How to Fire A Gas Kiln Efficiently: Hal Frenzel Answers this Burning Question

Though gas kilns are not as ubiquitous as electric kilns these days, there  are still many potters and ceramic artists who rely on them to create the wonderful effects created in a reduction, salt, or soda atmosphere.

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. In today’s post, Hal Frenzel explains how understanding fuel combustion will help you make the most of your gas kiln.- Jennifer Harnetty, editor.


What is perfect combustion? By reading the description in the North American Combustion Handbook, it seems rather simple: 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.

When excess oxygen is present in the kiln, it plays no part in combustion. However, it does absorb heat energy that would otherwise help fire your ware. In this way, it does contribute to fuel consumption. (relieved of their oxygen molecules) by oxygen-hungry fuel. This typically results in a color change.

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 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 color 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 (see diagrams on page 38). 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.

In the early stages of a firing, excess oxygen helps in the decomposition of the organic and inorganic carbonates and sulfates. In researching this article, I was unable to find a potter/ceramist who could explain exactly how excess oxygen during the glaze maturity period enhances the glaze finish or color. This raises the question as to whether the results would have been the same if fired in a neutral atmosphere during this period. If, by chance, the results are the same, then an oxidation potter would save both time and fuel if he or she fired in perfect combustion during this period.

Oxygen to burn fuel in an artist’s kiln comes from the air. The air, however, is not all oxygen. Rather, it is far from it. By weight, air is approximately 77% nitrogen and 23% oxygen. What this means to the artist is that for every ONE pound of oxygen from air that is heated to kiln temperature to burn fuel in a kiln, THREE pounds of nitrogen have to be heated to kiln temperature. This is why using “excess” oxygen is expensive. Using a minimum amount of excess air in an oxidation firing saves both energy and money.

 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.

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.

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 than there is oxygen from the air to burn the fuel. For complete combustion to occur in a reducing atmosphere, t
he 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.

 


 

 

 

Comments
  • this is a good intro article in my opinion. It covers the basic terminology involved in firing gas kilns. Its dangerous to pinpoint advice and techniques since every kiln fires different. I build kilns all the time and even the same design, with same burners, exit and inlet flues fires different based on a variety of factors. One of the things that i will point out that is lacking is about atmospheric burners. The best way to set reduction with these burners is thru the primary air adjustment dial on the back of the burner. So many potters have told me they don’t use these dials, way better to use these than a damper to set reduction. I agree that an oxy-probe is a great way to fire a kiln, but even the low end axner model is very expensive. The best advice for firing gas kilns is to get involved with alot of different types of gas kilns, fire often!
    To anonymous, I hate to be skeptical, but I have an extremely hard time believing your firing a gas kiln to cone 10 for 18 dollars!!!! propane is currently 2.50 gallon in my town, thats less than nine gallons my friend! Unless you have a little gas kiln that is entirely fiber and very small?
    Renee, I understand your concern with the quickness of firings. yes of course an oxy-probe would help with reduction info, a cheap pyrometer (Euclid’s sells a cheap digital that works well) will help with temperature info. I would start with just pilots overnight, then one burner for the first hour or two, then switch on the second one after that. Don’t worry about temperature differences, its early in the firing and things even out quickly at that point.
    happy firings all!
    shane mickey

  • I have a 9 cu ft gas kiln I’m still “getting to know.” This article has been helpful as there aren’t many resources or readings out there on firing light to med reductions in gas kilns and what other potters’ experiences have been with theirs. The difference with this new second-hand kiln I have is that it is insulated with fiber instead of bricks. So my firing schedules have been very hard to slow down as I find this kiln to heat up way too quickly for my liking.
    I’ve managed some excellent results–I fire to ^6 (~1220C), but not without some stressful babysitting at the pressure gauge to slow things down.
    Would anyone have some tips, insight or words of wisdom to share regarding their experiences with firing in a gas, fiber-insulated kiln? Would it be handy to use an oxygen probe?

  • After years of firing other people’s gas kilns I finally bought my own kiln and bought an oxygen probe as well. It really helps to reduce the cost of the firing. With my kiln, it’s often difficult to tell the difference between the degrees of reduction (yes oxidation vs heavy redux is easy , but light redux vs med, vs neutral isn’t so). I have found that I can get really good results (awesome copper reds) with light reduction rather than heavy. I also fire neutral between body redux and the start of glaze redux. I learned to fire at a school where everyone was into heavy reduction and since owning the probe I have learned that it isn’t necessary, even for celadons. Last time I fired it cost me $18 bucks and took only 10 hours.

