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Hello,

I have recently set up my home studio to practice pottery alongside my course. I have a question about my glaze firing and wondered if anyone could help? I’ve just done my first glaze firing, I used Botz brush on glaze on my bisque fired white earthenware (bisque fired to 04) and glaze fired at 05. I’m wondering if I should of also done it on 04 as now they are out of the kiln they are pinging like a tinkling noise. The kiln was left for 24hrs before unloading.

Ive read that this occurs also when the glaze doesn’t fit the clay, but the glaze is an earthenware glaze so I don’t really understand. Does anyone have any ideas on what I’m doing wrong?

 Thank you 

Emma

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Edited by Emma Louise
Added photo

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Welcome to the Forum, Emma... If you were doing a Raku firing using a crackle glaze, that would be a delightful sound as the glaze would be cracking just the way you want it to before reducing it. I don't think that's what you're doing here. What was the temp of the kiln when you opened it? What was the condition of the glaze when you inspected your pieces? Can you send some photos? If the glaze was rated at ^05 there shouldn't be a problem.  Again, pix might help.

johnnyK

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30 minutes ago, JohnnyK said:

Welcome to the Forum, Emma... If you were doing a Raku firing using a crackle glaze, that would be a delightful sound as the glaze would be cracking just the way you want it to before reducing it. I don't think that's what you're doing here. What was the temp of the kiln when you opened it? What was the condition of the glaze when you inspected your pieces? Can you send some photos? If the glaze was rated at ^05 there shouldn't be a problem.  Again, pix might help.

johnnyK

Hi JohnnyK

Thank you. I've just added a photo. Its a bots glaze that has the crystals in it. The kiln was cold when I opened it, not sure of the exact temperature but there was no warmth there at all. The bisque firing was 04 (wondering if I should of done 03 to tighten things up a bit), then the glaze was 05. It's an old sitter kiln so I'm afraid I never know the exact temperatures, it just shuts off when the bar has gone. You can't really see any crackle on the pieces but I've tested some and they are not water tight so its definitely cracked. I really don't think I applied it that thickly and wondering if there were just too many crystal particles on my pieces. This is my first time glazing in this kiln and I've also ordered a plain glaze to test out.

Emma

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Glaze fit is a bummer isn't it.  Wish I had some good advice, but if you're set on using that glaze you need to change your body or vice versa.  

I have a red stoneware body that is a nightmare for glaze fitting and I have to adjust almost every glaze to fit it. But I mix my own glazes so it's not too bad now that I know what the issue is.  

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8 hours ago, Emma Louise said:

Ive read that this occurs also when the glaze doesn’t fit the clay, but the glaze is an earthenware glaze so I don’t really understand.

Just because both your clay and glaze are meant for earthenware temperatures doesn't mean they will suit each other.  The glaze needs to fit the clay in order to avoid crazing (or shivering). What fit is measuring is called Coefficient of Thermal Expansion (COE or CTE) and is the difference between the COE of the glaze and body.

When you read about clay and glaze “fit” an easy way for it to make sense is to think about how clothing fits. You are the pot and the glaze is a pair of jeans, they need to fit each other. If the jeans are too small they will stretch then tear, in ceramics this is called crazing. The glaze is under tension and crazes. 

If the jeans are too big for you they won’t fit either, they will not stay on. In ceramics this is shivering, the glaze is too big for the pot. The glaze is in compression and shivers (sharp slivers of glaze pop off of the pot). A little bit of compression makes for a strong pot but too much isn’t good. 

It’s especially important to have a non crazing glaze with earthenware. The glaze is what is keeping moisture, oils etc from being absorbed by the porous clay. With commercial glazes it's going to be trial and error to find a glaze that fits your claybody. I would start by asking the clay supplier for their recommendations for a non-crazing well fitting glaze for the clay you use and make sure you fire to the recommended cone.

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You mentioned doing a course . Can you find out what glazes the teacher uses on this clay body presuming that is the clay you are using.

Or post your clay body and ask if anyone will share what glaze they've found to fit it.

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