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Raku Kilns


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#1 centered

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Posted 25 August 2010 - 04:47 PM

what is the differences in raku kilns? portable vs. non portable? will your results be the same in any raku kiln?

#2 Seasoned Warrior

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Posted 26 August 2010 - 12:36 AM

what is the differences in raku kilns? portable vs. non portable? will your results be the same in any raku kiln?


I enjoy raku but it is not my primary technique. The raku kiln just needs to be able to heat up your bisqued piece so that you can drop it into a container with combustible materials that basically use up the available oxygen creating a reduction atmosphere. I have used things from a pile of bricks to form a container to a smelter to heat up things to raku temps so the kiln is irrelevant. I think that the things to look for are convenience and safety. There are kilns like the tophat that remove the main portion of the kiln from the base allowing one to grab the heated piece with tongs easily and with no obstructions. There are clamshell kilns and I've even used a scrap top-loading electric kiln with a hole drilled in the bottom and a Ward's venturi gas burner shoved in the bottom to heat up the ware. I think that the things to look for are ease of access to the ware and the ability to grab it with tongs easily. The work flow from the kiln to the raku container is probably the most important consideration; you don't want any obstructions. I like to be able to just grab the ware and drop it into the container and not have to worry about anything getting into the way. Another consideration might be insulation to reduce the amount of fuel needed to reach temperature and the temp rise rate.

Regards,
Charles

#3 hansen

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Posted 26 August 2010 - 04:59 AM

what is the differences in raku kilns? portable vs. non portable? will your results be the same in any raku kiln?


I once built the classic updraft, wood-fired raku kiln, but basically used it to fire burnished work in. The local clay was extremely fine particle 300 million year old earthenware clay, all you had to do was rub it with a soft cloth to get a burnished surface, the kiln was made out of brick covered with a layer of clay, and I had construction crews dump wood off at the house. The pictures are in the Leach book as well as the Cardew book, if you don't know the titles, google the authors with the word "pottery"
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#4 Seasoned Warrior

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Posted 26 August 2010 - 02:58 PM


what is the differences in raku kilns? portable vs. non portable? will your results be the same in any raku kiln?


I once built the classic updraft, wood-fired raku kiln, but basically used it to fire burnished work in. The local clay was extremely fine particle 300 million year old earthenware clay, all you had to do was rub it with a soft cloth to get a burnished surface, the kiln was made out of brick covered with a layer of clay, and I had construction crews dump wood off at the house. The pictures are in the Leach book as well as the Cardew book, if you don't know the titles, google the authors with the word "pottery"
h a n s e n




It never ceases to amaze me how many excellent and well connected potters are represented here on this forum. this forum is truly an incredible resource. I am a fan of Bernard Leach and I am impressed that your kiln was in his book. Thank you for mentioning it!

#5 Chris Campbell

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Posted 26 August 2010 - 04:16 PM

This is probably the simplest kiln use ... you just need to be able to get at the work quickly and safely.
The most important parts are probably the quality of your burner and your insulation.

I've seen them as simply built as using an oil drum to the weirdest one built in an old Volkswagon!

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#6 Idaho Potter

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Posted 26 August 2010 - 07:26 PM

The only difference in kilns is having something that fits your outside area and you can handle safely by yourself--if necessary. Temperature high enough for most raku glazes to flow is about cone 06. You'll have to keep an eye on what you're firing because there are subtle changes that take place quickly as the glazes mature. My kiln is low enough so when the top is removed tongs can easily access the pots and transfer them to the smoke pots (metal containers with a bed of wood chips or sawdust, and newspaper for the flash firing). After the smoking is finished, I usually immerse the pots in a bucket of cold water. Yes, it shocks the pot, but if you are using a raku clay body, it is made to accept thermal shocks of heat to cold. The joy of raku is the fact that planning doesn't mean as much as serendipity does.

#7 centered

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Posted 27 August 2010 - 03:38 PM

Thanks for all your thoughts on raku kilns.
centered

#8 Marcia Selsor

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Posted 27 August 2010 - 04:30 PM

when I started teaching in Montana in 1975, there was a brick wood-fired raku kiln that took 8 hours per cycle. I built one using a barrel, fired with wood that could fired ...after several loads, in 3.5 minutes. It is featured in the Steven Branfman Raku Book, a Practical Approach, 2nd. ed. and it is my design. Point is, American raku is a fast firing. You can use less fuel if you insulate well and have a good efficient fuel consumption.
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#9 Marcia Selsor

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Posted 31 August 2010 - 07:06 AM

