Ceramics can be fired in two ways: bisque and glaze firing. These two methods of firing clay present a different set of procedures and outcomes that are important for potters to be aware of. In this article, we will explain what these two firing methods involve, how they differ in terms of temperature, materials, and process times, and the pros and cons of each.
What is Bisque Firing?
Bisque firing is the first step in the traditional process of firing clay. During the bisque firing process, the clay is heated up to a temperature between 1000°F (538°C) and 1840°F (1000°C). During this high temperature firing, the clay loses its plasticity and becomes vitrified – meaning it becomes hard and non-porous. This step makes the clay hardened and much more durable but not yet ready to accept glazes.
When firing a piece in the bisque stage, it is important to use a low fire clay. Low fire clays typically refer to clays that are intended to be fired at temperatures lower than stoneware (cone 5-6). Low fire clays usually have a porcelain-like feel and a creamy white finish when fired in the bisque stage. Low fire clays are typically composed of kaolin, ball clay, quartz and feldspar.
What is Glaze Firing?
Glaze firing is the second stage of traditional clay firing. During glaze firing, the piece is heated up to an even higher temperature, ranging from 1800°F (982°C) to 2400°F (1315°C). During this higher temperature firing, a mixture of glazes and slips or underglazes, applied to the piece beforehand, melts and creates a glassy surface. This stage creates a permanent look and texture to the piece that would otherwise not be possible.
When firing in the glaze firing stage, it is important to use a high fire clay. These are typically referred to as stoneware or porcelain and include white stoneware, red stoneware and buff stoneware. High fire clays often contain combinations of kaolin, ball clay, quartz, feldspar and mesh. In addition to making sure the clay used is suitable for high fire, it is also important for the glaze itself to be formulated for high fire temperatures.
What Types of Clay are Best for Bisque and Glaze Firing?
When it comes to bisque firing, low fire clays are typically favored. Common low fire clays include porcelain, stoneware, white stoneware, red stoneware, buff stoneware and earthenware. Each of these types of clay has its own unique properties which will be expressed during the firing process.
When it comes to glaze firing, high fire clays are typically favored. Common high fire clays include cristobalite, mullite and kaolinite. These types of clays have a wide range of properties that can be used to create delicate or durable pieces depending on the firing technique used.
What is the Temperature Range for Bisque and Glaze Firing?
Bisque firing usually occurs at temperatures between 1000°F (538°C) and 1840°F (1000°C). Glaze firing usually occurs at temperatures between 1800°F (982°C) and 2400°F (1315°C). These temperatures should always be monitored during bisque and glaze firing in order to ensure that they are not too high or too low. Firing too low can lead to the clay being weaker or cracking during the cooling off stage.
What are the Pros and Cons of Bisque and Glaze Firing?
Bisque firing is a relatively simple process that only requires lowering the surface tension of nonporous clay so that it can shatter on impact. This method is especially useful for creating functional pieces such as mugs or plates because its easy hardness makes them easier to clean. It also creates an impressionable surface that can be decoraed with glazes or paints to create added dimension.
The main disadvantage of bisque firing is that it does not enhance the aesthetics of a piece as much as glaze firing does. Glaze firing, on the other hand, creates durable surfaces and interesting textures as well as a variety of visual effects depending on the glazes used.
The main downside to glaze firing is that it requires more time than bisque firing. Additionally, due to the extreme temperatures used in glaze firing, there is a higher risk of crack formation or other damages occurring. Finally, glaze firing requires much more careful preparation because a lot can go wrong if not done properly.
How Do You Prepare for Bisque and Glaze Firing?
All clay pieces should be properly dried before being fired in either bisque or glaze firing. If pieces are dried too slowly, they could crack or otherwise go wrong during the firing process. For best results, pieces should be dried slowly in a cool area for several days until most of their moisture has evaporated as slowly changing temperatures can cause cracking as well.
In addition to drying properly, pieces should also be fired in accordance with the kiln’s specifications to ensure that all pieces are fired evenly. Finally, glazed pieces should be fired in accordance with the glaze’s instructions in order to avoid melting or cracking issues.
What Types of Kilns are Used for Bisque and Glaze Firing?
The type of kiln used for both bisque and glaze firing will depend on what type of material is being fired. For example, electric kilns can be used for both types but gas kilns should be used specifically for glaze firing as they require much higher temperatures. For those looking to use electric kilns, take note that some electric kilns may not be capable of attaining certain temperatures or may require special materials depending on its design.
How Long Does it Take to Complete a Bisque or Glaze Firing Cycle?
The amount of time required for each stage of firing can vary depending on the type of kiln being used, the size and weight of the piece being fired as well as other factors. In general, bisque firings can take anywhere from 8-12 hours whereas glaze firings can take anywhere from 10-14 hours.
Are There Alternatives to Bisque and Glaze Firing?
For those who are looking for an alternative method to traditional bisque and glaze firing, there are several options available. As mentioned previously, electric kilns can also be used in low fire applications as well as in low temperature glazing techniques such as raku or salt & soda. Additionally, “cold-finishing” techniques can also be used to create one-of-a-kind finishes without needing to fire a piece at all.
No matter which method you opt to use, ensuring that your pieces are properly dried before any type of firing is key. Additionally, making sure to keep your pieces free from dust and dirt before and after both processes is essential if you hope to achieve smooth results and beautiful finishes.
In summary, bisque and glaze firing are two of the most common techniques for shaping ceramic objects into functional or decorative pieces. By understanding how these two processes differ as well as what materials and preparations need to be taken for each process, you will be able to achieve better results. Whether you opt for traditional firings such as bisque and glaze or more modern cold finishing techniques, you will be sure to obtain beautiful and long lasting results.
