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karan

Raku Reduction Yields Weak Claybody

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Hi everyone-

I do Raku firing with my students- not my personal work, and need some advice. I have a problem with the claybody becoming much weaker than the bisque state during the reduction. Anyone have any tips to prevent the weakening? Perhaps something during my firing isn't right? Any advice would be great. Their pots look fabulous, but they have a tendency to be more brittle than I'd like. Maybe it is par for the course for raku?

 

Thanks for any input!

 

Karan

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I am trying to understand what you mean by weakened state. Do you mean they tend to break during reduction or do they break after?

Breaking during reduction is one of the aspects of raku ... White hot clay being crash cooled is risky at the best of times but if your wares have uneven walls or thick and thin areas the stresses are higher.

I've never had raku pieces that were more brittle afterwards so I can't answer there.

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Strangely enough, my reply I made yesterday isn't showing- so please disregard if you see two responses from me here.sad.gif

 

So, I am mostly concerned with the post-firing/post reduction state of the claybody. Say, after the kids pieces are fired, and they are cleaning them, we find them to be more brittle and easy to chip. If the kids drop them into the sink from a short distance of a few inches... (when they are cleaning them) they will easily break. The breakage wouldn't happen if the clay body were just bisque fired though. It is clear that the reduction has weakened the forms. (the breaks occur within cracks that have a smoky look- where the smoke has permeated)

 

I will describe my firing process, and you can tell me what you think...

I preheat my kiln a little bit to take the chill off on cold days. Then, I load my pieces. I turn up the gas, and heat them slowly the first 10-15 minutes to reduce thermal shock. After the slow heating of the pots, I then crank up the gas, to bring the kiln up to temperature. Usually takes another 30 minutes to reach final temp. (I have a small 18" I believe Olympic raku kiln) I then take the pieces out, put them in my containers for reduction. Now, I do allow them to sit in the reduction for quite a while... maybe half an hour or so. (I pretty much do this on my class bell schedule... when the next class comes in with the bell, and I'm putting in their pots, I take out the previous classes' pots from the reduction chambers)

 

So, I'm wondering, is my length of reduction bad? Could that be causing a greater problem with a weakened state of the claybody?? I do get great glaze results with the long reduction...

 

I guess I just need reinforcement from someone who knows more about raku than I do...Maybe this is all within the expected norms for the outcome for raku? smile.gif

 

Thanks a bunch for input!

 

Karan

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I suspect you just see the chips more because they show up so vividly against the rakued clay. Dropped bisque would chip but you would not see it. Have a black marker handy so they can fill in the spots and soon no one remembers where it was.

 

 

I agree for the most part. And the fast cooling probably does weaken the body slightly. But that's another reason Raku isn't functional. I don't think you're doing anything wrong.

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Karan,

Most raku firing is done to cone 06-05--same as bisque. I agree with Chris that chips are only more evident against the raku glaze and the carbonized clay. Black permanent marker covers most damage, but if there are any rough edges that may snag clothes or skin, take some emery cloth (sanding material) to smooth out the break.

 

I have friends that fire raku and stack all the pieces in very large galvanized garbage cans--with straw or hay as a buffer between pieces--and remove the work 24 or more hours later. I, on the other hand, take the work--still smoking--and plunge the pieces into a bucket of cold water. Raku is serendipity in action anyway, so don't sweat the small stuff.

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