This is a picture of my latest kiln. This one is in 21st Century Kilns by Mel Jacobson. This is an extremely efficient kiln with two burners running off four propane tanks. I use 3.5 -4 pounds of pressure. The tanks don't freeze and I am in a hot , humid climate. I had a similar one I built in Montana that was designed to be portable. The frame assembled to the base which was anchored by the weight of the kiln. The frame was iron tubing. That one required two people to lift the kiln chamber on the pulleys. My new kiln is countered balanced with a bucket of sand and rocks. I put it inside my kiln shed to protect it from tropical rains. I can fire by myself. I enjoy firing with others but there aren't many raku people down here.Attached File  rakukiln.jpg   446.54KB   57 downloads
I think the results in firing raku kilns depends greatly on weather, the atmospheric setting of your burners and how fast you get the pieces into the combustibles.
The kiln proper shouldn't make a huge difference unless it is poorly insulated and takes a long time to fire and the burners aren't well controlled. Wood firing raku does have different effects as well.

#10 Seasoned Warrior

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Posted 31 August 2010 - 01:07 PM

Nice looking top hat Marcia! I like that you use a pyrometer, I've always like using a pyrometer myself although I was taught to look for signs of temperature such as the shimmer of the glaze to indicate when one is at temperature. I guess I just like the reassurance of a temperature measuring device. Are you using venturi type burners on this kiln? I use a couple of GACO venturis on my raku kiln which is actually an old metal pot smelter built a long time ago in a 55 gal drum. I like the top hat type since you can get to your ware without obstruction as you demonstrate in your photo. One of these days I may build a new one. I've used a portable kiln made with kaowool and it is a small arch with handles on each side so that two people could lift it off the base. I've used it a few times at the beach here when I've been joined by other local potters for a beach party kind of like Bernard Leach describes ocuring in Japan. The fire brick base and the top hat make it really easy to move and rebuild in various locations and I like the small GACO venturi's Wards sells: I use them for several purposes. I particualrly like the GACO's from Wards since they optimize the burner orifice for the specific burner.

Regards,
Charles

#11 Marcia Selsor

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Posted 31 August 2010 - 02:24 PM

Attached File  smrakukiln2.jpg   44.01KB   47 downloads
these are two portables I used for my workshops in Red Lodge and around Eastern Montana. The burners were Marc Ward burners plumbed for three tanks of propane. That reduced freezing problems.
Marcia

#12 Seasoned Warrior

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Posted 31 August 2010 - 03:35 PM

Attached File  smrakukiln2.jpg   44.01KB   47 downloads
these are two portables I used for my workshops in Red Lodge and around Eastern Montana. The burners were Marc Ward burners plumbed for three tanks of propane. That reduced freezing problems.
Marcia


Nice clean set up Marcia.

Thank you for showing it. I have a quick question however, It looks like you have individual high volume regulators on the individual tanks and you have one line going to one of the kilns and another discrete line going to the other. Do you have a manifold out of view behind the tanks that equalizes the flow?

Regards,
Charles

#13 Marcia Selsor

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Posted 31 August 2010 - 08:35 PM


Attached File  smrakukiln2.jpg   44.01KB   47 downloads
these are two portables I used for my workshops in Red Lodge and around Eastern Montana. The burners were Marc Ward burners plumbed for three tanks of propane. That reduced freezing problems.
Marcia


Nice clean set up Marcia.

Thank you for showing it. I have a quick question however, It looks like you have individual high volume regulators on the individual tanks and you have one line going to one of the kilns and another discrete line going to the other. Do you have a manifold out of view behind the tanks that equalizes the flow? My new four tank/two burner setup is simpler.Again one regulator per burner, not per tank.

Regards,
Charles

If I remember, each burner had a regulator. One regulator is visible over the center tank. The three hoses came together and two went out to the burners. The regulators were on the two hoses. I don't know what is sitting on top of the right tank, but it isn't a regulator.
Marcia

#14 Idaho Potter

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Posted 01 September 2010 - 05:44 PM

I've read with interest about the raku kilns. They must be as individual as we potters are.

My kiln is made of expanded steel and angle iron (rolled for shaping), lined with ceramic fiber, and sits on a base of fire brick covered by kiln bricks. It only measures 20" in diameter and 24" high. The burner is a weed burner modified with pilot safety feature and runs off a 100 gallon propane tank with regulator. Once I get the kiln up to proper heat 1800 to 1860 degrees (approx. one hour) the subsequent firings take 10 to 12 minutes each. The kiln breaks down (top section is 18" deep and sets on the 6" base) and fits in the trunk of my car for demonstrations or art fairs. My first kiln was made in much the same pattern, except it was made of galvanized sheet metal.

I used to use the smaller propane tanks, but got tired of them frosting up and becoming unusable. Made for some really short firing days. Most propane providers are reasonable in setting a large tank and will work with potters on the minimum filling/billing requirements.

