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First Firing of My Homemade Raku Kiln


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#21 Marcia Selsor

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Posted 05 November 2012 - 01:16 PM

I agree with Kohaku about firing faster. I had a wood fired raku kiln that could hit temperature in a few minutes ..after several hours for firing batch after batch.
The coals were well established and it just needed a few pieces of additional wood to melt the glazes.
I glaze a day or more ahead of the firing and if you are going to blast 'em, then preheat the pieces on top of the kiln or in another kiln.

Marcia

#22 JLowes

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Posted 05 November 2012 - 02:03 PM

I am a little behind in geting in on this, but I thought relating some recent experience could be of benefit.

I fire with a pyrometer and then visually check the melt once I am close. My rule of thumb is to keep the temperature rising at one degree F per second. On a previous firing I had some bubbling, or really more like frothing, left on a couple of pieces when others had reached maturity. The glaze is a clear crackle I use in almost, if not every, raku firing session. This weekend I refired those along with three kiln loads of companions.

On my first kiln load I had one piece that still had some bumpiness to the glaze while others were fine, even though I went well over my usual temp, 1850, for this glaze. For my second load I increased the top temp to 1960 and held it a little and made a closer observation at melt. My glazes were very shiny and wet, and the profile was smooth. This was at around 100 degrees F over my "normal" temperature, with a hold. On my third load, I added my pieces from the previous firing, the one with bumpiness from this session, and the remainder of my new pieces. Again I brought the kiln up to 1960, added a hold at top temp, watched the melt and had a perfect firing. Interestingly the previously fired pieces with "defects" melted and went smooth close to the 1850 mark, while the new pieces had to have the higher temperature and a hold. At 1850 they looked like they were covered with foam, but eventually smoothed out when diligence and experience were brought to bear. Upon reflection, I had made a new batch of the clear crackle which has as its majority constituent Gerstley Borate. I had recently bought out the chemical stock of a retiring potter, including the Gerstley Borate I used to batch the clear crackle. That may have been the reason for the odd performance. It may be time to switch to a frit based recipe. Having fired this kiln many times definitely helped me to feel comfortable in changing the variables while looking for a result. You will ge there too.

My point for you is that you can refire the pots with the bubbling and it may smooth out the bubbling. Or, it could also be that you are not getting to and staying at the maturing temperature long enough for your glaze to settle. I had all of the isues you had until I got familiar (and comfortable) with my home built raku kiln. When I started firing my own kiln all I had was color and constant checking the melt, but having a pyrometer has been a big plus for me.

I am also wondering if your kiln exhaust port may be too small for the fire power of your burner setup. Your burner port could also be small and cause some of these problems you describe. If you have an inch all around your burner at your burner port, then that is probably sufficient. If your exhaust port is a little larger than the burner port that is also probably sufficient. If it is a good bit bigger, you could always use a kiln shelf as a damper. I use part of one as a damper to hold in some heat until I get up above 1000 and then start openiing up the port as I go higher.

Hang in there and you will be firing like a pro in no time.

John

#23 Lockley

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Posted 05 November 2012 - 04:07 PM

You never did say what you were using for a burner. From the troubles in set up it sounds like you may be using something like a homemade or Weed burner.

While you eventually achieve satisfactory results with that grade of burner, I strongly recommend that you use one of the purpose made Raku burners from Ward Burners or ther commercial manufacturer.

Commercial burners offer benefits in consistency, control, efficency. Commercail burners normaly provide a ready means of flame chemisty control and and some adjustment of the btu output. I think you will find it desirable if you intend to fire student work.

Lockley

#24 Benzine

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Posted 05 November 2012 - 08:05 PM

My intake and exhaust ports are four and five inches respectively. The intake port is an inch larger, than the end of my burner.

In regards to the burner, it is a Red Dragon 100 000 BTU weed burner.
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#25 Benzine

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Posted 09 November 2012 - 11:48 PM

I completed my second firing today, with my students.
In some ways, this was more successful, in some ways, no so much.

