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AnitaMarie

bisqued porcelain keeps breaking during glazing

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Hi - I am using porcelain for the first time, and while I knew it was persnickety, I am having an unexpected problem: it keeps breaking while I'm glazing.  While one pot was a very thin (too thin probably) vase, I have had multiple cup handles break in the middle or away from their attachment site.  The handles were not pulled - they were coils that I smoothed with a sponge, so I wondered if maybe the clay was not compressed enough?  The handles were relatively small but the cup was small and thin, too, so I don't think that is the problem. It is a cone 10  Narra porcelain, and they were bisqued to cone 06.  They seemed very stone-like and sturdy, no visible cracks, so the breaks were a big surprise.  I'm getting really frustrated...any ideas would be greatly appreciated!

Thanks, Anita

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how are you packing ware? How are you handling it after making it?

drying too fast

readsorption, bisque to higher cone, this may be the one!

cracks existing in raw/bisqueware prior to firing, then glazing?

too rapid early bisque firing

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Thanks for your responses, everybody.  I'll work on getting a picture.

These mugs were carried briefly, in an open box with newspaper stuffed around them.  No bumps that I'm aware of.  The handles break when I am holding them with my thumb and fore finger, at various angles, as I paint all the way around the mug (and no glaze on the handle yet).  So there is some torque going on, I guess.  The one thin vase that broke, I dipped it using my hands, not tongs - all five of my fingers around the rim and my palm above the opening.  That broke when the thin part got wet.  All pots were dried fairly slowly, under plastic.  Does porcelain need to be bisqued at a higher temperature than white stoneware, generally?  

Also, how slow should the bisque fire be exactly? They mugs were not fired by me, so maybe it was too fast.

Thanks for your help! - Anita

Edited by AnitaMarie
forgot a question

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If you are convinced that you are handling them gently enough given their thinness, natural weakness of bisqued-only attachments, etc.  than it sounds like they may be under bisqued.  I would try firing them a cone or so higher in the bisque.

as an observation, I have seen bisqued ware weaken somewhat when it gets too water-logged by dipping or spraying...

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1 hour ago, neilestrick said:

Are you sure they reached cone 06? Even very thin porcelain shouldn't break when it gets wet or picked up.

I’m thinking this too ... I bisque to Cone 010 and the work is very fragile at that stage. But, I have never had any break just because I glazed them. They are almost acting like greenware.

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Thanks, Everybody -

Unfortunately, I don't have the cones to show you because they were fired by a friend and she didn't save them, but I will try firing some at cone 06 and 04 in my own kiln and see if that helps.  I'll also beef up the handles - perhaps they were too delicate.   But nobody is worried that the handles broke because I didn't pull them?  I thought the lack of extra compression might have been a cause. Thanks, Anita

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If the vases and the rims are breaking too, it indicates it's not just the handle. And no, things won't break like that if they're not compressed enough. Lots of people handbuild a lot of different kinds of perfectly functional handles all the time.  In the absence of pictures, it's possible that the pieces are too thin and delicate, but like curt said, if you think you're handling things gently enough that's probably not it. 

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I have always avoided handling pieces by their handles, as I had always believed they were too brittle at that stage to take a chance. I am skeptical of the ^06 temp, as all too often, I have seen setters and programs that have cold spots in a kiln depending on the pack of the ware. A lower shelf with a short shelf above, especially if using full shelves in a smaller kiln can be off by at least 3 cones at times. I would request that the next firing have a cone pack next to your work. . . even if you have to provide it.

 

best,

Pres

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1 hour ago, AnitaMarie said:

Thanks, Everybody -

Unfortunately, I don't have the cones to show you because they were fired by a friend and she didn't save them, but I will try firing some at cone 06 and 04 in my own kiln and see if that helps.  I'll also beef up the handles - perhaps they were too delicate.   But nobody is worried that the handles broke because I didn't pull them?  I thought the lack of extra compression might have been a cause. Thanks, Anita

Photos would help immensely. Some folk here can detect if you sneezed whilst making,stacking or glazing your pots just by the appearance and placement of the cracks.

