Kiln firing differences

[kevinpleong]

Hi,

I am at a community studio currently and would like to get my own kiln soon but not sure what kind is the best for my work and what the differences between the types are. I was wondering if someone would be nice enough to explain what the difference between wood firing, gas, electric, reduction, and oxidation kilns is.

Thanks in advance

[Bill Kielb]

Without going into many paragraphs, all those firing techniques have characteristic looks with traditional wood being the hottest and likely the most random to let’s say cone 12 ish. Gas and electric are just different types of fuels, but gas traditionally would go to cone 10 and is also ideal for reduction where a handful of oxides are affected by a carbon monoxide rich atmosphere which causes variation in color and influence some oxides to be less refractory than in oxidation. Electric is mostly reserved for cone 6 and below and can be associated with brighter colors, less energy use …. Cone 6 oxidation electric is probably most popular for studios and hobby. 

Of course there are exceptions to all the above.I think the best way to begin to understand is to know the typical cone and look at as many pieces of each as you can fired with different fuels. This will give you an idea of the look and typical temperature, clay and glazes required.

That IMO  would be a good start, but as with most things in ceramics one can spend a lifetime learning the firing skills, glazing techniques, throwing techniques etc…. To provide a desired artistic look. For a start learn the look in common with each technique then if interested, learn the materials and process.

Edited December 14, 2022 by Bill Kielb

[Hulk]

Good question Kevin.
Starting out, I was convinced that a natural gas fired kiln would be the best choice for me.
Down the road from there - there isn't a place to set one up at our house, the neighbors are too close, a large enough gas kiln that works well would be heavy and expensive... I went with a used Cone 10 rated electric kiln to fire Cone 5/6 ware. I like it well enough that I'm looking to replace the unit with a new one when we move upstate.

Electric may be more portable as well.

The space, setting, and infrastructure available will likely be factors, also what kind(s) of work you are interested in, and your budget.

Edited December 14, 2022 by Hulk
added portable (I just want a new kiln tho', heh)

[Pres]

I like @Hulk thought that a gas kiln was my dream kiln. I would burn fuel, and like any fuel burning kiln would use up the oxygen thus being able to reduce the clay as in reduction. Fuel burning kilns could also do Oxidation, by making certain that plenty of oxygen was allowed into the kiln during firing. All of this could be done by adjusting the air coming in and going out. However, reality hit while teaching, electric was easier in a city/town situation. Further that with the cost of a gas line to the back of the house to the garage, and I caved for an electric kiln. I did make certain that the kiln could fire at least to the stoneware ranges so bought a kiln designed to be able to reach ^10. even though I wanted to fire to ^6 as that is the same as where I fired in the HS I taught at. The rest is history. 

In the beginning, firing at ^6 was difficult as many were not doing it, and even the glazes commercially available were limited. However, ^6 has become a very good area to be after the exploration of the 70's and 80's in the firing technique and with many esteemed potters making the switch to ^6 Oxidation, and others still doing reduction, but firing at the lower temp in their fuel burning kilns.

best,

Pres 

[Mark C.]

I am a gas  reduction fire cone 10 guy with an electric kiln that is only occasionally used to bisque only in.

I started 50 years ago and made my own gas kilns-I have three right now-a downdraft car kiln -gas-35 cubic feet a small updraft -12 cubic feet-gas a  24 cubic foot gas salt kiln and a skutt 10 cubic foot electric.I started when you could run your own gas lines and build a kiln and the building  inspector would sign off after looking it over.

In most places in the US those days are long gone-now you need a factory made kiln with all the safety  stuff and the spec plate  that says so.

These days you can find a high quality used gas kiln for 4k$ to 7k-say a Geil or a Baily if you look hard.

I recently found one for a friend a Geil for 6 k -18 cubic feet and saw a few largher for about the same $

As us old timers retire kilns are poping up on the market all the time these days

If looking for new look into Cooper kilns in Colorado as they cost less new

https://www.cooperworkskilns.com

 

For a gas kiln you need space  on your property and gas either Natural (best) or propane

Kilns (all need to be protected from the weather (shed ,shack, roof ,etc)

The kilns mentioned all have safety equipment on them and a inspector will sigh you off on the install, You will need a plumber to run the gas line

All these kilns will need to be trucked to you.

Edited December 14, 2022 by Mark C.

[neilestrick]

The simplest and most affordable way to fire is with an electric kiln. They are easy to use, easy to set up, can be installed in basements and garages and studios. They don't take up much room and are easy to vent. Firing to cone 5/6 you'll have durable, vitrified pots for food use, and there are hundreds and hundreds of commercially available glazes.

