Reduction Firing

I have long used the discoveries of Pete Pinnel to facilitate my reduction firings, and although my results speak for themselves, I rarely talk about my process. When I give a workshop I only talk about reduction firing when asked, and even then I see blank or angry stares from the crowd. It is truly amazing the degree to which opinion becomes entrenched, and potters’ unwillingness to try a new way of firing is astonishing. I want to share this method with you because I know from experience that it works.
Reduction firing is the process of allowing more gas than air to be present inside a kiln, and the real workhorse of reduction is carbon monoxide, not smoke. When enough carbon molecules are agglomerated together to be visible as smoke, a very poor agent of reduction has been created. Carbon Monoxide is a very tiny molecule, and has no trouble getting into a glaze or clay and making the carbon dioxide exchange, creating the beautiful colors that are the hallmark of high-fire reduction firing; celadon, copper red, shino and tenmoku glaze colors to name only a few. Creating smoke inside a kiln at the time of reduction is also extemely inefficient and wasteful of natural gas or propane, something to think about in these times of rising gas prices. Pete Pinnell has said: “If you are creating smoke in your firing, you should see dollar signs disappearing from the top of you stack.”
The object is to create an abundance of carbon monoxide at the time of reduction within the atmosphere of the kiln, and the way to do it is to increase the efficiency of combustion between gas and air, NOT to decrease air to the burners, as has been taught for decades in the United States. It is also critical to start reduction early, before the glazes have any chance to begin to melt (sinter). By increasing the amount of gas and air at cone 012, a short, bushy flame is created, increasing the amount of carbon monoxide that is a byproduct of efficient combustion, creating an even and effective reducing atmosphere throughout the kiln, while at the same time causing the temperature rise that is needed and wanted to carry the firing through to completion. It is the damper that is used to help create reduction and ensure even distribution of heat, not decreasing air to the burners.
I fire a downdraft, insulating firebrick kiln, using natural draft venturi burners, and I fire in straight oxidation to cone 012, allowing about six hours between lighting the burners and the time cone 012 falls. I use natural gas, and I am using three water column inches of pressure at the time cone 012 falls, with primary and secondary air ports wide open by that time of the firing. All burners are turned up to eleven water column inches of pressure as I see the fast and bright blue flame appear and hear the roar of combustion. I always begin with the damper halfway in, so I can go either way, push it in or pull it out, whatever is needed to create reduction and keep the temperature rise even top to bottom. For my particular kiln, I rarely have to adjust the damper very much, but always remember this simple rule: push the damper in, heat up the top; pull the damper out,heat up the bottom.
I watch the oxy probe readings as the atmosphere climbs into medium reduction, where I keep it all the way to cone 9 down. If you don’t use an oxy probe, adjust the damper so you get licks of blue flame from the bottom peep. When cone 9 is down, I turn all the burners down to five water column inches of pressure, putting the kiln in oxidation and slowing the firing. I take a half hour to 45 minutes to allow cone 10 to fall, and this time of oxidation is critical to create the orange color from my kaki glaze and brighten all my glaze colors, as well as give any escaping glaze gasses (pinholes) time to heal over. I get stunning copper reds, shinos, celadons, tenmoku’s, wonderfully subtle white glazes, my special chun, etc., etc., all in the same firing. When cone 10’s are down top and bottom, I shut off the gas and close the damper to within two inches of being completely closed. I also shut the secondary air ports, which are designed to allow some air to continue to be pulled through the kiln, so a very quiet circulation of air is allowed to continue throughout the kiln. Cooling occurs neither too fast nor too slow, a period of time that I never push, and I allow a full 30 hours before opening the door.

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