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Try, Try Again...


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Last night was my third attempt at a Raku fired Anniversary Bowl.  It was not "A Charm"...

 

The previous two attempts cracked/ broke in reduction.  Those were fired, when it was much colder outside around 20-30 degrees F.  Weather around here, has been in the 90s recently, and was probably 80s last night, when I fired.  

 

I wasn't sure if the first two attempts and subsequent failures, were a result of a glaze fit issue, too rapid of cooling, or just a hidden structural issue, that didn't appear until the Raku process.

 

I'm leaning away from glaze fit, as most of the glazes I use, have never given me trouble before.  I have NOT used the exact same glaze combination each time, as I wasn't thrilled with some of the results.  The consistent glazes are a Turquoise Crackle on the inside, Wheat Flash on the outside, and a standard low fire clear.

 

Reduction material was dried leaf bits the first two times, and fine saw dust the last (No access to dry leaves this time of year, and apparently, despite my recollection that I saved some from last Fall, I did not...).

 

The clay body is one that I have used for years.  It's a specified Raku clay.  I've had some wares crack before, but never this consistently.  

 

I've made bowls like it before, with no issues under normal/ traditional firing methods.  

 

This last time, one of the cracks actually formed during the heating portion of the process.  This was not the case with the other two.  

 

I'm thinking the form, or my process for creating it is the issue.  I throw the bowl normally, then flare out the rim, so I can add the text.  I will say, this last rim was a bit on the thin side.  Maybe that's why it cracked and warped a bit.

 

I've included some pictures of the results, plus one of the crack right before unloading.  More cracks did form during the reduction, as happened with the other two.  But none of those were visible, when I unloaded it.  There are also some small cracks noticeable along the rim, inside and out.  They look different than the other glaze crazing, so they could be smaller structural failures as well.

 

Eventually I'll figure this out, and have one I can keep...

 

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I have never seen a raku piece tear like that during firing. How quickly are you ramping up the temperature?

 

My generic advice to avoid problems with Raku is to glaze your pot the day before, so that it is dry before firing. Second, ramp the temperatures up slowly, especially if you have hot and cold spots. Allow time for the heat to distribute inside the clay evenly.

 

One way cracks in the rims like that can form is drying the bowl too quickly. If the rim dries out faster than the bottom, it can cause stress as it shrinks faster than the wall close to the foot. Wrap the bowl in plastic, to allow it to dry more slowly. Also place it rim side down when it is firm enough to support the bowl. This might help prevent future cracks.

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Used to do raku years ago, lots of breaks, I liked doing bowls and commemoratives. You could try a thicker rim, arcing out, then carve name and date into flat part of flared rim. This may give it more strength for the firing, double tongs are less harsh when lifting the bowl or a bowl lift which is like a rounded two tine fork that angles into the handle. 

 

best,

Pres

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Thanks for the replies all!  I am sorry, that I have not replied, I've been away from the computer for the last few days.

 

Is there any way to fire it vertically? I think the lip is getting blasted by the flame. Or maybe go very very slowly to not shock the lip as it is overhanging the shelf.

Marcia

 

Marcia, that is one issue, that I did not consider.  That could definitely be the cause, at least with this latest firing.  I just looked at some photos, that I took, during one of the previous firings.  It doesn't look like the rim was overhanging the shelf, as this one was.  It still cracked, but not during the firing.  Making the bowl a bit smaller, to avoid that, could be the fix.

 

I have never seen a raku piece tear like that during firing. How quickly are you ramping up the temperature?

 

My generic advice to avoid problems with Raku is to glaze your pot the day before, so that it is dry before firing. Second, ramp the temperatures up slowly, especially if you have hot and cold spots. Allow time for the heat to distribute inside the clay evenly.

 

One way cracks in the rims like that can form is drying the bowl too quickly. If the rim dries out faster than the bottom, it can cause stress as it shrinks faster than the wall close to the foot. Wrap the bowl in plastic, to allow it to dry more slowly. Also place it rim side down when it is firm enough to support the bowl. This might help prevent future cracks.

 

I have also never had a Rakuware do that either, until a couple days ago...  I've never even had "hastily" built student projects experience that kind of failure.  I've honestly never even really experience too many issues with any type of cracking or dunting, with my Rakuwares, even with my earlier works.

I am usually pretty good about drying any of my wares.  I won't say this isn't the cause, as this particular bowl/ project was put on the back burner multiple times, over the course of a few months.  So I could have forgotten about it for a bit, and not dried it properly.  I honestly don't remember at this point.  

 

writing on the rim puts stress on that part of the bowl.  could you try again with the rim supported  on top of some kind of padding inside another bigger bowl?

 

I have thought of this.  The writing on this one, was done by first stamping the letters on, with black underglaze, then simply outlining them with a small carving tool.  With the previous attempts, where the rims didn't crack, I actually used Sgraffito for the entire lettering.  So I would say I applied more pressure overall on those versions.  

 

I do like the idea of a bigger bowl, not so much for support, but to act as a shield against the heat, during firing.

 

Used to do raku years ago, lots of breaks, I liked doing bowls and commemoratives. You could try a thicker rim, arcing out, then carve name and date into flat part of flared rim. This may give it more strength for the firing, double tongs are less harsh when lifting the bowl or a bowl lift which is like a rounded two tine fork that angles into the handle. 

 

best,

Pres

 

I will most likely go with a thicker rim.  I don't think I realized how relatively think it was, until it was too late.

 

Looks like that's a big bowl that nearly fills the width of the kiln, leaving the rim exposed to the ravages of the rising direct flame vs. the belly of the bowl that is somewhat "protected" by the shelf.

 

Like I mentioned to Marcia, this could very well be the cause, and something I will try to avoid the next time.

 

A follow up question -- what cone does you clay vitrify at? Is it low fire, or cone 6? 

 

I bisque to Cone 04, and according to the supplier, the clay body can be fired all the way from Cone 6 to 10.  I realize that is a broad range of firing temps.  I like my supplier, but they are not overly specific about vitrification temps, absorption rates, shrinkage, etc.

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