  • Shane, many thanks for your advice. My fiber gas kiln has four burners and to slow things down I have resorted to turning on one burner at a time, with hours in between but I have been concerned about uneven temp differences when doing this. So far, I have these quite nice flashings on the pieces that are along the outer most part of the shelves. I rather like these and have accepted these as my kiln’s little kisses, for lack of a better way to describe them;)
    And reading that you too suggest I turn burners on gradually is good for me, its the exchange of ideas and experience that keeps us safe and improving. Thanks for your time in doing so, its hugely appreciated.
    As for dollars and fuel, well, I live in Australia and my firing costs are not cheap. But I love what I do so I make it work; I’ve almost got the firing schedules I need for my ^6 ox & red down to a ‘fine art’ for my kiln and with these, the pricey gas used will just have to be factored into my sales or gift giving values.
    Do keep the conversations about gas firings going though. I think there are a lot of us out there who will want to read more about anything productive anyone has to offer about it.
    Kind regards to all.

  • I live and teach pottery classes in Mexico. A good friend bought perfectly acceptable pyromete on Ebay for $35 and brought it down.
    I have been trying to find an explanation for a problem I’ve been having and have not found an answer to. I am using a cone 05 formulated clay used by facotries in Mexico to slip cast a variety of products. I use it for handbuilding. MOst of the time it has worked well but on occassion for no reason I can discover, one or two of the pieces will begin to disintigrate. Out of a load of 10 or 15 pieces, two or three will begin to disintigrate after a day or two, some wil be perfectly fine with a nice ring to them for 2 or 3 weeks and then just dissolve almost overnight into small fragments, most of the work remain intact. All the peices are made with the same clay, all fired in the same kin, in different locations in the kiln,some even made by the same student. On one piece the base dissolved and the figure sitting on it remained intact. It’s the strangest thing I’ve ever encountered and cannot get to the bottom of it. I fire in a gas kiln to 08 or so as I don’t want to loose the burnish that exists on some of the work.
    Anyone have a clue about what might be happening?
    Thanks
    Dan

  • A friend of mine owns a foundry here in Gaudalajara and suggested I assist the upper end of the firing by adding an outside source of oxygen (welding O2) to the propane. Any advantage to that?

  • pete,
    every kiln is different, but in mine neutral is achieved by going into reduction a little, judged by the orange flicker in the flame viewed through the burner quarls, and then adjusting air until I only just lose that orange flicker. At cone 10 I get a slight flare at the top spy in neutral. There’s got to be a cheap pyrometer out there – anyone?
    Brian.

  • Dan I also get those “Little kisses” sometimes on my earthenware in my small fiber/gas kiln. But I learn the hard way and for the pieces where terra sig made out of the same clay can get a rough feeling-I’m gonna cover with clear glaze and try to fire again, but I broke down and got real lucky- picking up a good working L&L kiln for $175-K type about 30 yrs old.
    The part about learning the hard way was making a real nice peice and trying to fire it in the gas kiln that goes to 1,800 degrees in an hour or less and it made a real nice explosion. I bought the electric with a kiln sitter and go REAL slow. And actually met a guy who sold me a wheel who bought out another mans pottery shop that was over 25 yrs old. He has the original L&L top and bottom for the kiln I bought STLL IN THE CRATE! lUCK PLUS.

  • What would be the indication of a neutral fire, either at the flue or at the spy-holes? I cannot afford an oxygen probe nor a pyrometer, so what do I look for? Anyone care to answer?

  • This is great to hear all the information about firing your gas kilns, I have been firing with propane for over 30 years and for the first 18 years I BUILT AND FIRED A 28 CU FT kiln with 4 low pressure burners and would adjust the primary air with the dial on the back of the burner and used the damper to get the reduction I needed, it was a great kiln. For the past years I have fired a 30 cu ft. kiln with four high pressure burners and it is like night and day. It sounds like most of you are firing in low pressure. My kiln is a down draft type and fires great. I have been firing carbon trap Shinos and had to learn how to fire my kiln all over. I have found shino glazes (carbon trap} are more involved than copper reds. I bisque to 08 for better absorbtion of the soluable materials in the glaze and go into reduction at 012 and stay there until cone 9 and oxidize for 10 to 15 minutes and then back into reduction to cone 10 on bottom and 11 on top. Does anybody have a better way? The copper reds are better than ever. Thanks and keep up all the dialogue on gas firing.