What type of kilns are used at your studio?

[Jeff Longtin]

I'll explain it this way: the choice between various kilns has a lot to do with the appearance you want you work to have. The type of fuel you chose to use will affect the appearance of your work. Electric kilns fire oxidation and mostly create a "neutral" atmosphere. That means glazes colored with stains mostly. Cone 6 is the popular "electric" temperature. Electric kilns are great for porcelain because the clean atmosphere maintains a white clay. 

The second type of fuel that is often used is either natural gas or propane.  (Propane tanks if you don't have a gas line nearby.) Natural gas/propane allows you to reduce the atmosphere and bring out warm colors in stoneware and porcelain clays and glazes. Celadons and tenmoku glazes look best in reduction atmospheres.

Wood is the third fuel and its a lot of work. Like the gas fuels it allows you to reduce the atmosphere and get beautiful warm colors. As electricity is a newer invention most of the world's beautiful "ancient" pots were fired in wood.

If you want a kiln that is easy to set up in a basement or garage, and can be moved easily at a later date, then an electric kiln is a good choice.  If you are in a more rural setting and have room for such things, outdoors in a kiln shed, a gas/wood kiln is a nice choice.

[Babs]

16 hours ago, Jeff Longtin said:

I'll explain it this way: the choice between various kilns has a lot to do with the appearance you want you work to have. The type of fuel you chose to use will affect the appearance of your work. Electric kilns fire oxidation and mostly create a "neutral" atmosphere. That means glazes colored with stains mostly. Cone 6 is the popular "electric" temperature. Electric kilns are great for porcelain because the clean atmosphere maintains a white clay. 

The second type of fuel that is often used is either natural gas or propane.  (Propane tanks if you don't have a gas line nearby.) Natural gas/propane allows you to reduce the atmosphere and bring out warm colors in stoneware and porcelain clays and glazes. Celadons and tenmoku glazes look best in reduction atmospheres.

Wood is the third fuel and its a lot of work. Like the gas fuels it allows you to reduce the atmosphere and get beautiful warm colors. As electricity is a newer invention most of the world's beautiful "ancient" pots were fired in wood.

If you want a kiln that is easy to set up in a basement or garage, and can be moved easily at a later date, then an electric kiln is a good choice.  If you are in a more rural setting and have room for such things, outdoors in a kiln shed, a gas/wood kiln is a nice choice.

Glazes coloured with stains, not sure about that. Maybe these days, with lots of folk buying commercial glazes

My shelves still full of the mineral chemicals needed for the colours. Been firing midfire forever it seems. Amazing the effects using an electric kiln, though I pine for my gas one  still.

Electric kiln a good starting point if you're doing it by yourself.

Enjoy it all!

[PeterH]

And I understand that slow-cooling can add to the available effects from some glazes in electric kilns, by giving them more time to develop crystals.

Super Cool! Slow Cooling in an Electric Kiln
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln

From https://digitalfire.com/picture/bestenazoh

Firing for Atmospheric-like Effects in an Electric Kiln  https://tinyurl.com/cf7ef94y
Fuel-burning kilns tend to be much larger than electric kilns. Because of their size, they usually have a slower heat rise, a soak at the top temperature, and slower cooling cycles. If you want similar results from an electric kiln, especially when firing to cone 6, one of the most important things you can do is emulate the heating and the cooling cycles of larger kilns. This means slowing the temperature gain to about 100°F (38°F) an hour during the last several hours of the firing, soaking the kiln at the top temperature, and then down-firing to slow the cooling cycle.

Electric kilns are built with thinner insulation and legs (to allow for air circulation), and they cool very quickly, especially at higher temperatures. If you are seeking buttery, matte surfaces but have trouble achieving them in the electric kiln, it is most likely due to fast cooling. Matte surfaces are usually caused by microcrystal growth during cooling, and, if the cooling cycle is too steep, there isn’t enough time for crystals to develop. In extreme cases, I’ve even seen matte glazes go glossy and transparent.
... note that the 100F(38C) above is "finger trouble", see 
What is the difference between fahrenheit degrees and degrees fahrenheit? https://digitalfire.com/picture/2704

Edited December 15, 2022 by PeterH

[Min]

Another factor that could make a huge difference in what type of fuel your kiln uses is insurance. Absolutely no way I could get home insurance for a gas kiln, it was hard enough to find an home insurance that covers electric kilns for a home based business. (I use BCAA in Canada) Also, where I live wood burning wouldn't be allowed. Even open wood fireplaces will be banned within 3 years.