#15 ~janie

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Posted 01 September 2010 - 06:00 PM

We are interested in building a raku kiln, and had planned to use natural gas for our fuel. Do natural gas/propane produce the same results?

Everything else seems to affect the finished product so much, I was just wondering about this.

#16 Username

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Posted 01 September 2010 - 11:35 PM

We are interested in building a raku kiln, and had planned to use natural gas for our fuel. Do natural gas/propane produce the same results?

Everything else seems to affect the finished product so much, I was just wondering about this.



For you, or anyone else building a kiln, there is a lot of information here and in the other sections of Marc Ward's site :

http://wardburner.co...esarticles.html



#17 Seasoned Warrior

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Posted 02 September 2010 - 06:00 PM

We are interested in building a raku kiln, and had planned to use natural gas for our fuel. Do natural gas/propane produce the same results?

Everything else seems to affect the finished product so much, I was just wondering about this.


Hi Janie:

Fuels are can be compared by a measurable unit referred to as "specific heat." Each type of fuel can be categorized by it's specific heat value. Specific heat is most usually defined as the amount of heat per unit mass required to raise the temperature by one degree Celsius. Looking up the specific heat of gasses we find that propane is about 1630 J/kg (Joules/kilogram) and that Methane (what CNG is mostly) has a specific heat of about 2260 J/kg. Put another way propane has about 94,000 BTU/gal while CNG has about 75,000 BTU/gal and so is a bit less efficient. Prices of CNG tend to be more stable than Propane in my experience because CNG is a petroleum reserve located with oil while propane is as a result of processing oil and the avaialbility varies hence the price. You may wish to have your your burners' orifi optimized for the fuel you are using since some of the physical properties are different.

Best regards,
Charles

PS: I edited this down because I don't think anyone really wants to know much more, or any more at all if anyone wanted to know any of it anyway. I remember my daughter once telling me that when askd for the time I launch into a tutorial on how to build a nuclear clock. So I'll just spare everyone the mind-numbing details! Posted Image

#18 Marcia Selsor

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Posted 03 September 2010 - 12:28 PM

I use small tanks for portable raku kilns as well as my top hat kiln. I can fire numerous firings and several days' worth I fire propane on 3.5-4 pounds pressure on two burners. . I fire my top hat which is 32 x 32 x 29 and can go taller or shorter without any problem. The first batch takes about 45 minutes. I keep the firing time slower because I usually am firing large flat slabs and try to avoid warping and cracking. I can get up to 25 firings from 4 30-gallon tanks. When I taught raku workshops in Red lodge, I took the three tank system with 2 burners for two kilns there. I fired for two days and never refilled.
Marcia

#19 ~janie

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Posted 03 September 2010 - 12:29 PM


We are interested in building a raku kiln, and had planned to use natural gas for our fuel. Do natural gas/propane produce the same results?

Everything else seems to affect the finished product so much, I was just wondering about this.


Hi Janie:

Fuels are can be compared by a measurable unit referred to as "specific heat." Each type of fuel can be categorized by it's specific heat value. Specific heat is most usually defined as the amount of heat per unit mass required to raise the temperature by one degree Celsius. Looking up the specific heat of gasses we find that propane is about 1630 J/kg (Joules/kilogram) and that Methane (what CNG is mostly) has a specific heat of about 2260 J/kg. Put another way propane has about 94,000 BTU/gal while CNG has about 75,000 BTU/gal and so is a bit less efficient. Prices of CNG tend to be more stable than Propane in my experience because CNG is a petroleum reserve located with oil while propane is as a result of processing oil and the avaialbility varies hence the price. You may wish to have your your burners' orifi optimized for the fuel you are using since some of the physical properties are different.

Best regards,
Charles


PS: I edited this down because I don't think anyone really wants to know much more, or any more at all if anyone wanted to know any of it anyway. I remember my daughter once telling me that when askd for the time I launch into a tutorial on how to build a nuclear clock. So I'll just spare everyone the mind-numbing details! Posted Image



Thank you so much for this information. Also...for the editing. I am not an engineer, my eyes cross at the amount of chemistry I need to know for ceramics, so every attempt to make it easier for me is greatly appreciated.

#20 bellonart

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Posted 05 September 2010 - 03:02 PM

Coming from a not-so-experienced-point-of-view... I think there are only 3 things to keep in mind when firing a raku kiln (portable or non)...

1) Kilns don't have to be necessarily expensive, but they should be well made.
2) Results will never be exactly the same
3) Make sure you're using the correct clay/glaze materials.

So to answer your question, if your kilns are well made, you created a good reduction atmosphere using a container and combustible material, you used the correct clay body and glazes, and you fired them to maturity, then you should (in essence) see very similar results no matter what kind of kiln you use.

This is how I understand it to work anyway, but I'm still learning. Hope this helps!




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