Introduction to the Firing Process
When you start considering firing, remember that clays and glazes are mixtures of different materials, each with different behaviors at different temperatures. The firing process can be seen as a series of simultaneous events, each affecting the ware in different ways. Within the kiln, there is an environment that includes smoke and gases from fuel combustion, as well as the oxidation or reduction of the atmosphere. Each firing has different effects on the final product of clay and glazes.
Firing is far more than just heating clay in a kiln at high temperatures; it is the pivotal stage in the ceramics process that solidifies the durability of both clays and glazes. Incorrect firing can lead to the destruction of hours of meticulous work, making the many variables involved critical to understand. The complex interactions between clay and glazes during this stage are crucial for achieving the desired qualities in the final product.
Porcelain, stoneware, and earthenware, though distinct, share a common trait: their resilience to wear, which can be both advantageous and challenging. As you deepen your understanding of various clays and glazes, considering the intricacies of the firing process becomes essential. This step is not just a phase in the creation of ceramic items; it is fundamental to their development and final quality.
Understanding Bisque and Glaze Firings
Bisque firing is the pottery process where work is fired once it is dried completely. This process is essential to the success of your work. Sometimes pieces explode in the kiln if they contain moisture. This can be dangerous for the pottery and the kiln. Exploding work can also destroy other pieces in the kiln. Exploding work is sometimes due to poor craftsmanship, but often it is the result of a lack of understanding the process of drying and of firing the work. Once pottery is bone dry, it is okay to fire in a kiln. If work is fired too soon, it can explode because of steam build up from moisture within. High-quality craft is one where the craftsmanship is balanced with a good understanding of the materials and processes involved. Bisque firing also enables the potter to find any faults within the forming or drying process. Missed joins and surface imperfections are easy to see. An unfired work can be returned to a bag of clay and reformed. An effective way to fix cracked bisque ware is to heavily dust the crack with baking soda. When the piece is refired, the soda will turn to a carbon dioxide gas and effectively fill the crack with a similar material to the bisque itself. Bisque firing is a simple process because all that needs to be done is fire the work to the temperature in which it will be fired in the glaze firing. Bisque ware is still quite porous and is fragile to mishandling. High-temperature bisque or stoneware bisque can sometimes be mistaken for the fired ware. Bisque ware is still distinguishable by its white to orange chalky surface and fragility.
Key Differences Between Bisque and Glaze Firings
Bisque firing is a long, slow process. In industry it can take anywhere from 8 to 24 hours. A slower firing schedule with a longer hold at the end is used because the ware still contains chemically combined water and other volatile materials that need to be burned off during the final stages of firing.
Bisque firing is generally done at a lower temperature than glaze firing. The exact temperature can vary depending on the materials used in the clay body, but is usually in the area of 1700°F to 1900°F. This is hot enough to vitrify the clay and glaze together into a hard, durable piece, but without melting the glaze to the point that it runs off the piece. Because it is easier to control the rate of temperature change in an electric kiln, many potters choose to bisque fire using electric rather than fuel burning kilns. This is also done to save the ware from having to go through extreme thermal shock, which occurs when pieces are put in a kiln that is too hot, or when the heat is turned up too fast.
When trying to understand differences between bisque and glaze firings, it is helpful to think beyond how the pieces look when they are done and think more of the process as a means to an end. Bisque firing is the initial firing and has several purposes, the most important being to create a piece that is durable enough to withstand the test of time. Glaze firing is done to vitrify the glaze coated surface of a piece, making the clay impervious.
Tips for Achieving Optimal Firing Results
Firing pots successfully is not so much a matter of following a specific firing schedule, as it is developing a much deeper understanding of the processes involved, and learning to observe and control these processes during a firing. This takes time and experience, but we’ll try to help direct you by pointing out things to look for and a few specific tests and measurements you can make. In any firing, the end result of the pots depends a great deal on the understanding and cooperation between the potter and the kiln. With experience, one learns to develop a sort of instinct about what to expect in firing the pots based on the condition of the ware and the kiln, the rate of temperature increase, and the visual observation of the interior of the kiln during the firing.
The key is to learn to make sound decisions about adjustments to the firing process based on the observable effects on the ware in progress, and to eventually be able to predict and obtain specific results in glaze and/or clay body effects. Step number one is to establish rapport with the kiln. This may seem a bit odd, but it is no jest. A very useful practice to learn more about kilns is to make small tests in the kiln with cones and/or glazed test tiles. Coil built mini-kilns work fine for this. By observing the effects of various firings on the test cones or tiles, one can gain some valuable insights about what is happening in the larger kiln. A bit more advanced method is to use small witness cones in the kiln in addition to the self-supporting cones placed on the ware. This can measure the amount of heat work actually occurring in any given location inside the kiln, but still does not give a complete picture of the overall heat distribution in the kiln.
A few potters get quite involved in learning about their kilns and heat work by placing commercially available “pyrometric bars” in various locations in the kiln. Pyrometric bars are composed of materials which soften and bend at specific temperatures. These bars can then be observed and tested to give a very detailed picture of what is happening with heat work in the kiln. Most potters won’t go this far, but it does work.
This is the final installment of a series of articles designed to help you better understand the process of firing pottery. In the first part of the series, which appeared in the January/February issue, we identified the major areas of consideration for the potter in striving to produce high quality work. Subsequently, we have begun to take the mystery out of attaining better results through many specific technical and procedural guidelines and suggestions. Now we will focus specifically on firing and discuss some practices which can help you achieve much better firing results. We will examine the concept of the “ideal firing” and ways to better understand and control the complexities of heat-work in a kiln. And, we will take a very close look at both oxidation and reduction firing with an eye towards achieving the best possible results in each type of atmosphere.
Conclusion
This essay has outlined three different stages of pottery clay and how to fire it, enabling a complete understanding of how to handle the different kinds of clay bodies to achieve the best results. Understanding potter’s clay is important, as the process of making it into a ceramic body is completely irreversible. You can always add water to dry clay and make it plastic again, but once it has been fired it cannot be changed. The more you understand about clay, the better your control and success will be in making it into pottery. With both bisque and glaze firing, the important factors include knowing the nature of the clay body and what you want to achieve with it, and being able to identify the point that the ware has reached the desired state. Bisque firing should produce a porous, yet strong enough body to withstand glazing and a high temperature glaze firing. Glaze firing should cause the glaze to melt properly, and surface defects such as pinholing or blistering can often be traced to faults in the greenware or bisque firing. With knowledge and experience, you will be able to make better judgments with the firing process to obtain the results you want.