 

[Mark C.]

Yes on the insurance issue with  at least a homemade kiln-my potshop and kilns are not covered by my home owners insurance (this was done  in the 70s or early 80s) but the shop (detached) is covered for fire (12k) through my business liability insurance. Now 12 k will not build a new shop and replace my 5 Brent wheels and two Peter  puggers  and an electric kiln and huge roofed area and all the materials- grinders and two gas kilns ,buts it something.

In terms of wood fired  kilns our county in rural areas does not restrict them (but we are in a very rural part ) in extreme NorthernCalifornia

wood buring is still a go as not much population is here and we have the cleanest air in the state usually (unless it a wildfire nearby)Which is about every other year now.

Edited December 16, 2022 by Mark C.

[Callie Beller Diesel]

On 12/14/2022 at 11:10 AM, Jeff Longtin said:

: the choice between various kilns has a lot to do with the appearance you want you work to have

I disagree with this one. I’m not going to claim that all things are possible in all kilns, but there are ways to get vibrant colours in fuel burning reduction firings, and there are LOTS of examples of folks that make work in a cone 6 electric that have a more subtle, earthy palette. While it isn’t common, you can use scrap pallet wood simply as a fuel to get to any temperature you want, although it’s usually associated with things like ash built up over days. Remember that a lot of firing techniques evolved and continue to evolve because people use the materials that are most readily available. Like Min, I cannot access a gas kiln either. Mark however, pays far more for electricity than he does propane, so that’s a deciding factor in what he uses. 

I think everyone here is answering this question with their biases, based on how they fire themselves, and which glaze chemistry they prefer. Fuel firing vs electric has an effect on the kiln’s atmosphere, which in turn affects certain chemicals in the glaze. Which chemicals? It’s pretty situation dependant. Your kiln is a tool used to apply heat and atmosphere. That’s only 2 variables among many.

I think the best way to help @kevinpleongis to first ask them some questions so we can advise them better. Kevin, what kind of work are you making, what temperature are you used to firing to, and how much experience do you have? How much work are you making, and do you anticipate that going up or down in the next 5 years-ish?  Our answers will be different based on how you plan on using your kiln. 

[Callie Beller Diesel]

For anyone who thinks that an earthy aesthetic is confined to a wood kiln, I just want to point them at this thread, and @Joseph Fireborn’s images. 

 

I have a piece of his, and if I didn’t know better, I’d swear it came out of a wood kiln. 

[kevinpleong]

On 12/14/2022 at 9:54 AM, neilestrick said:

The simplest and most affordable way to fire is with an electric kiln. They are easy to use, easy to set up, can be installed in basements and garages and studios. They don't take up much room and are easy to vent. Firing to cone 5/6 you'll have durable, vitrified pots for food use, and there are hundreds and hundreds of commercially available glazes.

What type of kilns are used at your studio?

I just finished a hand building class at Foothill College in Los Altos, Ca. I am going to be taking a wheel throwing class there this winder quarter. I have a community studio that is about 3 blocks away from me (about 10 mins) which is amazing. The only problem is they have an oxidation and reduction kiln but the glazes are very dark and mostly browns in the reduction kiln. I am not sure whether this is because of the glazes or the way they fire the reduction kiln. For the most part, I like the glazes at Foothill college, the closest community college to me, and they only do reduction firings to cone 10. I love the reductions at Foothill but only use the oxidation kiln at the local community art center.

[kevinpleong]

I want to produce more work but don't do as much right now as I am at a community studio or at a community college mostly just focusing on ceramics. I appreciate cone 10 because the clay is completely vitrified at that point. At the community art center I have been mainly using stains either as slips or as a body stain. Unfortunately, stains are pretty expensive but that is the only way for me to get nice colors at the community art center. I just started using glazes since I just had my first community college hand building class and their reduction kiln produces much better glazes. The community art center really only has browns and really dark blues, greens and reds. They are all very earthy and as stated above in another reply I'm not sure if that is because they are not firing the kilns properly, the glazes they make aren't good, or they just aren't consistant. Whenever I fire at my community art center and try and replicate a glaze I liked it almost always turns out a different color than the first time.