The kiln still seemed to take a while to get up to temperature, and even then, it didn't seem to get quite hot enough.
I took the advice, regarding, pulling back the burner, which did help, at least in regards, to reducing the soot issue, I had the first time.
The glazes did look "wet", when I checked them, yet some still had the same "foamy" look to them.
I may enlarge the burner intake on the kiln, to see if that helps, with getting the kiln up to temperature.

I did two firings, back to back, and I'd say, the second batch turned out the best.

Another interesting point, as the second batch was finishing, the burner shut off. At first, I thought I was out of gas. Then I realized the tank was coated in frost. So it may have froze up?.....

Well, I promised my students, we'd do the firing, and I was a man of my word. Now, with that out of the way it's time to perfect my process, so we can do it again, with even better results.
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#26 Marcia Selsor

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Posted 10 November 2012 - 08:21 AM

Glad you had success. When I began teaching my students and I built a wood fired raku kiln and would spend the day firing batches. It brings back great memories of being set up on a ridge above the campus and the city with mountains beyond in beautiful Big Sky country. We would do many many firings per day. At the end of the day, the bed of coals only needed a few split pieces of 2 x 4.
Raku is a great experience for students and can teach much about the processes. Refiring bubbled glazes is a good example.

Marcia

#27 GMosko

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Posted 10 November 2012 - 09:13 AM

Just read your post, if a bit late. I wish you hearty congratulations! Successfully firing your own built kiln is one of the true pleasures in life! If you continue to have carbon buildup on the insulation, I would like to offer whatever experience I have, since building seven of my own kilns (some very large). Would it be possible for you to include a photo of the area in question? Aside from any possible tips I could offer, I am just very curious how your new kiln looks.

Anyway, thanks for the post, and keep the faith.

Sincerely,

Gil Mosko
Pueblo, CO
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#28 GMosko

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Posted 10 November 2012 - 09:20 AM

I totally agree. One simply cannot go wrong with ANYTHING from Ward burners. Mr. Ward's knowledge is so vast it is scary. And he is helpful, too, freely sharing information.

Sometimes it is hard to believe, but a properly designed piece of equipment almost always saves you money in the long run.

Sincerely,

Gil Mosko
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#29 Benzine

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Posted 10 November 2012 - 10:15 AM

I'll get a picture of my kiln, and the ports put up.

I really did want to get a Ward burner, because of their reputation, and because I heard, they were relatively inexpensive (some people said they picked one up for thirty five dollars). But I couldn't find anything that cheap, which is why I went with the weed burner.
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#30 buckbuck

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Posted 11 November 2012 - 09:58 PM

I use a weed burner however, it's good for close to 300 K btu. I use a pyrometer for a sanity check. Pulling the burner back from the opening does help the dramatic but wasteful orange flame coming out the top. Keep tweaking and you'll be amazed at your results. If you want some fun, try some sagar wrapped ferric chloride pieces. It is quite toxic when fired but the results are awesome. Fire to around 1160 F. You'll see the aluminum foil start to melt. Pull it let it cool and enjoy. Spray with semi-gloss clear for the best pop in color.

#31 Idaho Potter

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Posted 13 November 2012 - 07:01 PM

Congrats on your success!

I've been using a weed burner since 1988, and haven't had a problem with it. The freezing of the propane tanks is a real pain. I've seen some placed in a bucket of water to stop the freezing, but I got a tank that held 3 times the propane and quit having that problem. Now, I have a 100 gal tank set up outside my studio, and get it refilled once a year (propane provider comes by and pumps it in). I relize this probably won't work in a school situation, but consider it if you set up a kiln at your studio/home.

