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Sakura Ceramics

Strange Raised Bumps On Plates - Help

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I am hoping someone can shed light on this strange trouble I found in this monring's glaze kiln run.  I threw small plates of stoneware (IMCO 1C) which were bisqued to cone 04 and then sat in a box for about 11 months. I washed them gently with a damp sponge and allowed them to dry in dappled sunlight for about an hour. Then brushed on 2-3 coats of various commercial glazes. Most come out wonderfully. Two developed the strangest raised welts (largest is about 2 inches long by 0.5 inches wide) I've ever seen. These are NOT blisters in the glaze. They appear to be like balloons which have inflated within the clay warping the inside bottom of the plates around where the flat bottom transitions to the angeled rim.  The glaze over these ballons appears to have a tiny separation exposing a thin line of clay, but is otherwise undisturbed. I've been throwing plates of all sizes for two years; this is the first I've seen such a thing.  Any insight is welcome. Thank you.

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Hi Sakura,

 

I'm with Pres. Sounds like bloating to me. I've been dealing with this issue off-and-on for about a year with a few different clay bodies... I was actually greeted by some heavily bloated pots this morning when I arrived to the studio. First time in a while. :(

 

Are you venting during the bisque? I believe the source of my clay's bloating issues is linked to a lack of offgassing during the bisque. How fast is your bisque program? Moving very slowly up to 1600F has helped significantly. And leaving the top peep open.

 

As Pres mentioned, overfiring can be a culprit of bloating as well.

 

Good luck!

 

Chris

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Hi Pres and Chris and Marcia.  Many thanks for your insights.  I found the camera, and attempted to upload photos.  Okay, I don't know how to upload photos to this forum. I tried including them from Word and got an error message. Could you please instruct me on how I can include the photos?

 

I'd never heard of bloating. (Down side of being self taught). It's a great word to describe what I see.

 

Let's see. Marcia: Glaze run is to cone 6 ramped quickly on my manual L&L 1970's kiln with kiln sitter, two rings, two knobs, low medium high -- setting knobs simultaneously to each setting with about 70 minutes in between clicks.  Glazes are Amaco Potter's Choice PC-33 Iron Luster cone 5/6 (shades of brown) and one from our local shop Leslie's Ceramics Supply Niji Purple Cone 5 brushing glaze.

 

Bisque (over a year ago) and while I keep a record of how I fire, I'm not certain which load these two came from.  Best guess looked like: 1230 lower ring on low, upper ring off, lid propped and lower ring peep hole plugged, upper peep open.

1530: low and low, 1845: Med and low.  2015: med and med, lid closed, open peep still open. 2115 hi and med 2230 hi and hi and both peeps plugged. 

 

Next question if this is bloating, can you describe what's happening and the clincher: how to prevent it in a manually fired kiln? :)

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Thats bloating of the clay body

The cures can be hotter bisque fire -slower fires- cooler glaze fires

It can also mean change your clay body if these above do not fix it or your glazes need this temp.

 

I fire higher temps and bloating can also mean a bad batch of clay as that has happened to me more than once.

Mark

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This may sound weird but could a rubber band or rubber band pieces maybe gotten into your clay? This random bloating happened to me when i was in college using the class clay slurry to wet down clay i dug so i could mix it in the clay mixer. Investigating i found a few pieces of a broken rubber band in some of that clay batch. I continued to use the batch just kept an eye out for more rubber band and the few pieces that managed to sneak by didn't ruin enough ware to justify throwing the whole clay batch out.

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I saw this topic the other day, but until I saw your photos, I didn't realize that this was IMCO Stoneware 1C.

 

Please check out my post about this (link here). Turns out the manufacturer was aware of the problem at certain temperatures, and reformulated their clay. You probably have an older batch. (EDIT: Just read your full post... Helpful! Yes, if these are 11 months old, they are the old formula for 1C).

 

There are ways to work around it if you have a lot of this clay. Let me know if you have any other questions, I use this clay heavily.

Edited by mregecko

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