Edited December 19, 2022 by kevinpleong

[kevinpleong]

On 12/17/2022 at 10:27 AM, Callie Beller Diesel said:

I disagree with this one. I’m not going to claim that all things are possible in all kilns, but there are ways to get vibrant colours in fuel burning reduction firings, and there are LOTS of examples of folks that make work in a cone 6 electric that have a more subtle, earthy palette. While it isn’t common, you can use scrap pallet wood simply as a fuel to get to any temperature you want, although it’s usually associated with things like ash built up over days. Remember that a lot of firing techniques evolved and continue to evolve because people use the materials that are most readily available. Like Min, I cannot access a gas kiln either. Mark however, pays far more for electricity than he does propane, so that’s a deciding factor in what he uses. 

I think everyone here is answering this question with their biases, based on how they fire themselves, and which glaze chemistry they prefer. Fuel firing vs electric has an effect on the kiln’s atmosphere, which in turn affects certain chemicals in the glaze. Which chemicals? It’s pretty situation dependant. Your kiln is a tool used to apply heat and atmosphere. That’s only 2 variables among many.

I think the best way to help @kevinpleongis to first ask them some questions so we can advise them better. Kevin, what kind of work are you making, what temperature are you used to firing to, and how much experience do you have? How much work are you making, and do you anticipate that going up or down in the next 5 years-ish?  Our answers will be different based on how you plan on using your kiln. 

@Callie Beller DieselI want to produce more work but don't do as much right now as I am at a community studio or at a community college mostly just focusing on ceramics. Both places are not the easiest to work at as there open hours are weird. I appreciate how vitrified pieces are at cone 10. At the community art center I have been mainly using stains either in slips or as a body stain. Unfortunately, stains are pretty expensive (although cobalt is too) but that is the only way for me to get nice colors at the community art center. I just started using glazes since I just had my first community college hand building class and their reduction kiln produces much better glazes. The community art center really only has browns and really dark blues, greens and reds. They are all very earthy and as stated above in another reply I'm not sure if that is because they are not firing the kilns properly, the glazes they make aren't good, or they just aren't making the glazes consistent. Whenever I fire at my community art center and try replicating a glaze I liked it almost always turns out a different color than the first time.

 

I know a lot of people enjoy reduction kilns but from my experience at my local art center I can't quite understand why as most of the time the glazes turn out a lot darker and a lot more earthy.

[neilestrick]

7 hours ago, kevinpleong said:

The only problem is they have an oxidation and reduction kiln but the glazes are very dark and mostly browns in the reduction kiln. I am not sure whether this is because of the glazes or the way they fire the reduction kiln.

Probably a combination of their glaze choices, firing technique, and clay choices.

7 hours ago, kevinpleong said:

I appreciate cone 10 because the clay is completely vitrified at that point.

Cone 6 clays are also vitrified. There's no difference, assuming both are properly formulated.

 

[kevinpleong]

2 hours ago, neilestrick said:

Probably a combination of their glaze choices, firing technique, and clay choices.

Cone 6 clays are also vitrified. There's no difference, assuming both are properly formulated.

 

Yes and no. My understanding is that there are many commercial cone 6 clay that technically are not fully vitrified. They still allow them to be produced but they still technically have up to 2% absorption.

[Hulk]

I'm seeing the range of tested absorption - very low (1% or less), low (2%), somewhat low (2-4%), not low (5%+)* - for high fire clays and mid fire clays.
I'm still thinking absorption has to do with formulation/ingredients and firing to the specified cone.

*these range labels reflect my opinions!

[Callie Beller Diesel]

@kevinpleongWhen you’re early on in your clay career, many teachers and mentors will give you a lot of generalities that sound like absolute rules. That’s because clay knowledge is vast. Think really, really big. (Insert your own Douglas Adams reference). Because of that, teachers have to limit what they show to learners, or they’d wind up with a permanent case of decision paralysis from the sheer overwhelm. 

As you learn more, it’s a good idea to question those generalities and find out your specifics.  Not all cone 10 bodies meet a <1.5% porosity threshold either, but few people test for it. Clay is mixed from ingredients that can and do vary on the regular, and your firing cycle will matter in how close to maturity you get. If you fire differently than the way the manufacturer did under their testing conditions, your results can vary. 

 The generality of cone 6 clays not being vitreous usually comes from manufacturers that sell clays that are rated for cone 6-10. If you only fire that clay to cone 6 though, no it won’t be fully mature. Not everyone buying that clay is making production pottery and is concerned about long term durability, so that’s why they do that. If you go through the catalog that your supplier has, you can find many clays that are formulated to be mature at cone 6. If you’re making functional work, that’s what you need to stick with.

All that said. 

