonding agent. Water no longer holds the clay particles together. When the clay has lost its water content, another bonding process occurs. This is known as ‘sintering.’ Sintering takes place when forming pottery. It’s part of the process of making pottery.
Sintering is the process of fusing particles into a single solid mass using pressure and heat without melting the materials. Metal, ceramic, plastic, and other different materials are commonly sintered together.
When clay is fired, it is transformed from its simple, soft beginnings into a new, durable substance called ceramic. Ceramics are durable and sturdy.
The temperature required to convert soft clay into hard ceramic is quite high and is often generated by a kiln. Pottery cannot be fired in a home oven because ovens can not reach the high temperatures of more than 1,500 degrees Fahrenheit required for firing clay.
What Is Firing?
The technique of heating clay and glazes to a high temperature is known as firing. The ultimate goal is to heat the object to the point when the clay and glazes are “mature,” or have achieved their ideal melting point. Pottery is said to have reached its maturing temperature. This is the desired temperature, or what I refer to as the firing temperature.
Pots and other clay objects do not appear melted to the naked eye. The melting occurs at the molecular level. This is done in two steps: bisque firing and glaze firing.
1. Bisque Firing
Bisque firing or biscuit firing is the first time newly formed clay pots (unfired pottery, unfired clay) or greenware are subjected to high-temperature heating, usually in a kiln. Firing is done to vitrify, which means to render glasslike, so that a glaze may now be applied to the surface of the pottery.
Biscuit porcelain, also known as bisque porcelain or bisque, is an unglazed, white porcelain that has been processed as a finished product and has a matte look and feel. It was frequently employed in European ceramics, mostly for sculptural and ornamental items that were not intended for use as tableware and so did not require a protective glaze.
Bisque is clay that has been fired once but not glazed
Ed Shears
Greenware (referred to as the “Dry Clay Stage”), is delicate and must be completely dry (and in the desired form) before it’s fired. I have broken a lot of greenware when loading it into my kiln. Great care must be taken when loading. Only totally dry clay should be bisque fired. Once the kiln is loaded, then it’s closed and the firing process begins.
The importance of a gradual temperature rise cannot be overstated as water evaporates from the clay shape without cracking. I use an electric kiln with a controller that I set to a firing schedule. Atmospheric water is pushed out of the clay at the start of bisque firing. When water is heated too rapidly inside the clay body, it turns to steam, which might cause the clay to explode. When this happens clay particles undergo a thermal shock. It can clause damage to kiln furniture and or your kiln shelf. Who wants damage to their expensive electric kiln, right?
Clay Transformation During Bisque Firing
Chemically bound water will begin to be pushed out of the clay when the kiln reaches around 660 degrees Fahrenheit. The clay becomes dried by the time it reaches about 930 degrees Fahrenheit. Firing has transformed the clay into a ceramic substance at this point.
Bisque fire continues until the kiln achieves a temperature of around 1730 degrees Fahrenheit. The pot has sintered at this temperature, which means it has been changed to the point where it is less brittle while still being porous enough to receive glaze application.
The kiln is switched off after the temperature has been attained. To avoid shattering the pots due to stress from the temperature change, the cooling is done slowly. It’s important at this stage to decrease the temperature very slowly, usually over a day or so. I am in no hurry to open the kiln until it has cooled fully. At this time, the newly formed bisqueware can be removed.
2. Glaze Firing
A ceramic glaze is an impermeable layer or coating used to color, embellish, or waterproof bisqueware. A glaze is required for earthenware, such as fired clay pottery, to contain liquid.
A thin coating of glaze is applied (depending on your decorating technique) to the bisqueware, which is then allowed to totally dry before being loaded into the kiln for its last phase, glaze fire.
The process for firing glaze is like bisque firing. For the glaze fire, I carefully put the glazed objects into the kiln. It must not come into contact with other pots, otherwise the glazes will melt together, permanently fusing the pots. The kiln is gradually heated to the correct temperature to mature the clay and glazes, then slowly cooled again. Firing at too high a temperature is not good. And firing at too low temperature is not good either. To achieve the desired results, glazes should be fired at their proper temperature. Each ceramic glaze should be fired to a specific temperature range. If fired at too low a temperature, the glaze will not mature. After the kiln has cooled fully, it is opened and the finished piece is removed. Again, I am not in a hurry to open the kiln until it has fully cooled. I like to be over cautious and prefer a longer cool down stage.
The clay and glaze alter dramatically after the second fire. It completes the chemical change of pots or pottery from a soft (organic matter), organic material to one that is rock-hard and resistant to water and the passage of time. Fired pottery and artwork can last for centuries or more! Did you know they have found some of the oldest pottery fragments in caves in south China dating back to over 20,000 years old?
What Happens If Clay Is Fired Too High?
Clays and glazes are designed to mature at certain temperatures, and any deviation can result in inadequate durability or color in ceramics. Did you know clays can distort or even melt if fired to high a temperature? Firing clay to very high temperatures results in glaze runoff. And if fired to low temperatures, your pots or artwork will be dry, rough, and perhaps unsolidified.
References
Kipfer, B. A. (2021). Bisque Firing. Encyclopedic Dictionary of Archaeology, 172-172. https://link.springer.com/referenceworkentry/10.1007/978-3-030-58292-0_20438
Robson, J. T. (1933). Rapid Firing of Semiporcelain. Journal of the American Ceramic Society, 16(7), 296-298. ceramics.onlinelibrary.wiley.com/doi/abs/10.1111/j.1151-2916.1933.tb16981.x
Rawson, P. (1984). Ceramics (Vol. 6). University of Pennsylvania Press. https://books.google.com/books?hl=en&lr=&id=1e79xqbRqEEC&oi=fnd&pg=PA1&dq=Is+Pottery+Art&ots=JYVyRyIXYR&sig=AQZbSLk4xQyCjy9e1dLnol83yQA#v=onepage&q=Is%20Pottery%20Art&f=false
Peterson, S., & Peterson, J. (2002). Working with clay. Laurence King Publishing. https://books.google.com/books?hl=en&lr=&id=hJ4BOhm-XicC&oi=fnd&pg=PA8&dq=How+To+Wedge+Clay&ots=sqI8oBwnGm&sig=NnNyDhF8yak-UJJ5I-QHkp99uJU#v=onepage&q=How%20To%20Wedge%20Clay&f=false
Earthenware clay, potters wheel
Bisque ware, raw state, slip stage, low fire, high
Clay melting temperature, cone temperatures, cone 01, cone 04, cone 1, cone 10
Leather hard stage, leather hard, greenware
