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Sofusryge

Raku - Cooling Dunts

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Hi

 

This is such a nice place to gather information and inspiration.

 

Lately I have experienced problems when finishing a series of rather large slab-built 2-sided elliptical bottomed pots in my raku kiln. They got what i guess is called cooling dunts going more or less straight down through the two faces of the pot. I have so far fired three and broken all. It doesn't seem to matter much, if i take the temperature slowly up (about 1 hour 20 minutes from cold), or fire as usually (about 45 minutes from cold). The breaking is clearly happening as the pots cool down in the reduction bin - sharp edges and smoke impregnated lines. I need the smoke for the design, so cooling down in the kiln is not an option.

 

I'm using a raku clay from a local supplier, containing 20% chamotte. My raku-kiln is an oil drum lined with refractory fiber. The design of the pots is partly naked, partly glazed with Soldner clear.

 

Any suggestions how to prevent this breaking? I've just mixed up a batch of paper-clay, as it should have good resistance to thermal shock. Is it likely that pots made from this will withstand the cooling better, or am i mistaken in believing that as an solution?

 

Regards,

Sofus Ryge Petersen

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I can't see paper clay having better thermal shock resistance. The paper fibre gives better green strength, and allows for some interesting things to be constructed, but after the paper fibre burns out, it leaves a more porous (and therefore brittle) clay body behind.

 

If you think that it's thermal shock that's happening in the reduction chamber (and this is common), have you tried insulating the bin on the outside to slow the cooling in there?

 

Are you bisquing the piece before you raku it?

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Don't ya just hate it when nice raku pieces crack?

 

Might try different clay bodies. The ones made for raku are pretty groggy but work. A friend of mine swears by danish white with sand and does larger things.  Clay planet makes a good raku clay and also a sculpture clay that works well.

 

I have occasionally done the post firing reduction in the kiln itself with objects too large to lift hot.  I open the top hat kiln and put a trash can over the piece that has newspaper taped inside it. works pretty well. Carbon burns out the next firing.

 

Another option is to put a brick or piece of old broken kiln shelf  in the kiln when you fire the piece. then put the hot brick in your trash can, set the piece on it and add your reduction material.  Allow a long time to cool.

 

Another thing i learned is to have channels under the piece so a little air can get under the piece and relieve temperature differences.    I make a lot of animal figures in seated yoga positions and they used to crack up the sides until I started cutting little grooves into the design that allow air to escape.  I do this with my high fired stuff too. 

 

Hope this helps.  Rakuku

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You need to slow down the cooling. You can't necessarily just wrap a blanket around the reduction can, though, as it may burn. Someone with more experience in raku than me will surely chime in soon.

If the OP perhaps has some leftover fibre blanket from their kiln construction, that could be used though. (With precautions)

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I'd add quartz sand to the clay about 30% by column and try larger particle sand as another example. Try quartz sandbox sand and the larger contractors sand or find your own along a creek or river sand bar. Too little or too much sand will cause things to crack, that's why you should experiment and keep records.

Good luck,

Alabama

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Thanks for the remarks and suggestions. I will try to insulate my reduction-bin next time i fire one of the pots (still have 2 left to experiment with). I imagine that a container inside a bigger container, fillede with perlite in the hollow wall will do the trick. If not, i´ll try smoking directly in the kiln.

 

The pieces have been bisqued to around 900c before firing in the raku kiln.

 

Rakukuku: I would imagine that a hot pieces of kiln-shelf under the pot in the reduction-bin would make the temperature-difference between bottom and top of the pot greater, and maybe amplify the problems? I have propped the pot up on little bent pieces of steel (don't know what they are called), to get proper airflow under the pot in the kiln. Maybe i should do the same in the reduction phase.

 

Chris Campell: i'm pretty sure the design itself is partly to be blamed, it really begs thermal stress problems. I'll post a photo. 

 

Shame about the paperclay, had high hopes that it would do miracles :-) Sand in the clay - duly noted. Perlite in the mix maybe worth a try too?

 

Regards,

Sofus

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I use a piece of soft brick in the reduction kiln. It does sound like the clay doesn't like the fast cooling. Also I think Chris may be right about the form itself not doing well in raku. Are they fairly closed forms?

 

Pictures would help. 

 

Marcia

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I've used both a stoneware and a specified Raku clay, and had only a hand full of issues with either.  I would say a good 80-90% of things I've fired have turned out without any dunts.  

The times I have had issues, I attribute it more to the form, or glaze application, than I do the clay body, or the cooling.  I've fired in 70 degrees F and had things crack, and have fired in 20 degrees F with no cracks.  I've even dunked wares in water, right out of the kiln, and still had no issues.  

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Here's the latest cracked pot. The horisontal lines coming in from the sides of the pot is merely glaze crackles, it's the vertical lines coming down from the top of the pot that is dunts in the ware.

 

As mentioned earlier, i'm quite aware that the design is not optimal when it comes to thermal stress, but it's the design i want to go with here, so i'm quite eager to make it work.

 

I'm not sure about the effect of pre-heating, the dunts are happening when cooling down.

 

 

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Seems like this form should be able to survive raku. Maybe put the piece in the reduction can upside down or sideways.

 

Also, when I do crackle pieces, I let them cool a little in the open kiln before getting them into the reduction chamber.  I do this to get a better crackle pattern but it might help your dunting.  I actually mist my crackle pieces with a bit of water before I remove them to encourage crackle but this might make it worse for you.  Also I heat up my tongs a little before pulling pieces so I am not grabbing a red hot piece with cold tongs.

 

Also my bisque temperature is cone 06, same as my raku temperature.  

 

One thing I have noticed that seems to cause raku cracking is uneven thickness of pots. Don't know if you make a bottom thicker than the sides but consider it.    Hope some of this helps.  rakuku

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How much difference in thickness is there between the rim and the bottom of the pot? If the rim is fairly thin and the bottom is thick, it could be retaining more heat and the thermal differences could cause the cracking. I cannot tell from the pictures - have you trimmed out part of the foot? If you have not tried it, that might help equalize the temperature variation as well.

 

I also agree with those saying to try a different clay body, or add some sand.

 

Good luck!

 

Susan

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Thanks again for the remarks, it's much appreciated.

 

The bottom of the pots are cut from the same slab as the sides, uneven thickness ought not be the problem.

 

I'll try play around with the clay mix a little, and insulate the reduction container to see if it will fix the problem. I'll keep you posted if i succeed.

 

Regards,

Sofus

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Well, had some succes with the latest two pots. Went directly to the "reduction in the kiln" option,  and it seems to do the trick. No dunts. But cooling in the kiln gives me a new challenge. Usually i just transfer the pots to the reduction bin, let the combustibles combust, and then put the lid on. Nice reduction, nice black color. In the kiln, the temperature is so high, that at first when i introduce combustibles, the carbon just burns out, leaving the ceramics as white as the day they came out of the bisque firing. I know it's a question about timing, the first pot got a nice black, as i fed the kiln combustibles at regular intervals. The second pot didn't get black at all, but som sticky soot that i can't seem to get off the glaze on the inside. Apparently, i missed the interval were the carbon has a proper effect on the raw ceramics, as i fed the kiln lots of combustibles, and there where both white and black paper ashes in the kiln afterwards.

 

At what temperatur does the carbon penetrate the ceramics and produce the stable black color?

 

IMG 2607

 

Regards,

Sofus

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