As to glazes -- One way to make sure your glazes mature at the same time is to fire the different glazes individually. I have one glaze (Shinju pearl) that matures at a much lower temp than others, so I always fire it separately. It bubbles quite a bit, but when it glistens, a matter of two minutes can change it from a subtle blue/white with pink overtones, to a mauve tweed when it's overfired. As the sheen appears, I shut the burner down to pilot immediately as the heat factor continues and matures the glaze.

When you fire the glazes separately, your eye becomes accustomed to the "look" and if you're using a pyrometer, make note of the temp at maturation so you can repeat and get the same results.

Using some small cup forms, try underfiring and overfiring every raku glaze you have. The results will sometimes quicken your pulse. Take notes!

Shirley

#32 Benzine

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Posted 14 November 2012 - 07:19 PM

Congrats on your success!

I've been using a weed burner since 1988, and haven't had a problem with it. The freezing of the propane tanks is a real pain. I've seen some placed in a bucket of water to stop the freezing, but I got a tank that held 3 times the propane and quit having that problem. Now, I have a 100 gal tank set up outside my studio, and get it refilled once a year (propane provider comes by and pumps it in). I relize this probably won't work in a school situation, but consider it if you set up a kiln at your studio/home.

As to glazes -- One way to make sure your glazes mature at the same time is to fire the different glazes individually. I have one glaze (Shinju pearl) that matures at a much lower temp than others, so I always fire it separately. It bubbles quite a bit, but when it glistens, a matter of two minutes can change it from a subtle blue/white with pink overtones, to a mauve tweed when it's overfired. As the sheen appears, I shut the burner down to pilot immediately as the heat factor continues and matures the glaze.

When you fire the glazes separately, your eye becomes accustomed to the "look" and if you're using a pyrometer, make note of the temp at maturation so you can repeat and get the same results.

Using some small cup forms, try underfiring and overfiring every raku glaze you have. The results will sometimes quicken your pulse. Take notes!

Shirley


I had no problem with the tank, the first time I used the kiln, nor with the first load I did, during the second firing. The tank didn't freeze, until the second load was almost done, which was after the sun had set, and the temperature began to drop outside.

To simplify the glaze problem, I might just limit the glazes that get used. I just had the students use our normal low fire glazes, because we already had them. But I have thirty of them. I think there are just too many to make that work well. So I'll either just use the ones that worked the best, or buy some specific Raku glazes. I've been wanting to get a white crackle glaze anyway.
"Anything worth believing, is worth questioning"

#33 Idaho Potter

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Posted 17 November 2012 - 07:58 PM

Benzine,

Because I use a white body clay, I've been applying clear crackle glaze (works great with underglaze designs) and it looks white because anything unglazed is black. Whether you use a white or clear crackle, light application makes for smaller crackle, heavy gives larger crackle. You can come up with some nice variances when you do both on the same pot.

Did you get much crackle with the lowfire glazes? I've only used them when I wanted a particular color in a particular place. Sounds like I need to move from my comfort zone and do a little more experimenting.

Shirley

#34 Benzine

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Posted 20 November 2012 - 06:33 PM

Benzine,

Because I use a white body clay, I've been applying clear crackle glaze (works great with underglaze designs) and it looks white because anything unglazed is black. Whether you use a white or clear crackle, light application makes for smaller crackle, heavy gives larger crackle. You can come up with some nice variances when you do both on the same pot.

Did you get much crackle with the lowfire glazes? I've only used them when I wanted a particular color in a particular place. Sounds like I need to move from my comfort zone and do a little more experimenting.

Shirley


I got some crackle with the low fires. I'll double check, and I can tell you which brand and colors, they were.

Today, I just ordered some Raku glazes from a supplier, one of which was a white crackle glaze. I'll be interested to see if those turn out better with my kiln, than just using the low fire. If they don't turn out, then I'll have to do some extensive kiln tweaking.
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#35 Benzine

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Posted 20 January 2013 - 11:04 AM

Alright folks, I finally got around to getting some pictures of my burner and exhaust set up, for my kiln. I really had no excuse not to, as yesterday, I did another firing with the kiln. It was unseasonably warm, and I had some gifts I started on back in November, that I finally finished glazing.

