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Biglou13

Fixing Crazing...

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I've read, possibly here, that you can try and amend a crazing glaze by adding silica.

 

What percentage "jumps" should I start my testing with?

 

I'm still trying to wrap my head around the COE thing.

 

Why/how does silica lower the COE.?

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Hi Biglou

 

Have just done this myself with a blue/green transparent glaze that crazed unreasonably on a ^6 porcelain body.

 

Used increments of 5%, 10%, 15%, 20%, 25% silica to 5 x 100ml sample size containers of glaze, tested as a 'pie chart' circle on the inside of a medium size bowl of same the porcelain.  5% a little improvement,10% had the tiniest cracks (decorative but not for my functional ware), 15% was was craze free but a little too matt for what I wanted 20% very matt, 25% bubbled, retested 11%, 12%, 13%, 14% found 12% did the job.

 

Also tested the original glaze on 2 stoneware bodies I have in the studio, 1 on white s/w was unremarkable but 1 on buff coloured s/w had very, very fine crazing and changed the colour to a beautiful pale soft blue. Now searching my supplier for a buff coloured ^6 vitrified stoneware!

 

When you test your glaze by all means adjust the silica levels to eliminate the crazing but also test on other clay bodies you have....you might also be pleasantly surprised

 

Check out the Digitalfire site... 

 http://digitalfire.com/4sight/education/index.html#GlazesThermal Expansion

 

Irene

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Why/how does silica lower the COE.?

Silica has a COE of 3 point 5.  Whiting, for example, has a COE of 14 point 8. By adding silica you are lowering the overall COE of the glaze. If the glaze can take more silica and the crazing isn't too severe it works. Problem is that it doesn't always work.

 

From Tony Hansen,

"Add increasing amounts of silica

Yes, this may work in some cases of slight crazing where the glaze can tolerate more silica. However adding enough to simply hide out-of-the-kiln crazing may not deal with the deeper fit problems, the internal stresses between body and a glaze of higher expansion will weaken the ware and eventually craze it anyway. Also, it is often surprising how much must be added to reduce expansion to any degree. Why? Yes, silica is a low expansion oxide, but if the glaze is full of high-expansion fluxes like sodium and potassium, adding it is a little like adding white paint to dilute the color of black. The amount of silica present is usually what is required, so adding more can introduce unwanted gloss, higher melting temperatures, and change in surface character. Since mattes, which usually maintain a critical ratio of silica:alumina, are the ones that often craze, adding silica is not a solution. In addition, highly melted reactive glazes typically depend on low silica for their unique character. What about functional glossy glazes? Many high temperature glossy glazes will tolerate silica additions, but again, to put enough in to solve the problem will normally detrimentally affect melting. Adding silica to middle or low fire glazes will almost always mean less melting."

 

Best investment you can make for glaze testing is buying one of the glaze calculation programs. 

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Best investment you can make for glaze testing is buying one of the glaze calculation programs. 

 

Amen to that. Have to add...... and spend the time to learn to use it.  It is NOT that difficult.

 

best,

 

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

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I had a supposed matt glaze that crazed and ran on Frost. I added 3% silica for the crazing and added 2% EPK for the running and I got a nice matt glaze that doesn't run and passed the lemon test for leaching.You need to test and alter for specific clay bodies. The adjustments can be small like 1-5%. It just depends...as always in clay.

I don't use a computer program. I had two semesters of Glaze calculation; one in college and one in graduate school.It depends on the clay being used, the firing and the chemicals in the glaze.

I understand the ratios between the fluxes, stiffeners and glass makers, known as the radicals.

RO, RO2 and R2O3

 

Marcia

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Matt Katz gives away an abbreviated version of his glaze program on this clay company website.. Google Matt and Dave's Clays, it's under the science tab. It's an excel spreadsheet with all the data entry done for you, but it's a great tool for doing all your umf calculations.

 

 

It's pretty important to understand the materials and the products you're formulating so you don't harm anyone with improperly formulated materials. Certain materials have some pretty grizzly msds sheets, and innocent experimentation or casual science can lead to some rather unpleasant results...

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I had a supposed matt glaze that crazed and ran on Frost. I added 3% silica for the crazing and added 2% EPK for the running and I got a nice matt glaze that doesn't run and passed the lemon test for leaching.You need to test and alter for specific clay bodies. The adjustments can be small like 1-5%. It just depends...as always in clay.

I don't use a computer program. I had two semesters of Glaze calculation; one in college and one in graduate school.It depends on the clay being used, the firing and the chemicals in the glaze.

I understand the ratios between the fluxes, stiffeners and glass makers, known as the radicals.

RO, RO2 and R2O3

 

Marcia

Marcia, so the silica is a true addition, or do you adjust the glaze formula to accomodate the increase in silica?

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Matt Katz gives away an abbreviated version of his glaze program on this clay company website.. Google Matt and Dave's Clays, it's under the science tab. It's an excel spreadsheet with all the data entry done for you, but it's a great tool for doing all your umf calculations.

 

 

It's pretty important to understand the materials and the products you're formulating so you don't harm anyone with improperly formulated materials. Certain materials have some pretty grizzly msds sheets, and innocent experimentation or casual science can lead to some rather unpleasant results...

 

You might want to get that free calculator now.  I see on their website that they are closing down.  

"Happy Trails
 
We are writing from Matt and Dave’s Clays with news of the future.  Sadly, that news is that this is the end of the line. Matt’s family has a new opportunity that they have decided to follow and we can’t continue without him. So we are going to be closing our doors.
 
We have loved our time helping everyone make great things and we have loved seeing the things that you have made. We will miss you all. We will be accepting orders until Wednesday the 26th of March. If you would like to stock up, please place all order by then. "

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