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Upate On Glazes


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#1 oly

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Posted 08 February 2014 - 09:42 AM

I'm making  anew post, simply because I could not seem to 'attach' new images to the old post, for some reason, anyone know?

 

I wanted to share these pics, the semi-crystal glaze where I was getting a 'burnt' effect inside the pot, by using a much thicker layer of glaze as kindly suggested on the this forum, the glaze has worked nicely. the only tech issue I think is some small bubbles that have broken the surface and of course it is crazed, thogh that is okay I think.

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#2 Marcia Selsor

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Posted 08 February 2014 - 09:49 AM

that type of crazing is a defect, especially for functional pots. Beautiful color.
I would worry that the pool in the bottom could actually explode eventually.
Try to balance the coe.
Marcia

#3 oly

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Posted 08 February 2014 - 11:35 AM

Yes, there is a lot more to this glazing than first appears. The tutor at the evening classes I went to suggested I 'keep things simple', so I'll be keeping away from these sorts of tricky glazes at least for a while!

 

I've done some tests on some more standard cone 6 glazes which are going to give me something to work with while I continue to improve my throwing skills.

 

What is 'coe' please? : (

 

that type of crazing is a defect, especially for functional pots. Beautiful color.
I would worry that the pool in the bottom could actually explode eventually.
Try to balance the coe.
Marcia



#4 JBaymore

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Posted 08 February 2014 - 12:11 PM

Coefficeint of Thermal Expansion....... I usually make sure to list "reversible" in there......... Coefficeint of (reversible) Thermal Expansion.

 

This relates to the amount that the ALREADY SET liquid glaze layer shrinks upon cooling.  (Glass is a super cooled liquid.... not a solid.)  If the glaze shrinks more then the clay body under it .... it puts the glaze in tension.  Glass is very weak in tension.... so it fractures.... which we call crazing.  Or we call "crackle" if we want to make a "defect" a "feature". (Glass half full/ half empty .... "you say toe may toe, I say tah mah toe".)

 

If it is the opposite situation.... the glaze shivers when it gets in too much compression.

 

When the COE of the glaze and the clay are VERY close to a match or actually matched....... the glaze "fits" the clay body.

 

You adjust the COE in glazes by altering the relationships of the OXIDES in the fired liquid glaze.  Diffeernt oxides impart differeing OCEs in the final melt.  Sodium oxide has the highest COE.... lithium oxide the lowest.

 

best,

 

..................................john


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

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#5 Benzine

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Posted 08 February 2014 - 11:42 PM

John, I thought glass being a super cooled solid, was a misconception?
People believed it, partially because they noticed old window were thicker at he bottom, and assumed it is because the glass flowed down there over time due to gravity. Instead it was just due to the methods for glass fabrication at the time, not being as precise as they are now.
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#6 Benzine

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Posted 08 February 2014 - 11:53 PM

I actually looked into it, glass is considered an "Amorphous Solid", a solid with no order.
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#7 Marcia Selsor

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Posted 09 February 2014 - 09:07 AM

I had a student melt some glass in a large pot fired to ^9. The glass was at least 1/2" thick. After a few weeks at home sitting on a shelf, it exploded. We work with some interesting materials.
So be careful when you have excessive pooling inside a piece.

Marcia

#8 Benzine

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Posted 09 February 2014 - 10:15 AM

Marcia, when you say "exploded", how much force are we talking about?
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#9 Marcia Selsor

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Posted 09 February 2014 - 11:17 AM

shattered with force. Scared the crap out of the student. I wasn't there, but that is what he told me.
Marcia

#10 Benzine

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Posted 09 February 2014 - 11:24 AM

shattered with force. Scared the crap out of the student. I wasn't there, but that is what he told me.Marcia


Wow!

I've only ever heard the pinging, associated with cooling wares too quickly. One of my recent pieces, that had the issue, cracked pretty bad, but I didn't heat. thing.
"Anything worth believing, is worth questioning"




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