The term “bisque firing” is one of the most commonly used pottery terms. It’s one of the most important steps in the pottery-making process.
The purpose of bisque firing is to transform greenware (unfired clay) from its fragile state to a porous and robust state. Without bisque firing, clay work will generally disintegrate, crumble and crack. There is not many uses or purposes for clay work that has not been bisque fired.
Once bisque fired, glaze and paint can be applied for a lasting and durable piece.
It is important to know that from clay’s original state to the bisque fired state, your work may “shrink” anywhere from 5%-15% of its size after you first complete it.
As stated above, pottery is usually fired in two stages. The first stage is the bisque fire and the second firing is the glaze fire.
You might be wondering why pottery is fired in a kiln at all. Well, the first fire is used to drive out moisture, organic matter, and carbon compounds from the clay.
As the clay and clay particles lose these elements, the clay shrinks and becomes rigid. This is called “sintering”. Once the clay has undergone this change, it has become ceramic and can no longer be dissolved in water. In short, the bisque fire changes the clay into hard ceramic material.
After the clay has been bisque fired, glaze can be applied in liquid form. It dries on the ceramic surface. The pottery is then glaze fired.
During the glaze fire, the glaze melts and forms a glassy non-porous coating on the pottery. This can make the pottery waterproof and therefore usable for dinnerware and holding water. But also, glaze can make your pottery look beautiful.
What we have covered here is very brief. We highly recommend you research and read more about the amazing chemical changes that occur during firig, the complexity of the process and about “cones”.
Bisque (Low vs. High Fire)
The term low-fire in pottery refers to firing processes that are done at a relatively low temperature, typically cone 04 to cone 06. The term also describes clay bodies and glazes that are suitable for low-fire.
When bisque firing low-fire clay bodies, you can fire the clay slightly higher than the glaze. Usually, this means you will bisque fire to cone 04 (make sure you get the zero in there!), which will tighten the clay body just a bit more than a lower temperature will. After bisque firing is complete, the clay contains no chemically bonded water.
As an example, low-fire glaze firings are usually between cone 06 and 04, with cone 05 being very common. A bisque firing on average takes around 10 hours. This involves slowly ramping up the temperature, holding at certain temperatures for a period of time, eventually reaching the required temperature, then allowing the kiln to cool down below 100C before safely removing the cooled pieces.
Note in the chart below different temperatures for different types of clay and outcomes.
