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Raku firing process


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#1 Idahopotter

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Posted 05 February 2013 - 12:33 AM

How long should it take me to fire a trash can raku kiln with a weed burner.

#2 Marcia Selsor

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Posted 05 February 2013 - 08:36 AM

If this is a 30 gallon trash can and you have good insulation, the appropriate size burner port and flu, and you increase the gs regularly, the first batch should be ready within an hour and each batch after that will take less time. There are a lot of variables, so that is an estimate.

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#3 R Fraser

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Posted 05 February 2013 - 05:21 PM

My first raku kiln was a brand new garbage can with 1 inch of 2300 degree cerawool liner held in place with porcelain buttons. I used a 300K BTU high pressure propane weed burner for the first few firings then a MR 750. Like Marcia says about an hour for the first load and 30 to 45 minutes for following loads. It still amazes me that even though I routinely hit 1900 degrees on my pyrometer the paper lable on the can never burned off, only just slightly browning over time. The temp probably could have come up faster but I always try to go gently through 950 to 1100 degrees so I slow down the process, really letting it go fast after 1200 degrees F.
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#4 Benzine

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Posted 05 February 2013 - 09:27 PM

My first, and current Raku kiln, is similar to those mentioned. I'm still having some issues with mine, in regards to getting it up to temperature. It will get to around 1500-1600 and just hang out there.

I'm slowly trying to solve the problem, with help from multiple people on these boards.
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#5 Bill R.

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Posted 09 February 2013 - 11:28 AM

I had the same problem and realized I had the burner to close to the burner opening which restricted oxygen. I pulled it back a couple inches and problem solved.

#6 R Fraser

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Posted 09 February 2013 - 02:56 PM

It is true that if the secondary air is restricted by having the burner too close to the inlet port, and or the burner port diameter is too small relative to the burner diameter the temp rise will quickly stall. On my current raku kiln I run my MR 750 around 2 inches out, and the inlet diameter is an inch to an inch and a half greater than the burner diameter. I usually leave the burner damper open fully. The exit flue is 8 inches and when I want to reduce the glaze during firing I can adjust the exit flue with an IFB to create a little back pressure. My kiln is 24 inch diameter cylinder by 26 inches high insulated with 1 inch 2600 Cerawool. I have had it as hot as 2230 degrees F. Don’t ask why.

My trash can kiln I used about the same inlet diameter and I think the exit flue was at least 6 inches, again restricted if need be with an IFB. The base was dry stacked 2300 IFB. I never had trouble with stalling as long as it could breathe well.

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#7 Benzine

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Posted 09 February 2013 - 11:39 PM

It is true that if the secondary air is restricted by having the burner too close to the inlet port, and or the burner port diameter is too small relative to the burner diameter the temp rise will quickly stall. On my current raku kiln I run my MR 750 around 2 inches out, and the inlet diameter is an inch to an inch and a half greater than the burner diameter. I usually leave the burner damper open fully. The exit flue is 8 inches and when I want to reduce the glaze during firing I can adjust the exit flue with an IFB to create a little back pressure. My kiln is 24 inch diameter cylinder by 26 inches high insulated with 1 inch 2600 Cerawool. I have had it as hot as 2230 degrees F. Don’t ask why.

My trash can kiln I used about the same inlet diameter and I think the exit flue was at least 6 inches, again restricted if need be with an IFB. The base was dry stacked 2300 IFB. I never had trouble with stalling as long as it could breathe well.



R Fraser, now that's a kiln! Very nice.

I think enlarging my inlet and possibly the outlet is probably the answer for my problem. Why would you want to create back pressure?
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#8 Idaho Potter

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Posted 10 February 2013 - 11:09 PM

I used a garbage can kiln for two years, switched to a homemade sheetmetal kiln for 12 years, went to an expanded steel kiln (all of these were lined with one-inch ceramic fiber) two years ago. Burner inlet measures 4" x 5" with a lid flue 6" x 8" that I can adjust with a small kiln shelf. When firing, the pots that are the most delicate or important go into the first load which can take at least one hour to an hour and a half. After that, I run a load through every 15 to 20 minutes--length of time determined by how long it takes to get the fired pots into the smoking cans, and new items loaded.

