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Diane Puckett

Crystal formation in under fired glaze?

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After two and a half years, I have finally gotten my glaze problems solved. I am beyond relieved. Now I can focus on making pottery that will not end up in the landfill.

 

In my last glaze tests, I got some really nice, small crystals with Opulence Bamboo. Someone had indicated it was matte because it was under-fired. Is it possible to have crystalline formation in an under-fired glaze?

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neilestrick    1,381

No. It is most likely matte because of the crystals, which is the best way to achieve matteness, because you get a fully matured glaze that is usually more durable.

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No. It is most likely matte because of the crystals, which is the best way to achieve matteness, because you get a fully matured glaze that is usually more durable.

 

Thanks, Neil.

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Marcia Selsor    1,301

There are macro crystals and micro crystals in glazes. Micro crystals are what cause matt glazes as Neil described. The large decorative crystals are macro crystals usually in a zinc silicate glaze. They are very runny and the temperature is dropped rapidly and held at a lower temperature to grow the macro crystals.

 

Marcia

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There are macro crystals and micro crystals in glazes. Micro crystals are what cause matt glazes as Neil described. The large decorative crystals are macro crystals usually in a zinc silicate glaze. They are very runny and the temperature is dropped rapidly and held at a lower temperature to grow the macro crystals.

 

Marcia

 

This are big enough to see clearly, maybe 1/4 inch, not as bit as the ones I have seen in decorative crystalline glazes. I did a slow cool and shut off the vent fan at 1500 degrees. I would not use this glaze inside a functional piece, but I did wonder if it was possible to have crystal formation in an immature glaze.

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Marcia Selsor    1,301

There are macro crystals and micro crystals in glazes. Micro crystals are what cause matt glazes as Neil described. The large decorative crystals are macro crystals usually in a zinc silicate glaze. They are very runny and the temperature is dropped rapidly and held at a lower temperature to grow the macro crystals.

 

Marcia

 

This are big enough to see clearly, maybe 1/4 inch, not as bit as the ones I have seen in decorative crystalline glazes. I did a slow cool and shut off the vent fan at 1500 degrees. I would not use this glaze inside a functional piece, but I did wonder if it was possible to have crystal formation in an immature glaze.

 

 

I really can't say anything about commercially made glazes. I have never used them except for ^04 for a class project decades ago.can you post a photo of the glaze with the crystal in it?

 

 

Marcia

 

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There are macro crystals and micro crystals in glazes. Micro crystals are what cause matt glazes as Neil described. The large decorative crystals are macro crystals usually in a zinc silicate glaze. They are very runny and the temperature is dropped rapidly and held at a lower temperature to grow the macro crystals.

 

Marcia

 

This are big enough to see clearly, maybe 1/4 inch, not as bit as the ones I have seen in decorative crystalline glazes. I did a slow cool and shut off the vent fan at 1500 degrees. I would not use this glaze inside a functional piece, but I did wonder if it was possible to have crystal formation in an immature glaze.

 

 

I really can't say anything about commercially made glazes. I have never used them except for ^04 for a class project decades ago.can you post a photo of the glaze with the crystal in it?

 

Marcia

 

 

Will do so ASAP. I have an almost dead camera and almost dead PC. My iPad is fine but takes lousy photos.

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That glaze is supposed to be matt at cone 6 you know?.

 

 

Yes, but some glazes which are matte at cone 6 are actually under fired, this getting their matte finish from being under fired rather than crystals. Someone had told me this was one of those glazes, but since I got visible crystals, I think it is mature at cone 6. I was just wondering if you can have crystals in an under fired glaze.

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That glaze is supposed to be matt at cone 6 you know?.

 

 

Firing schedules can greatly alter the matteness of a glaze.

 

 

Cone 6, 15 minute hold, turned off the vent at 1500.

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neilestrick    1,381

Cone 6 plus 15 means closer to cone 7. If you fired down you could probably get larger crystals. Even just a little slower, like 175 per hour down to 1500 would be enough to make a difference.

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Cone 6 plus 15 means closer to cone 7. If you fired down you could probably get larger crystals. Even just a little slower, like 175 per hour down to 1500 would be enough to make a difference.

 

Forgot to put that I did a controlled cool, which is probably why I got the crystals. Now that I have gotten my glaze problems solved, I plan to skip the hold next time, or maybe do a 5 minute hold.. Cone 7 was nearly flat on 2 of my shelves, so even though my thermocouple offsets are already increased from 18 to 30, I think they need to be increased more.

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