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My Gas Kiln. Firing a Gas Kiln Efficiently-What is perfect combustion?

My gas Kiln. Home made Second hand Kiln.

One gas inlet with Chimney.

Firing a Gas Kiln.

Your Glaze Firing -Oxidation/Reduction.

My kiln has 3 shelves.  you can see on the left, there is only one, gas inlet area, at the bottom of the kiln.

Some Kiln shelving has been put on the side, so the gas fire goes up and around.

Turn the gas  burner on to KPA5.

We put the bung in at the front of kiln, when temp reaches 500. (1hour)

Turn up to KPA10. It takes a while for the temp to go to 900. (2 hour +).

Start reduction. Turn up gas to 15kpa, start to close off chimney. ( I have a piece of kiln shelf that can be pushed across), black fire will flow high up chimney, let it settle, So only do a little at a time.

You can not close the chimney completely. You will end up with black pottery if you do.

When you reach your temp we do 1280.put every thing back and turn down to 10KPA and let sit (soak) for two hours before turning off.

Next Day you will be able to open your kiln.

What happens when you open, when  your kiln is to hot.

Is it to hot to open the kiln.

I did not do it, my husband opened it after turning the gas off, he wanted to see inside. All he seen was red hot. Only opened kiln up about 6 inches.  Results were not good the top shelf was pieces of clay every where. All my pottery I had done was broken or had pieces of pottery embedded in them, You could say the pottery had a smashing time.

Do not open your kiln until it cool down.

What is perfect combustion?

Perfect combustion exists when one carbon atom is combined with two atoms of oxygen to form one carbon dioxide molecule, plus heat. But when you are firing a kiln to achieve a certain consistent atmosphere, it becomes a little more complicated.

To achieve complete combustion, the exact proportions of fuel and oxygen are required with nothing remaining. In a gas kiln firing, this is often difficult to attain because of the many variables in fuel and oxygen (which is derived from the air) and the equipment used to mix the two.

The most common fuels used today are natural gas and propane. These are hydrocarbons and when they are properly mixed and ignited, they produce heat, carbon dioxide and water vapor.

Air is a combination of approximately 75% nitrogen and 25% oxygen by weight. Unlike oxygen, the nitrogen does not react (combust) but it still absorbs a portion of the heat and therefore creates a cooler flame.

During the firing of a gas kiln there are a trio of atmospheres that have to be controlled to achieve both a rise in temperature and the desired glaze results. The first, and most important, atmosphere is neutral. It is only in a neutral atmosphere that perfect combustion can be attained. A neutral atmosphere is the most fuel-efficient firing possible.

If the amount of air is increased, or the amount of fuel is decreased, from a neutral firing, the mixture becomes fuel-lean and the flame is shorter and clearer. The gas kiln has now entered an oxidizing atmosphere and the rate of temperature rise will decrease.

If the fuel supply is increased or the air supply is decreased the atmosphere becomes fuel-rich and reduction begins. The flame becomes long and smoky and incomplete combustion occurs. The result is an excess of carbon, which combines with the remaining oxygen and creates carbon monoxide. To convert back to its natural state of carbon dioxide, the carbon takes oxygen from the metal oxides in the glaze, thus altering the finished colour of the glaze. The rate of temperature rise will also diminish under these conditions.

Regardless of the atmosphere necessary for the results you desire for your work, a higher level of efficiency and fuel savings may be attained by firing to a neutral atmosphere whenever possible . With the enormous increases we have seen and will continue to see in fuel costs, it might become highly desirable to buy an oxygen probe and maintain a neutral atmosphere for at least part of your firings.

Oxidation/Reduction- is the result of the colour in your glaze.

Oxidation Atmosphere: A mixture of fuel and air where there is a significant excess of oxygen from the air relative to the fuel; defined (somewhat arbitrarily) as more than 3% excess oxygen.

Neutral Atmosphere: A theoretical mixture of fuel and air where there is a perfect balance between the amount of fuel and the amount of oxygen from air necessary to burn that fuel.

Reduction Atmosphere: A mixture of fuel and air where there is more fuel present than there is oxygen from the air to burn the fuel. For complete combustion to occur in a reducing atmosphere, the fuel must react with all the oxygen from the incoming air and with oxygen from other sources. For a ceramics artist, the important “other” sources of oxygen are oxides of iron and/or copper in the ware being fired, as those oxides are reduced.

When an excess of carbon (fuel) or a shortage of oxygen (air) is introduced, incomplete combustion takes place. Carbon monoxide (as opposed to carbon dioxide) is produced along with heat, though not as much as would be produced during complete combustion. The carbon monoxide then looks for more oxygen, which it takes from oxides in the clay and glaze in the kiln. This is also the reason yellow flames shoot out through spy holes when a kiln is in reduction—the carbon-rich fuel is following the oxygen supply.

Gas Kiln Firing – Defining the Terms

Oxidation Atmosphere: A mixture of fuel and air where there is a significant excess of oxygen from the air relative to the fuel; defined (somewhat arbitrarily) as more than 3% excess oxygen.

Neutral Atmosphere: A theoretical mixture of fuel and air where there is a perfect balance between the amount of fuel and the amount of oxygen from air necessary to burn that fuel.

Reduction Atmosphere: A mixture of fuel and air where there is more fuel present

Firing a gas Kiln.

Cone 10. Glazers.

I glazed all my pots, high firing glazers.

What happens in the kiln is different on each shelf, different at the front, back and middle of each shelf. The temperature is different all over the inside of the kiln.

Firing at high firing gas kiln is a lot of luck. When I get great results from a firing, I end up doing the same glaze in a different part of the kiln.

Using red glazers, at the bottom of the kiln you get red. At the top you get, a beautiful green. That the different between Oxidation and reduction. If only we let the kiln sock longer. We might of got red on top to.

Every firing is different, great fun to open the kiln, and see the results.

Happy potting have a great day.

Wendy’s Pottery Journey.

Wendy

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