There are a few terms that potters use when talking about firing that are confusing when you first hear them. The purpose of this section is to provide some clarity.
We have a few different types of kilns at BHC. They are described briefly as follows:
- Gas Kilns: We have three 40 cubic-feet gas kilns . These are the primary kilns that we used to fire bisque and glaze. Our staff loads, unloads and operates these kilns.
- Electric Kilns: We have 2 programmable electric kilns. Our staff loads, unloads and operates these kilns.
- Raku Kilns: We have two Raku kilns. One stays in storage and is only taken out for large workshops. Once students have taken the Raku workshop and have demonstrated that they know how to operate the Raku kiln, students are allowed to use the Raku kiln on their own.
Cones (AKA: Pyrometric Cones)
Cones are a way of measuring heat that has been standardized and is used by potters around the world. It’s important to note that cones, unlike thermometers, also account for time. If you put cookie dough in an oven that is 375 degrees, the cookies are not instantly done. It takes time for the heat to do it’s work on the cookies. In the same way, it takes time for the heat inside of a kiln to do it’s work on the pots. Cones measure “heat-work.” Cones tell you the temperature of the pots, rather than the temperature of the kiln.
The chart below will help you understand the relationship between cones and degrees Fahrenheit
Potters use cones to describe 3 things: The temperature that a clay body is designed to be fired to. The temperature that a glaze is designed to be fired to. The temperature the kiln is fired to.
Clay bodies and Cones: It is essential to match the clay you are using with the cone you intend to fire to. BHC is a cone 10 studio which means that we fire our kilns to 2381 ° F. For this reason, only cone-10 clay can be used at BHC. If cone 08 (1753° F) or cone 5 (2205° F) clay ends up in a cone-10 firing, the clay will melt and cause extensive damage.
We do some cone 5-6 firings. However, we do not allow cone 5-6 clay in our studio. The risk of accidentally getting cone 5-6 clay in our cone-10 kilns is simply too great. Some of our cone-10 clays fire just fine at cone 5-6. Please ask if you want more information.
Glazes and Cones: Glazes are also formulated to be fired to a specific cone. Since we are a cone-10 studio, our glazes are referred to as cone-10 glazes. If a cone 5-6 glaze ends up in a cone-10 firing, it will melt and cause extensive damage. This is why we do not let students bring in their own glazes.
We have a cone 5-6 clear glaze in the studio. This is the only cone 5-6 glaze that we have. It is designed to be used over underglazes. You can learn more about this in the learning to glaze section of this handbook.
Bisque Firing: All pottery is fired two times. The first firing is known as a bisque. In the bisque firing your pot will be transformed from an easily breakable piece of dry clay into a much less fragile, but still porous object that is ready to be glazed. (We bisque fire to cone 08.)
Mid-range Firing: This is a glaze firing (done after a bisque) to cone 5-6. We only offer a cone 5-6 clear glaze that is meant to be put over an under glaze (see Learning to glaze). It is important that a pot glazed for cone 5-6 does not end up in the high fire. Please ask your teacher for help. Our staff loads and unloads these firings.
High-Fire: This is a glaze firing (done after a bisque) to Cone 10. We are a cone-10 studio and all of the glazes and clay bodies in the studio (except the cone 5-6 clear) are designed for cone 10. Our staff loads and unloads these firings.
The term reduction refers to the amount of carbon in a gas kiln while it is firing. The amount of reduction in a firing is increased by decreasing the amount of oxygen available to the burning fuel (propane). Reduction is decreased by increasing the amount of oxygen available to the burning fuel (propane). Electric kilns, since they do not have a burning fuel source, produce zero reduction. It is important to note that there is no correlation between temperature (cone) and reduction.
Our regular weekly firing is moderately to heavily reduced (lots of carbon). In general, our glazes respond well to reduction.
Occasionally we do a light reduction firing. We will inform you when this happens and provide you with clear instructions.
Raku means “enjoyment”, “comfort” or “ease”. The technique was developed in Japan during the 16th century through the joint efforts of a tea master and a tile-maker. Raku, which got its name from the family that produced the wares, has been passed down from generation to generation until the present day.
The type of Raku that we do was made popular in the US during the 1950’s by William Soldner. Soldner and his contemporaries maintained the general firing process of Raku (heating the pottery quickly and then cooling rapidly) but continued to form their own unique styles. This is why the type of Raku we do is frequently referred to as “Western Raku” or “California Flash Firing.”
The Raku Process:
- Glazed pots (use special raku glazes) are heated rapidly (about 45 minutes) to cone 06.
- Once the kiln is turned off, the pots need to be moved to a reduction chamber (a garbage can filled with newspaper). Since the pots are extremely hot, this is done with tongs.
- the reduction chamber is allowed to cool for about 30 minutes before the pot is removed.