An excerpt from The Clay Lady’s Lesson Book -Practical lessons for the studio and the spirit of the potter by Danielle McDaniel~The Clay Lady
First a little history on cones and clay.
It takes the earth about 10,000 years to make clay. Small particles, made from decaying rock, make the journey from the top of mountains to the ground below. Time gently pulls these particles down the mountainside with rain and, finally, left on the side of a changing riverbed to our hands.
Humans have been making things out of clay for about the same amount of time. Although the process of making vessels and firing those vessels is quite different from today. What we use in the studio with the introduction of manufactured clay and electric kilns has only been around for about 75 years. Historically, people dug clay from the ground, today called ‘wild clay’, to make not only their pots but the bricks for their kilns. As we progressed in the quality of our vessels, it became the practice to dig clay from the ground, dry it out, pulverize, sieve to clean out debris, and then water was added back to make a consistent and quality clay.
The potters would make their vessels and then load them into a kiln. It was unknown at what temperature the clay would need to reach to ensure the vessels were vitreous. Too cool of a temperature and the vessel would be soft and breakable. Too hot of a firing and the clay would pass its melting point and slump. A good firing brings clay just to its melting temperature, turning it into a ceramic form. To know when the best firing temperature was met in the kiln, the potter would take a handful of clay and create a cone, similar to an upside down ice cream cone. They would set this cone inside the kiln by a small opening called a peep hole. During the firing, the potter would look inside the peep at the cone of clay. When the potter saw it was starting to bend over, or melt, they would stop feeding wood or fuel to the kiln because the melting signaled the clay would now be vitreous and the walls solid.
During the industrial age when gas kilns were developed, a manufacturer, Orton, started manufacturing cones. Each mechanically formed cone was manufactured to melt at a specific heat rise rate and temperature. They started with cone 1 (C/1) which is the temperature that Terra Cotta becomes vitreous. Then the cones are numbered from there to C/42, the melting point of Alumina, the building block of clay.
Cone 10 correlated to the temperature where porcelain becomes vitreous and is the common Cone used in wood and gas kilns. Over the last 75 years, once electric kilns were in use, Cone 6 became prevalent because it was the top temperature easily reached with electricity.
As cones and kilns became manufactured so did clay. Potters had already discovered how to mix different raw materials into wild clay to optimize their results. It was a short step for companies to mine individually all the different raw materials that naturally occur and come together in wild clay. Following recipes of naturally occurring clay, then chemically engineered for optimal firing results, these raw materials are now mixed in a large mixer. After the dry materials are blended, just the right amount of water is added before being de-aired. This creates the clay bodies that we use today. Each clay body mixed for a specific use and/or look by current day clay artists.
With the invention of electric kilns, bisque firing became prevalent as part of the firing process. This initial, first firing makes a pot “hard as a biscuit” so that is ready for repetitive dipping in liquid glazes before the glaze fire. Bisque firing brings the clay to a point where the silica molecules melt just enough to tack the clay platelets together, leaving space for liquid glaze soak in. On a microscopic level a bisque pot would look like a dry sponge. This is different from the final firing which fully melts the silicate. This full melt fills the space between the clay platelets, creating a vitreous pot.
Since manufactured cones numbers started when Terra cotta becomes vitreous but bisque firing is at a cooler temperature, the manufacturer had to develop a different numbering system for lower temperatures. They created the system of Cone 01, Cone 02, Cone 03, etc, similar to negative numbers. Just as the value of a negative number increases the closer to 1, the temperature of 04 is hotter than the temperature of 06. Studios commonly bisque fire between 06 to 04. At The Clay Lady’s Studios we bisque at Cone 05 just two keep the confusion down between C/06 and C/6 (the high temperature in an electric kiln).
Clay bodies, manufactured surface decorating products, manufactured glazes, and glaze recipes are now chemically engineered to 3 main firing temperatures, or Cones, of 10, 6 and 06. These temperatures not only meet the required firing temperature of manufactured clays, but also assure the combination of these products fit together at these temperatures. It would not be successful to put a C/06 low-fire glaze on a Cone 6 clay pot because the clay and the glaze would melt and shrink at different rates. Therefore the “fit” would not be successful.
It is interesting to note, however, that a bad fit between clay and glaze is how modern-day raku firing is done. The low fire Raku process uses C/06 low-fire glazes on a high fire, highly grogged, clay. During the firing process, the glaze shrinks more than the clay. When pulled from the fire, the glaze, not fitting the clay, crazes or crackles. The next step in the process is to place the pot into burning combustible material. The cracks in the glaze are filled with the dark, black carbon from the fire. So, what is not desirable in one instance is a decorating option in another!
There are different reasons for choosing different clay bodies that fire at different temperatures. Wood and gas kilns are traditionally fired at C/10 for the atmospheric surface decoration of the flame from the gas and wood on porcelain and stoneware. Mid-Fire clays are C/6 and fired in easily available electric kilns using manufactured glazes which are also abundant. Bisque firing for both these final temperatures is between C/06-04.
There are earthenware clays and decorating products that mature at the low-fire C/06-04. The advantage to low-fire is the glaze colors stay colorful and bright. Also, the wear and tear on your kiln is lessened. Many sculptors fire at this temperature to lessen cracking and warping. Traditionally low-fire is reserved for kids and community classes.
Currently there are potters changing commonly held traditions. Firing only to C/6 in gas and wood firings as well beautiful earthenware work taken to the higher heat of Cone 2. These practices of separating traditional cone and fuel source connections creates an endless array of possibilities!
If you are in a community studio, it is imperative that you buy your clay and decorating products for the Cone that your studio is firing. If you have a home studio it is recommended that you choose one temperature of clay. If you choose two different firing temperature clays and glazes, it is especially important to keep them completely separate is the studio processes. If there is a mix up and you put a low-fire pot in a Cone 6 or 10 kiln, it will over melt and look like a pile of ice cream on your kiln shelf!
The Cone Mountain is a simple analogy that I use to explain how to remember the different firing temperatures for clay.
Clay is made from disintegrating rock. The weather and wind beat on the high, exposed rock over time, sending small particles to the ground. The purest of clay, porcelain, is found in the highest mountains. Porcelain becomes vitreous at Cone 10, the highest measurement of firing for clay.
Cone 10, the common firing temperature of wood and gas kilns, is 2350 degrees and known as High Fire.
Over time these pure clay particles roll from the highest point down to the middle of the mountain, gathering up different minerals and stones, creating stoneware clay. The mid-firing range is Cone 6 or 2,242 degrees. This is the common temperature of electric kilns, known as Mid-Fire.
The rain flow continues the journey of these particles down the mountain to rivers, leaving the larger bits behind. As these rivers change course over time, these particles, now smooth and small, are left on the banks creating earthenware. Earthenware firing temperature is C/06-04 or 1890 degrees, known as Low-Fire.
This analogy is an easy way of remembering the three main temperatures of clay. Cone 10 is the highest firing temperature for pottery; Cone 6 is mid-fire temperature and earthenware (or low fire) is fired at Cone 06-04.