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Clear Glazes and Crazing

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I am in the process of testing clear cone 6 clear glazes to fit my cone 10 porcelain clay body.

Why this odd combo?(This is the clay I have right now 500 + pounds, when it is gone I may choose to switch to a cone 6 clay and testing will resume again but at least I will know what to do)

Clays tend to change over time and I am seeking a clear understanding of glaze formulation in order to be flexible.

 

I am looking to alter my glaze to meet the following requirements.

Does this glaze make for a strong durable functional product without crazing?

and

Does it look nice over slips?

 

My results were mostly good but not perfect. Crazing or Shivering in all tests.

Some tiny cracks some large.

Q 1. (How can you tell if a glaze is shivering or crazing?)

 

Q 2. When tinkering with a recipe to reduce crazing how do you know when you are getting close to a good fit? What does it look like when you cross the line of fit into the opposite problem?

 

Q 3. What should I change in this glaze to be rid of the crazing or shivering?

 

This recipe was the most successful out of 6 different formulations including 2 Amoco clear varieties.

 

(Very small Crazing)

Katie's Clear

Nepy Sye 39

Gerstly Borate 27

Flint 18

Whiting 8

EPK 8

 

 

2nd runner up....did not make colors look as good. Crazing large crackles.

CDG Clear

Frit 3124 85

EPK 15

------------------

Zircopax 2 (for bright colors) *I found little difference in using the pax myself.

 

I love the simplicity of the second choice as well as there is no gerstley borate to complicate the recipe.

 

Thank you for your thoughts.

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Can I recommend Michael Bailey's Glazes ^6, and Mastering ^6 Glazes by John Hesselberth and Ron Roy. Lots of answers. Using a porcelain body marketed at ^10 glazed with ^6 glazes will always be tricksy. There will others with more immediate knowledge of the chemical problems with vitrification and glaze fit who will give you chapter and verse why you're getting crazing and shivering: frankly an incredibly serious problem and you should not be selling or giving pieces away if they are for kitchen use if you're experiencing shivering where pieces of glaze are falling away, or can be pried away from the body. In the long run you'll want to consult the above mentioned texts. Invaluable.

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I am in the process of testing clear cone 6 clear glazes to fit my cone 10 porcelain clay body.

Why this odd combo?(This is the clay I have right now 500 + pounds, when it is gone I may choose to switch to a cone 6 clay and testing will resume again but at least I will know what to do)

Clays tend to change over time and I am seeking a clear understanding of glaze formulation in order to be flexible.

 

I am looking to alter my glaze to meet the following requirements.

Does this glaze make for a strong durable functional product without crazing?

and

Does it look nice over slips?

 

My results were mostly good but not perfect. Crazing or Shivering in all tests.

Some tiny cracks some large.

Q 1. (How can you tell if a glaze is shivering or crazing?)

 

Q 2. When tinkering with a recipe to reduce crazing how do you know when you are getting close to a good fit? What does it look like when you cross the line of fit into the opposite problem?

 

Q 3. What should I change in this glaze to be rid of the crazing or shivering?

 

This recipe was the most successful out of 6 different formulations including 2 Amoco clear varieties.

 

(Very small Crazing)

Katie's Clear

Nepy Sye 39

Gerstly Borate 27

Flint 18

Whiting 8

EPK 8

 

 

2nd runner up....did not make colors look as good. Crazing large crackles.

CDG Clear

Frit 3124 85

EPK 15

------------------

Zircopax 2 (for bright colors) *I found little difference in using the pax myself.

 

I love the simplicity of the second choice as well as there is no gerstley borate to complicate the recipe.

 

Thank you for your thoughts.

 

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It has been my experience to not mix glazes and clay that fire to different cones. In fact it is a very bad idea. I say this because making the stuff is fraught already with so many variables where things could go wrong so we have to keep as many of them as constant as possible. Even with my experience I cannot say just off hand why the two are not fitting. The feldspars in both the clay and glaze probably do not match and using a prepared frit alone in the second glaze is a problem because it is difficult to find out what is in it. One can only control some of the outcome by understanding what materials are interacting with each part of the process. High fire glazes (stoneware) are easier to make with fewer ingredients because everything melts - but lower fire calls for more ingredients some of which are kept from melting entirely, so we have to better balance things out. The opposite of crazing is crawling and that looks even worse than crazing which is a flaw we sometimes want for visual effect. I had a bunch of shivering on every edge (including the throwing ring edges) on lots of my pots I made years ago in South Africa. Colin Pearson explained that I was not bisquing high enough and therefore created a sort of dust between the pot and the glaze - so I started to bisque slightly higher. But that was with clay and glaze vitrifying at cone 10. I have consulted in the past, the labs at Alfred, especially when I ran into impossible probems in Africa years ago. They seemed to welcome a challenge and I learned a lot in the process. Certainly Richard Zakin would have something intelligent to give to you about the problem. You know you have a shivering problem when after the pots have been standing around from the kiln for an hour or so you begin to hear little pings and when you pick the pots up there are bits of glaze lying in a perfect circle. I stopped some of this by simply waxing all the edges and corners so they remained unglazed. But this is not a solution. Shivering weakens the pot and crazinng weakens the glaze and people will bug you with: how will I get the food out of all those little lines? Personally I would just finish up my cone 10 body with cone 10 glazes and then start again with a 6 body and a 6 glaze. Or sell your cone 10 body on craigslist or somewhere. All the best, Ruth

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Is your clay body vitrified at cone 6? Might be something to test out. If it is vitrified at cone 6, then a "mild" amount of crazing isn't the end of the world. If the body is not vitrified, then even a small amount of crazing will allow liquids through.

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I have a similar problem in that I have been slaking down 800 pounds dried up ^10 Porcelain and plan to mix it with paper pulp for sculptural clay body. I plan to test additions of 2-5% of feldspar, stir it with a heavy mixer and dry to workability on plaster. Will fire to ^6.

Marcia

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Cone 10 porcelains can be used at cone 6, but you will get more crazing than with a cone 6 porcelain. There's a good chance that even at cone 6, the cone 10 porcelain will be every bit as tight as a cone 6 stoneware. Check the charts from the manufacturer.

 

Crazing is when the glaze crackles. Shivering is when the glaze flakes off the pot.

 

 

If you are getting crazing, start adding silica in 3% increments (by weight). If it starts getting too glossy, add equal parts kaolin and silica (by weight). The crackle pattern will get larger as you get closer to a good fit.

 

To correct shivering, reduce the silica or increase the feldspar.

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Ya I agree to someone's above. Crazing and shivering is the matter of expansion and contraction between body and glaze, but mostly happen in cooling process. Crazing is determined by glaze which have more expansion and contraction relative to the body. Shivering is the reverse of crazing, a glaze is under too great compression.

 

May be I'm the old fashion. You can read the classic book which title Clay and The Glazes for the Potter by Daniel Rhodes. This is the holy book to me.rolleyes.gif

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