  • What effect would a fan has in firing a gas Kiln

  • interesting because my raku kiln has a huge gap between burner and kiln and it does roar at 10psi, in a nat gas geil id start light to med reduction as early as 1350 or so, keep it slow and steady the rest of the way and finish how u like. It seems as long as u catch early reduction u have better odds for deep copper reds and purples. carbon trap shinos i let nature sort out cuz im clueless but they seem better thin

  • Dan,
    That is a really strange situation. It sounds like the clay is being over-fired. Is your clay high in iron? Is there any chance that a flux is getting mixed into the clay slip (possibly as a deflocculant)? Are you using a release agent in your molds? Are the glazes interacting with the clay body causing it to flux?
    DeBorah

  • Folks – if you don’t get the answers you need here, you might want to try posting your questions on the Ceramic Arts Community Forum: /community/

    -eds

  • Adding welding oxygen could be dangerous, and is an unnecessary expense. Try using a blower instead. But…..if your orifice is the correct size for propane, you should have enough gas velocity entering the venturi to create an efficient oxidizing or neutral flame, without any of these “cheats”. City gas requires a much larger orifice than propane, because it’s delivered at a much lower pressure. Also, city gas yields only a fraction of the B.T.U.’s of propane, all other things being equal.

  • A word of caution here, about trying to achieve a “neutral” atmosphere to save on gas. I spent $800 to buy an oxyprobe, in the year 2000. Fitted it to a little test kiln (just a single burner). It did what it claimed to do, but $800 was a lot to spend.

    Later developed a “kiln exhaust sniffer” to do the same job for peanuts. Full of new enthusiasm, I fitted an exhaust sniffer to our bigger kiln (27 cubic feet, four burners). Then, expecting to save gas by firing “neutral”,I carefully adjusted gas and air for the next firing. The load included several shelves of little frogs glazed in a copper green. Surprise, surprise! Frogs on one side of the kiln were lovely green. Frogs in the opposite corner were pink, obviously reduced. When you think about it, just because the atmosphere is neutral as it emerges from the flue, doesn’t mean that all four burners are operating with the same oxidation level. The flue gas is just the average. So I’ve learned to adjust the kiln a little on the oxidation side of neutral. I’d guess this same effect could apply to any kiln with more than one burner.

    Try Googling for “kiln exhaust sniffer”. Or find a pdf version somewhere on our website http://members.optusnet.com.au/~rogergraham

  • Daniel, I had the same problem of pots disintigrating in a firing but the circumstances were completly different. My pieces were fired in an Anagama kiln fired to cone 11/12. I have not gotton to the bottom of the mystery but I believe it had to do with the pieces being glazed inside and not on the outside of the pots. I use a flashing spray to encourage the fly ash to flash and make color. Sone of the pieces had to be swepted off the shelf and others fell appart 36 hours later, I took a picture of a piece and 5 minutes is was in a heap, needless to say I was shocked, never had this happen before. I was trying a new clay in the wood firing of which I have used in my gas kiln to cone 11. I believe it had to do with the pressure of the glaze on the inside not being compatable with the clay with no glaze on the outside. I will test this again the next time we fire the Anagama. Are your pieces glazed in and out? Does anybody have any ideas?