Would be well  worth attaching photos of the cracked pots.

I'd be tempted to immerse a bisqued pot in water prior to glazing to see if any cracks are apparent , obviously you'd then have to let it dry before glazing, just saying.

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Ok, here is the long awaited for picture - I was only able to salvage one piece, and this one didn't break in the middle of the handle like the others...but maybe this will help solve the mystery.  I'm doing a cone 04 bisque right now, so I will report back soon.  Thank you for your thoughts and suggestions!  Anita

IMG-3609.JPG

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Aha!

No, it's not too thin. If the handles are breaking away like that at the bisque stage, it means it wasn't attached as well as it could be. Porcelain doesn't like it when you try to attach pieces that have too much difference in their respective moisture contents. You really have to either work very wet and fresh, which takes a lot of confidence,  or employ the "wiggle" to attach and use the right amount of either slip or water. The pieces will also benefit from being wrapped in plastic for an hour or two to allow them to marry properly before being allowed to dry fully.

 

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Your handle is very stylish but looks like it might slip around in your hand when drinking a cup of coffee.  You could design a wilder thicker handle with the same curves.   Denice

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Scratch and slip (shallow scratches on thin ware, obviously) and a little wiggle when you join them will give you more consistent success with porcelain - and as Callie says, making sure that the pieces are of similar moisture content AND bagging for a while till beyond leather hard. Porcelain, she beautiful, but very unforgiving. Definitely do not lift by handles or rims until fired to ^10. Good luck. 

Rae

Edited by Rae Reich

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Rae, I should have explained more.  I lace drape dolls and often drape the lace after the doll foundation has dried or almost dry.  The Porcelain Magic acts much like the "porcelain glue" we used in the 70s which was a dry powder you added water to.  Unfortunately, it is no longer available and I have no idea of its compound.  In the cases of lace draping you are attaching very thin to fairly thick.  I agree with the hatching to attach handles, etc.  

I am also trying to understand if she is using Cone 10 porcelain and Cone 06 glaze.  Did she file the cup to Cone 10 and then apply the lower fire glaze or just fire the greenware to Cone 06...  If she didn't fire to Cone 10 first then the porcelain never reach maturity and could certainly fall apart with little weight or pressure.

 

Edited by RosieMom

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27 minutes ago, RosieMom said:

Rae, I should have explained more.  I lace drape dolls and often drape the lace after the doll foundation has dried or almost dry.  The Porcelain Magic acts much like the "porcelain glue" we used in the 70s which was a dry powder you added water to.  Unfortunately, it is no longer available and I have no idea of its compound.  In the cases of lace draping you are attaching very thin to fairly thick.  I agree with the hatching to attach handles, etc.  

I am also trying to understand if she is using Cone 10 porcelain and Cone 06 glaze.  Did she file the cup to Cone 10 and then apply the lower fire glaze or just fire the greenware to Cone 06...  If she didn't fire to Cone 10 first then the porcelain never reach maturity and could certainly fall apart with little weight or pressure.

 

Thanks for your clarification. I've made dolls all my life, but never of clay. Maybe I should try not being such a perfectionist about "classic" dolls. Lace draping sounds fun!

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anitamarie, i echo the rule to NEVER lift a pot by it's handle for any reason until it is safely fired to maturity.  you can paint your glaze on by stretching your hand to hold the rim by a finger and the base by the thumb.  it might take a little practice to do this without putting pressure on the piece and winding up with part of the rim gone.

Edited by oldlady
correction

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ye with Pres and Old Lady.

it s a skill to have the "holding" hand be just that and not a ever tightening vice..

better still place your cup on a banding wheel and keep hands off !

I don't ever use handles until out of the glost.

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