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Kohaku

Form Splits In Raku Firing

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I don't generally have a high failure rate when firing raku... even my more complex, assembled forms typically survive intact.

 

However, recently I've been trying to develop a new component for my water features. So far- after five attempts- every single effort has failed.

 

I want to make a 'mill wheel' shape- basically two shallow disc/bowls joined by an inner hub. I then add flanges cut from slabs in between the two.

 

The idea is for water to come out the top, and cascade down over the flanges.

 

Here's a diagram.

 

Twa_zpsd851f11f.jpg

 

Every one of these has split during the post-fire reduction phase- always at the join between the discs and the hub. I've tried with the concave side in and out... same result.

 

For joining, I score, slip, add magic water, and carefully seam the components on the wheel. However I obviously can't work the seam on the interior once the top disc is added.

 

Any suggestions on how to enhance the strength of this design? I like the idea... but I can't suffer this high of a failure rate....

 

 

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Kohaku,

Sounds like you work the first seam double-sided and the second single-sided. Can you confirm that it is always the

first seam that holds and the second that fails?

 

Can you expand on "score, slip, add magic water". I heard of "score + slip", "score + magic water" and "score + magic slip"

but not the combination you are using.

Regards, Peter

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I think the bigger issue is that there is an enclosed hollow space in the middle. The outer edges of the big wheels are going to cool many times faster than the interior of the hollow space, and the thing comes apart. It could also be that it is cracking during the heat up, as the outer wheels will heat much faster than the interior hollow space. I would fire up slowly, much more slowly than a typical raku firing, and figure out a way to insulate the container during the post firing reduction to slow down the cooling. Fiber blanket over the container comes to mind (wear a mask), or bury the container.

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Kohaku,

Sounds like you work the first seam double-sided and the second single-sided. Can you confirm that it is always the

first seam that holds and the second that fails?

 

Can you expand on "score, slip, add magic water". I heard of "score + slip", "score + magic water" and "score + magic slip"

but not the combination you are using.

Regards, Peter

 

Peter- it's usually the second (final) seam that fails... or both in some cases. There's no way that I can see to work the second seam double sided. I did have one (concave side out) where both seams failed.

 

My terminology on the join was a bit off-kilter (my wife was luring me out the door for a run, and I was hurried).

 

I score the join area- deeply. I then wet the region thought with magic water, and work it into a slurry with a brush.

 

The first possible 'fix' that I've considered involves making the inner hub thicker (more surface area for the join)/ Or... maybe I just need to score more deeply?

 

I've made many other assembled forms, however- all with wheel-thrown elements, and none have presented me with these problems...

 

Thanks for responding.

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I think the bigger issue is that there is an enclosed hollow space in the middle. The outer edges of the big wheels are going to cool many times faster than the interior of the hollow space, and the thing comes apart. It could also be that it is cracking during the heat up, as the outer wheels will heat much faster than the interior hollow space. I would fire up slowly, much more slowly than a typical raku firing, and figure out a way to insulate the container during the post firing reduction to slow down the cooling. Fiber blanket over the container comes to mind (wear a mask), or bury the container.

 

This makes sense to me... it seemed plausible that there was some asymetric shrinkage taking place.

 

There's some initial cracking that usually takes place as soon as I lift the top of the kiln off- so it's not just the post-fire reduction I need to worry about.

 

As an alternate fix, I've wondered about cutting out the top quarter of the hub. This would still allow for water to spill out... but would allow heat to vent a bit more...

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I would say can you get an effect similar to your desired raku finish in different way. It seems that the stress of a quick cool down in the raku process might be to blame not the joints. If you can slow the cooling in a gas or electric kiln you probably won't have the casualties.

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Nigich- I do a lot of surface carving... and the blackening of the carved lines during Raku is pretty integral to my work.

 

I may try some similar forms at cone 6... but I'd like to work the possibilities out with Raku as well.