  • In college I learned to fire on Johnson Forced air gas burners on NG. Burners were great, big blowers, tons of air, and plenty of power while still being finite in control. I was taught firing slowly as to not damage pots with glaze moisture in them, and to also give my glazes/pots enough time to properly fire. We had gauges for our air pressure and for our gas pressure, but they had long since been cooked by heat from burner ports, and no longer functioned. The way we fired was by creating an overnight preheat flame which was a blue cone in the center of our flame with tinges of orange and yellow flame surrounding it, and was not extremely forceful or overly lazy. During firing we would hourly “bump” our kiln up using “turns of air knobs” as our increase, and adding gas to get an efficient flame. We would do this until body redux, where we would remove air, and close damper down to go into redux, and would hold that there until the end of firing. We would tell the amount of redux by looking at the flame length which was exiting through our spy holes and damper (updraft kiln). After a few firings you learned what was the right amount of redux per your specific glaze palette. Really great kilns, with expensive burners, which would take around 16 hours (inlcuding preheat) to fire to ^10.(~35 cu.ft soft brick kiln, home built, 9″ walls)
    Now I fired in a home built, downdraft car kiln, soft brick, 9″ walls with big bertha weed burners. Burners are WAY cheaper than the Johnsons ($150 vs $5k), and are natural draft burners (burners have no adjustment for air control so we use our damper to control redux). We are also firing on LP vs NG. We use a PSI gauge to adjust our “bumps” up in temp and fire with low pressure. From room temp to cone 10 it takes just 7 hours and the kiln is ~55 cu.ft. Glazes come out looking beautiful and I never blow up pots or have fatal glaze issues. Our LP costs us $1.80/gal and we currently spend around $40 to fire one gas load. I utilize the same method of calculating redux by measuring flame length exiting spy ports and once I know what I want in my glazes I just repeat my firing method.
    Every kiln is going to be different as are all the glazes that each potter uses. If your concerned about what you spend on gas to fire one load you should look at what you spend vs what you earn in finished pots. $10-50 in gas is not something I would worry about if it means possibly getting a load of pots that comes out looking like poo. Likewise since there are so many variables from one potter to the next its almost impossible to say there is one way to fire every gas kiln. Everything in this business takes time and trial, and thats only something that can happen with you, your kiln, your pots, and your glazes. Its obvious the difference between heavy redux and straight oxidation. Not many glazes will be highly affected by 1-5% oxygen levels.
    Also, personal motto; dont begin to depend upon tools to tell you how to fire because the minute they break you’ll be unable to work. Learn to fire by sight. It may take time and effort but it will make you a much more versatile ceramic artist for doing it. After 16 years of making pots, and firing 70-80 gas loads a year I know what works for me. Part of the fun of this art form is experimentation with your glazes and your firings, it can be trying at times when your hunches dont turn out the way you thought, but remember, its only mud.
    —Dan, your friend suggesting to use welding gases at the end of your firing is setting you up for serious dangers. O2 is highly explosive and I would not want to be bringing tanks of explosive gasses near a 2,000 degree kiln. As well it takes Oxygen, a fuel source, and heat to create fire. If you’ve already got ignition there is enough oxygen present to create combustion. If you want more heat in your kiln at the end of the firing a small squirrel cage blower, or fan will help even out gas heavy natural draft burners better than a tank of explosives. Make sure you’ve got the right orifice size in your burner for the style of burner your using, the type of fuel your using, and the pressure at which your consuming it.
    –Pete, neutral at the spy ports will be hard to judge. Its easier to tell by looking at your flame. Too much gas = blue flame, too little gas will be orange or yellow. Blue cone in the center of yellow tinges is a good indication of neutral or efficicient firing.

  • On the subject of adding oxygen, adding oxygen to fuel to increase combustion efficiency is what makes a cutting torch possible. It can be done safely. Oxygen is not actually explosive. It makes gas explosive, but, you will not have any more significant risk of explosion than you otherwise would with normal air. I would imagine that it would make your kiln heat up faster, if, as the article states, you don’t have to heat up as much nitrogen. The bigger issues would be the faster heating and the extra expense of setting up an oxygen delivery system. I would guess, even after setup costs, the oxygen is more expensive than the gas. Unless you have a real problem firing oxidation in gas, it’s not worth it.

  • My husband is building a small gas kiln/Fibre, and the fibre blanket is rated at 1300 degree and the other blanket at 1260degrees ( we are using the two blankets, both 25mm thick) there will be two burners using propane gas. I am wandering after reading all the comments by potters, if this is going to make the firing too quick? Thanks for any advice.

  • Hi everybody, this is for Sam: could you give me some gas kiln plans? I just moved from Canada to Dominican Republic and can’t find a kiln so I’m planning to build one but so far, I didn’t found good plans…
    I said “this is for Sam” because he build his kiln but by all mean, if anybody can help me, I will be the happiest potter on the island!
    Do I have to mention the difficulty to find raw materials to make my own glazes?
    Thanks for all suggestions,

    Elisabeth.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.

Larger version of the image

Send this to a friend