Gas Kiln Design and Firing Integrating Material and Energy Efficiency into Gas Fired Kiln Plans
Learn how to efficiently build and fire a gas kin this FREE PDF!
If you want to build and/or fire a gas kiln, there are several things you need to know before diving in. Whether you plan on doing oxidation, neutral, or reduction firing, and regardless of the type of gas fuel you will be using, knowing how a kiln is designed and put together helps you understand what is happening during the firing. Gas Kiln Design and Firing Integrating Material and Energy Efficiency into Gas Fired Kiln Plans provides guidance and information critical to the success of your ceramic work.
Check out this excerpt:
Demystifying the Reduction Firing Process
by Ryan Coppage, PhD
The Reduction Firing Process
Almost as a standard, the process of “reduction” is described with some degree of equivocation no matter where you go or in which ceramic setting you work. Most pottery professionals don’t like to describe it, especially to a persnickety chemist. These descriptions vary from place to place, but the process of reduction is most commonly communicated as “reducing the amount of oxygen in a kiln,” such that the flame/fuel searches for more oxygen and will pull said oxygen out of clay bodies, etc. While this is absolutely parallel and incidental to reduction taking place, that phenomenon is not reduction and is not responsible for the vibrant, beautiful colors that are synonymous with the firing method.
Reduction is the process of electrons being donated to a metal/element/surface through some set of reactions, while another component in the same set of reactions is oxidized (electrons lost). This is your defined set of oxidation and reduction parameters. In a gas kiln, albeit natural gas or propane, you are using some set of hydrocarbons and oxygen. Your reaction breaks down to the following for an oxidation firing using propane:
C3H8 + 5 O2 4 H2O + 3 CO2
With access to enough oxygen to efficiently burn your fuel, you are almost exclusively producing water vapor and CO2, which will cause oxidation of your ceramic surfaces. This is the clean-burning, efficient, blue flame, which is why easy temperature gains are synonymous with oxidation atmospheres. Upon limiting the amount of oxygen access to the inside of your kiln via vents, flue control, or passive dampers, you are no longer producing purely CO2 from your fuel; you are now producing carbon monoxide (CO) and carbon black in the form of soot and char (C:H compounds in the ratio of 8:1 or so). Your flame is bright orange (1A) and you can see smoke and soot often rolling up and out of the kiln. Conversely, a fully oxidative flame is bright blue (1D), with transitionary flame compositions between the two (1B, 1C). CO and soot both deliver electrons to the surface of your pottery in the kiln. CO reacts with oxygen at the surface and forms CO2, leaving electrons behind. Soot will flow and adhere to the surface of whatever it touches in the kiln as incredibly small, free-radical black carbon particulate. As small carbon particulates build up on pottery surfaces, they begin to aggregate as soot and oxidize, causing a small flow of electrons into the ceramic surfaces. These processes result in the metals in your glazes gaining electrons, which means the net charge or oxidation state is reduced. Potters take advantage of this atmosphere for reducing oxidation states of metal colorants, soot production in carbon trapping, and more specialized glaze effects via soaking.
Efficient Gas Kiln Firing
by Hal Frenzel
Most anyone can figure out how to mix gas and air to produce heat in a kiln. What takes a little more expertise is firing a kiln with efficiency, regardless of what type of firing is being done. Understanding fuel combustion and the kinds of burners and other atmospheric controls that are available will help you understand the processes at work in a gas fired kiln, and will help you determine the best possible approach for your ceramic art.
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Jennifer Poellot Harnetty
Editor, Ceramic Arts Daily
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