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Celia UK

White Stoneware Low Fire Glaze Mistake - Pinging

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I've just taken a glaze firing out of the kiln - 1080oC (cone 04) transparent glaze on (mostly) a white earthenware clay. I can see that 2 pieces were in fact stoneware, which wasn't obvious after the bisque firing as my other pieces were white earthenware. The two stoneware bowls are pinging like mad and I can see the linear glaze cracks going round the bowls. The glaze can be pretty forgiving at higher temperatures and I'm wondering if these pieces might 'work' if refired to the clay's maturity temperature, or will the glaze fit always be a problem?

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Try it Celia, what have you got to lose?  I've got lots of ^04 glazes (commercial) that do work at ^6, and a few that "work" but change colour or fade.  Assuming they're still in one piece by the time you next run a ^6 firing  :)

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Maybe put the same 04 glaze on the top half of a test scrap piece (on a cookie / shard) of the same stoneware and fire that to the stonewares temp. Would save filling up the kiln with larger pots just to see what the glaze does.

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Thanks all - I don't think I was clear expressing my thoughts! I'm sure the glaze will survive as I've fired it higher before but I was wondering if the the glaze fit would be better if the clay was fired to maturity - alleviating the pinging and cracking of the glaze. OR will the glaze fit be poor irrespective of the firing temperature? Would a slow cool help at all?

Ann, I agree I've nothing to lose by trying - was just picking everyone's brain before putting them in with the next stoneware firing!

 

I am trying to develop a system to ensure I keep track of which clay is which - labelled my damp boxes and shelves but it's when pieces go into the bisque firing that I sometimes lose track - my white earthenware and stoneware clays look pretty similar when they come out and even the feel is much same! I mostly use the white earthenware, but had some stoneware pieces from a throwing workshop lying around. JUST in case anyone was wonder how on earth I could mix them up!!!

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Thanks all - I don't think I was clear expressing my thoughts! I'm sure the glaze will survive as I've fired it higher before but I was wondering if the the glaze fit would be better if the clay was fired to maturity - alleviating the pinging and cracking of the glaze. OR will the glaze fit be poor irrespective of the firing temperature? Would a slow cool help at all?

Ann, I agree I've nothing to lose by trying - was just picking everyone's brain before putting them in with the next stoneware firing!

 

I am trying to develop a system to ensure I keep track of which clay is which - labelled my damp boxes and shelves but it's when pieces go into the bisque firing that I sometimes lose track - my white earthenware and stoneware clays look pretty similar when they come out and even the feel is much same! I mostly use the white earthenware, but had some stoneware pieces from a throwing workshop lying around. JUST in case anyone was wonder how on earth I could mix them up!!!

 

I often fire stoneware pieces with underglaze that I paint on them as greenware. When they come out of the bisque all the underglaze is crazed. When I fire again the crazing disappears, never to return. So I would definitely suggest refiring the piece to completion. 

 

Re: the mix-up: Honestly it is easy to do, I can understand how it would happen once the piece was dry. 

 

As soon as I take clay out of the box I write what kind it is and my initials on the bottom since my dad keeps some clay in my studio for when he comes to throw. But that doesn't help when they look and feel the same dry! I've gotten so confused between my dad's porcelain and my B-Mix but of course when fired they are unmistakably different. 

 

Since you are obviously firing at both temperatures regularly, might I suggest, Celia, developing a system to prevent this? For example, as soon as you trim your earthenware pieces you have a tiny stamp you use to mark the bottom along with your signature or initials. Like a little flower or a heart. And you could mark the stoneware in a different way. Nobody else would ever know the reason for the difference but it might help. Or mark all your class pieces differently from pieces you make at home? 

 

 

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The COE of clay is significantly higher when it is immature: rather than vitrified. Silica primarily, but most all materials expand when heated until the flux melt begins to happen. COE values given for clay usually apply to the final product.

 

Visual aid: A slice of bread is filled with air and expanded. When you press down in the center you force the air out: and the bread flattens to less than half the original thickness. Same thing is happening in clay: but not as dramatic.

 

Nerd

 

Not sure where your post (above) disappeared to Nerd - pasted here from my notification email!

Thank you for the explanation - it makes me think a refire might well be successful. I'll try it for sure.

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