Generally, it is easier to get more vibrant colours at low and mid fire temperatures than it is at cone 10. It’s possible at cone 10, but you really have to do a lot of glaze testing to work out what you like. Reduction atmosphere and when in the firing it’s applied is a whole other layer of complexity to how glazes turn out, and is really good at adding visual depth to a piece without really having to layer your slips and glazes. The atmosphere creates more chemical reactions. One clay, one glaze and the right firing cycle, and you have some rich, satisfying surfaces. But they do easily drift towards a more earthy palette. If earth tones don’t make your heart sing, that part can be really frustrating. At cone 10, the clay and glazes are simple, but the firing is not. 

Cone six oxidation firings will usually make you a better chemist. Vibrant colours and neutrals are both pretty easily obtained.  While you can adjust the glaze’s outcome with carefully applied soak holds in an electric kiln for some really great results, the firing is simplified, compared to cone 10. But that means to get more visual depth, you have to pay a lot more attention to your surface applications, and how they work together. When working at mid fire temperatures, the firing is simple, but the clay and glazes are less so.

 

 

[neilestrick]

3 hours ago, kevinpleong said:

Yes and no. My understanding is that there are many commercial cone 6 clay that technically are not fully vitrified. They still allow them to be produced but they still technically have up to 2% absorption.

I said 'properly formulated'. There are commercially produced cone 10 bodies that have greater than 2% absorption, too. There is nothing about cone 10 that makes work any more durable or functional or safe to use. It's just hotter. That's it. Regardless of the temperature you're firing at, you can make vitrified, durable, safe pots.

[Min]

15 hours ago, kevinpleong said:

I appreciate cone 10 because the clay is completely vitrified at that point.

When ceramicists use the term vitrified it usually has a different definition than a person who works with glass. It's my understanding that the term "vitrified" for a glass worker / gaffer / glass blower/smith etc means that the materials have been transformed into glass or a glassy substance by heat and fusion. Potters have adopted the term vitrified to encompass not only glazes but ceramic bodies as well. 

There can be confusion with this.  Given that claybodies contain crystalline structures as such they cannot by definition be transformed into a glass or glassy substance.

IMHO a better description of zero porosity vitrified claybodies would be a claybody fired to maturity with zero absorption. Is zero absorption necessary for functional wares intended to hold liquids? Nope, in fact there will be a high number of stoneware or earthenware bodies that will slump and or bloat well before they reach 0% absorption. Are these bodies with higher than 0% absorption still suitable for functional wares intended to hold liquids? In many cases, yes absolutely they are. Under approx 1.5% (one point five) absorption bodies are used by many potters for such usage with no issues. 

Fritware bodies fired in the earthenware range can have absoption figures down to zero as can midrange bodies or any temp/cone you would like to fire at. Are cone 10 bodies vitrified just because they are fired to cone 10? Maybe yes, maybe no. Fire a raku or sculpture body to cone 10 and it's absorption will probably be well be over 1.5% still. Overfire a claybody and after it's densification is maxed out the absorption/porosity will actually increase as the body breaks down.

[Kelly in AK]

Most clay sellers (all I’ve seen) publish specifications on their clay bodies tend to that include: fired color, shrinkage at the stated maturing temperature, and absorption at the stated maturing temperature. If a clay is sold as ranging from cone 6-10 they should show absorption at cone 6 and cone 10. There are many cone 6 clay bodies available that I consider vitrified (1.5% or less absorption). There are some with higher absorption, say 2%, sold as good for functional pots, and still other clays made for non functional work. It’s a slight complication that fired strength and porosity don’t always correlate (mostly they do). You just have to draw the line somewhere. Some potters accept more porous clay as long as it’s sturdy, usually with the caveat “microwaving not recommended.”

As cone 6 oxidation firing has become more of a standard over the last twenty years issues of non-vitreous clay and problems with glaze fit in that range are much less a problem than they once were.

By the way, I’ve never seen a clay that’s vitreous from 6-10.

[Pres]

Over the years, it seems the answer to the question of vitrified depends on the beliefs of the potter.  Considering this, I believe that the best way to confirm your beliefs is by testing. I have found that most suppliers are willing to supply sample clays with orders if the potter asks. I know that the folks at Standard Ceramics have been very accommodating when I mention looking for a clay with specific numbers in relation to absorption and shrinkage rate at cone 6 in that they will suggest a few clays, and let me have some samples for testing. I usually only take one at a time, and use it on the wheel and handbuild a bit to get a feel for the workability, then check the clay out in firing and then with my glazes.  It is a really good idea IMO to make good connections with your clay supplier in order to get the best service possible.

 

best,

Pres