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Glaze Cracking On Bisque


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#1 yedrow

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Posted 08 October 2013 - 08:26 PM

I'm having an issue with one of my glazes cracking as it dries on the bisque. The problem is worse when it is put over another glaze. This of course leads to crawling on the fired ware. I've tried using Veegum T to toughen it up, but not to much effect. I have just inherited some CMC, but I don't know how to use it. Is that a possible solution to this problem? Or do I need to do something else?

 

Joel.



#2 bciskepottery

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Posted 08 October 2013 - 09:03 PM

Maybe try adding some bentonite to the glaze; the glaze may not have enough clay in it so it is shrinking and cracking. 



#3 Marcia Selsor

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Posted 09 October 2013 - 09:32 AM

Maybe thin the glaze a bit. Sounds like it is going on too think.
Marcia

#4 yedrow

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Posted 09 October 2013 - 09:31 PM

I'll try the bentonite. From what I've read though, bentonite can cause the same problem. I was trying to avoid this effect. Would a 1-2% mix avoid exacerbating the problem?

Marcia, It is pretty thin and I still have the problem. At the moment the glaze has a specific gravity of 1.47. I need a solution that will keep this sg. If I'm understanding the articles I've read about CMC, it can toughen the glaze. But, I don't know how to use it. From what I've read there is a learning curve to this material.

 

Joel.



#5 Mark C.

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Posted 09 October 2013 - 11:50 PM

I'm not a big gum user but have used it in salt kiln coatings for adhesion.

heres the ceramic arts daily stick-

 

Carboxymethylcellulose (CMC) — an organic gum used as a suspension/adhesion agent in glazes. Normally, a small amount of gum is added to a quart or so of warm water and left overnight. Once dissolved, this solution may be added in small doses to glazes, slips, and engobes to improve application performance. Source: Clay: A Studio Handbook

 

 


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#6 Min

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Posted 10 October 2013 - 08:34 AM

Is there a large amount of kaolin in the glaze? If so then calcining 1/2 of it would reduce shrinkage / cracking. 



#7 minspargal

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Posted 10 October 2013 - 08:43 AM

That usually happens to me with commercial glazes that get too dry in the jars. i have tried remixing with water, using the additive Amaco sells and nothing seems to revive them. Now i just throw them out if i see them cracking on the bisque.I hate to throw money into the trash can. Anyone have any luck with this?



#8 yedrow

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Posted 12 October 2013 - 10:14 PM

This is the recipe. There isn't a lot of EPK in it so I don't think that is the problem.

 

Custer Feldspar............. 24.1
Gerstley Borate............. 13.8
Wollastonite................ 5.8
Magnesium Carbonate......... 2.5
Barium Carbonate............08.6
FRIT 3134................... 8.7
FRIT 3195................... 2.3
EPK Kaolin.................. 9.9
Silica...................... 24.2
Rutile...................... 8.0
red iron oxide.............. 1.0

 

I'm working on a newer version of this glaze so the problem could fix itself. I'll do a test on it this week.

 

Custer Feldspar............. 25.17
Gerstley Borate............. 17.70
Magnesium Carbonate......... 2.61
Barium Carbonate............ 4.29
FRIT 3134................... 9.09
EPK Kaolin.................. 10.34
Silica...................... 30.80
Rutile...................... 8.00
red iron oxide.............. 1.00 



#9 hershey8

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Posted 26 October 2013 - 01:29 PM

I have two commercial  cone 6 glazes that have laid around and settled/dried up some. I've mixed them up, but they crack after being applied to bisque. They seem to have adequate water. Can anything be done to correct this?



#10 Mart

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Posted 27 October 2013 - 09:58 AM

....The problem is worse when it is put over another glaze....

We have few glazes that are "clay based" and they do not like frit based glazes when applied to bisque ware... or was it the other way around.




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