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Guest JBaymore

Are you taking these up from cold to temp in the kiln, or placing them into a pre-heated kiln?

 

How are they positioned in the kiln....... laying on one side (disk) or standing on the edges of both disks, or something else?

 

best,

 

...........john

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Nigich- I do a lot of surface carving... and the blackening of the carved lines during Raku is pretty integral to my work.

 

I may try some similar forms at cone 6... but I'd like to work the possibilities out with Raku as well.

I was also thinking that maybe a sager could work for you. That might give you the blackening reduction atmosphere that you want. Just thinking inside the box as it were.

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Are you taking these up from cold to temp in the kiln, or placing them into a pre-heated kiln?

 

How are they positioned in the kiln....... laying on one side (disk) or standing on the edges of both disks, or something else?

 

best,

 

...........john

 

John- I've done both (started cold and increased slowly, placed in a pre-heated kiln).

 

I usually give 45 minutes for a firing cycle to 1850, glaze (and ambient conditions) dependent.

 

All of these pieces were placed on end in the klln, not flat down. (small unglazed region on the base)

 

I removed them manually (kevlar gloves) rather than with a tongs... trying to be as gentle as possible.

 

The fracturing seemed to happen once the kiln was opened, or in the post-fire reduction chamber.

 

Thanks...

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Nigich- I do a lot of surface carving... and the blackening of the carved lines during Raku is pretty integral to my work.

 

I may try some similar forms at cone 6... but I'd like to work the possibilities out with Raku as well.

I was also thinking that maybe a sager could work for you. That might give you the blackening reduction atmosphere that you want. Just thinking inside the box as it were.

 

 

I've considered this. Would the unglazed surfaces blacken similarly in a sager? The dark lines (mosaic effect) are what I'm after as much as the surface reduction effects...

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Nigich- I do a lot of surface carving... and the blackening of the carved lines during Raku is pretty integral to my work.

 

I may try some similar forms at cone 6... but I'd like to work the possibilities out with Raku as well.

I was also thinking that maybe a sager could work for you. That might give you the blackening reduction atmosphere that you want. Just thinking inside the box as it were.

 

 

I've considered this. Would the unglazed surfaces blacken similarly in a sager? The dark lines (mosaic effect) are what I'm after as much as the surface reduction effects...

 

I don't know how the underglaze would turn out, but finding out will be an adventure in it's own and who know what you will discover. I like your fountains by the way and am looking forward to see what you come up with.

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Kohaku,

I assume that you do a bisc firing first. In which case it does look like a thermal shock issue, due to the

more rapid temperature shifts in the 2nd firing. As some cracking occurs in the kiln (as it opens?) the

shape may just be causing too much stress on air-cooling.

 

For this form, could you switch to some sort of smoke firing (e.g pit-firing)? Which can have a very slow

heat-up and cool-down, minimising stress.

 

For some of his piece Tim Andrews uses/used a three-stage process (bisc, e/w crackle glaze, raku smoke)

to get a more "refined" crackle, but still with smoke-blackened cracks. http://tinyurl.com/prtu6pa

I suppose I'm suggesting something similar but with a gentler 3rd stage.

 

Regards, Peter

 

PS Could you post a link/photo to some of your work showing the sort of surface effects you are after.  I'm

also interested in how you combine the rather non-functional porous raku surfaces with the demands of a

water feature (especially if it's outdoors).

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Kohaku,

I assume that you do a bisc firing first. In which case it does look like a thermal shock issue, due to the

more rapid temperature shifts in the 2nd firing. As some cracking occurs in the kiln (as it opens?) the

shape may just be causing too much stress on air-cooling.

 

For this form, could you switch to some sort of smoke firing (e.g pit-firing)? Which can have a very slow

heat-up and cool-down, minimising stress.

 

For some of his piece Tim Andrews uses/used a three-stage process (bisc, e/w crackle glaze, raku smoke)

to get a more "refined" crackle, but still with smoke-blackened cracks. http://tinyurl.com/prtu6pa

I suppose I'm suggesting something similar but with a gentler 3rd stage.