Sorry, I don't use a pyrometer or cones--strictly by sight on the glazes. Of the glazes I know really well (all commercial raku glazes), I can underfire or overfire to achieve same results each time. I still pull the fired work from the top (lid lifts off and is set aside).

If you are just beginning, only you can figure you the length of time for firing and the flow of work. I've found that I actually work more efficently by myself, as then I don't have to worry about someone being in the wrong place while I'm moving the hot pots. Anyone who works with me has to help with firings--just them and me--until a rhythm is established. There is are too many chances for injury with a bunch of inexperienced helpers. Better safe than sorry. Times when I've had as many as 20 people who wanted to "help" I learned early on to just say no thanks. Distractions lead to disasters.

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#9 R Fraser

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Posted 11 February 2013 - 01:04 PM

[/quote]

R Fraser, now that's a kiln! Very nice.

I think enlarging my inlet and possibly the outlet is probably the answer for my problem. Why would you want to create back pressure?
[/quote]

If I want to alter the kiln atmosphere during the firing, ie to fire the glaze to maturity in a reducing atmosphere can alter the glaze appearance, in which case I often let the glaze cool more before placing it in the reduction container. It mainly allows me more options to manipulate the final glaze appearance. Restricting the exit flue alters the flow of secondary air which changes the effeciency of the combustion and can shift it to a more reducing atmosphere.
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#10 perkolator

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Posted 11 February 2013 - 07:27 PM

i've got a question for those who raku often, which has to do with firing process. What is the orientation/angle of your burner to the kiln?

the raku kiln i use on occasion is a DIY 30gal trashcan kiln, but i've never had any "formal" training on how to use it for the most efficiency.

Through a few firings, I figured out the secondary air on my own, by pulling the weed burner out slightly so it can pull more air around it - but what I haven't been able to clarify is how the burner should be pointed in the kiln.

Should it be at an angle to "swirl" the flame inside and create turbulence, or should it go straight in? if it goes straight in, does anyone put a target brick inside (or place bricks holding shelf somewhere in path of flame) to keep from blasting straight into the kaowool on the opposite side? I've tried it 3 different ways (straight shot, straight/target brick, angled) but couldn't really tell the difference (probably because each load was different size/shape objects) and finally settled on a partial angle with the flame directed at one of my shelf posts. I also put a brick under the burner so it's not quite perfectly horizontal, and angled slightly upward by a few degrees.

Input on this would be appreciated. Thanks

#11 R Fraser

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Posted 14 February 2013 - 03:31 PM

i've got a question for those who raku often, which has to do with firing process. What is the orientation/angle of your burner to the kiln?

the raku kiln i use on occasion is a DIY 30gal trashcan kiln, but i've never had any "formal" training on how to use it for the most efficiency.

Through a few firings, I figured out the secondary air on my own, by pulling the weed burner out slightly so it can pull more air around it - but what I haven't been able to clarify is how the burner should be pointed in the kiln.

Should it be at an angle to "swirl" the flame inside and create turbulence, or should it go straight in? if it goes straight in, does anyone put a target brick inside (or place bricks holding shelf somewhere in path of flame) to keep from blasting straight into the kaowool on the opposite side? I've tried it 3 different ways (straight shot, straight/target brick, angled) but couldn't really tell the difference (probably because each load was different size/shape objects) and finally settled on a partial angle with the flame directed at one of my shelf posts. I also put a brick under the burner so it's not quite perfectly horizontal, and angled slightly upward by a few degrees.

Input on this would be appreciated. Thanks


I asked Marc Ward about this when I was planning my Raku kiln and he felt positioning the burner in the middle aimed straight in was fine, as the high pressure burner generates alot of turbulance on its own. I was going to try to point it along a tangent to the kiln interior to swirl and mix it up inside but it would have been difficult to get it aimed that way, and the flame tends to hug the wall it is aimed along anyway. Straight in has worked great for me and if there are any "cool spots" in my kiln I have not been able to find them. My octagonal shelf sits on three 8 inch posts which puts the shelf about level with the surface of the brick and the flame path is straight between the silts.
Richard
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