Anyway, here are a couple pictures of my set up. One from behind the burner, which allows you to see the size of the burner, compared to the intake. One of the exhaust port on the top. And one from the side of the burner, to show how far, I keep the burner from the intake.

In terms of the firing, it went maybe a little better than the previous two. I actually used specific Raku glazes this time, instead of the low fire glazes I had used previously.
The kiln still seemed a little slow in getting to temperature, and it really likes to hang out around 1500 degrees Fahrenheit. Some of the glazes became glossy, and I was trying to go a little bit more, but the torch went out. I thought, that my tank was finally out of gas, but I'm fairly certain it just froze up. So some of the glazes are a little bumpy, but I got some good color out of them.

So I'll leave you with the pictures. Any suggestions, for modifications to my kiln, or a different approach to firing, would be appreciated.

[attachment=1907:Burner Port.jpg] [attachment=1909:Exhaust Port.jpg] [attachment=1908:Burner Set Up.jpg]
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#36 Marcia Selsor

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Posted 22 January 2013 - 04:25 PM

How large is the hole on the lid compared to the burner port? It looks smaller.
If you have them closer to the same size you may get more air and hotter burning flame.
On second glance maybe they both could be a little bigger. What is the diameter of both holes?


Marcia

#37 Mark C.

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Posted 22 January 2013 - 05:52 PM

Those MR 100 burners are 80$ -I have 4 on my salt kiln-really nice burners considering the low cost. They are venturi. My weed burner is not a true venturi. Just make sure your weed burner has enough BTU's to work. Soot is not suppose to happen if the burner is working well. Run a little water over your tank while firing to prevent freeze ups.Or put tanks in water bucket-this works well for small tanks.
Mark
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#38 Benzine

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Posted 22 January 2013 - 09:26 PM

Where did you get your burners Mark? I looked, and couldn't find them that cheap, which is why I went with the weed burner. The burner, I do have, is 100 000 BTUs.

Marcia, the exhaust port is about four inches wide, though since I have a little blanket overlapping, it's probably slightly less. The intake is probably about three inches. I'll have to measure to verify......I knew the exact size at one point, as in when I built it.

The good news is, I didn't have any flame shooting out the top this time, and definitely no carbon build up.

One thing I did discover, is that two of the three kiln bricks I am using for shelf posts, cracked. Could this be from the rapid heating and cooling? Or could it be from the kiln not sitting perfectly level during firing? My back driveway is gravel, and not completely even.
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#39 Mark C.

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Posted 22 January 2013 - 11:45 PM

I think the bricks cracked from rapid heat and cool


I just googled MR 100 venturi burners
69$ here at clay planet
http://shop.clay-pla...uri-burner.aspx
78.95 at Wards-he has a great chart for propane in terms of BTUs on these burners scroll down under the ransome burners at top
http://www.wardburne...uriburners.html

I bought mine at this place -Gas applliance
My 8 -MR 75s back in the 70's my 4 - MR 100's around year 2001
Its a very old school business-I have been buying all size MR burners from them since the 70s-They moved several years ago from Long Beach Ca.
You will need to call them about pricing -they ship like everyone else-they also used to be less cost than say a ceramic supplier-Its an old school connection.
Many ceramic places buy from them.
I'll share it with all who read this and need burners.
http://www.gasapplianceco.com/
give them a call.Let us know what the prices are now?
Mark
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#40 Benzine

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Posted 23 January 2013 - 11:18 PM

Thanks for those links Mark. What else would I need to complete the burner? A hose, regulator, some type of flow control? Much like electricity, gas frightens and confuses me.


And in regards to Marcia's question, regarding the size of the ports, both are four inches across, measuring from the metal skin, but with the blanket overlap, they are about three and a half inches smaller. I could have swore, that when I cut the ports, I made one larger.
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