 

Regards, Peter

 

PS Could you post a link/photo to some of your work showing the sort of surface effects you are after.  I'm

also interested in how you combine the rather non-functional porous raku surfaces with the demands of a

water feature (especially if it's outdoors).

 

Peter- here's a recent fountain. To answer your question- there are multiple components, and the base pedestal (that holds the pump) is cone 6.

 

On this one, the water cascades out of the hole in the side. In terms of both design integrity and sound quality, I think that the 'mill wheel' design would be preferable.

 

I'm intrigued by the sager idea, and may give that a try.

 

9206491_orig.jpg

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To follow up on the sager idea... can anyone answer the following question...

 

If I fired to cone 6, with a piece (similar to the above) (with carved, wax resist-filled lines and glaze in between) nested in a sager... and stacked the sager with combustables... would the glaze free surfaces come out blackened?

 

Alternatively, what if I used my standard Raku Palette, and Sager-fired to 1850?

 

I'd worry that the carbon would burn off at the higher temps... but maybe if it's a tight seal, this wouldn't happen?

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Kohaku,

 

If I fired to cone 6 ... would the glaze free surfaces come out blackened?

What temperature does your body vitrify at? I suspect that it is simply not possible

to blacken anything other than an porous body. [unless you are doing sophisticated

things like carbon-trapping. Which I would love to hear about.]

 

 

Alternatively, what if I used my standard Raku Palette, and Sager-fired to 1850?

I'd worry that the carbon would burn off at the higher temps...

I share your concern about burning off. AFAIK those going for smoke-blackening don't

mix it with simultaneous glaze firing (delighted for evidence that I'm wrong here). Don't

know if this is for aesthetic or technical reasons.

 

 

Regards, Peter

 

You may be interested in this method of aluminium-foil wrapped "saggar" firing at

500F-ish as a possible way of blackening your lines.

http://users.skynet.be/russel.fouts/Files/OhYesYouCanPMIJA09lr.pdf

... and this video of a 1300F tin-can saggar firing

 

 

Although both would only be useful if you had previously glazed the pot; either in an

e/w glaze firing or using a suitable cold-in/cold-out kiln programme for your raku glazes.

 

 

 

 

 

 

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if you stick with raku....all of the suggestions have been good, i would add:  extended drying time ( a form like that i would keep covered for a week or more, then start airing it gradually , very slow drying has solved almost all my raku cracking problems...also, the addition of 3-5% kyanite to my clay mix has allowed for thin, open-formed slab pieces with 0 cracking (with slow drying too)

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Heya Cass...

 

Thanks for the feedback. Can you clarify what you mean by 'extended drying time'? Not sure if you mean 'time post glaze application' or 'time in the reduction chamber' or something else...

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Just an update on this topic...

 

There are two things that I've tried to ramp up my success rates with this form.

 

First- I added kyanite (as per Cass's suggestion above).

 

Second- I changed how I was constructing these fountain components. Originally, I was joining two shallow bowls to an inner hub, as shown in the diagram below...

 

Twa_zpsd851f11f.jpg

 

As I've described, this form was repeatedly fracturing when raku fired.

 

As an alternative, I tried joining two flaired bowl shapes (see below) and then adding texture to the rim.

 

My goal- in each case- was to craft an upper vessel for a water feature where the water would cascade around the rim of the vessel.

 

Anyhow, here are a couple examples from recent firings. No fracturing, really nice functionality in both cases.

 

gallery_16050_715_1322195.jpg

 

 

gallery_16050_715_217028.jpg

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Still a big fan of your designs. The colors are great. Also, is that an otter?

 

Thanks. I still feel like my forms have a ways to go to match my ideas... but I guess that's the process.

 

Otters both... although a bit abstractified, certainly.

 

Harbor seals and otters pretty much battle it out in terms of being my favorite subjects.

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