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Crystal formation in under fired glaze?


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#1 Diane Puckett

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Posted 18 April 2013 - 04:28 PM

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?
Diane Puckett
Dry Ridge Pottery

#2 neilestrick

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Posted 18 April 2013 - 04:46 PM

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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#3 Diane Puckett

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Posted 18 April 2013 - 06:57 PM

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.
Diane Puckett
Dry Ridge Pottery

#4 Marcia Selsor

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Posted 19 April 2013 - 06:27 AM

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

#5 Diane Puckett

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Posted 19 April 2013 - 11:43 AM

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.
Diane Puckett
Dry Ridge Pottery

#6 Marcia Selsor

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Posted 19 April 2013 - 04:40 PM


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


#7 Diane Puckett

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Posted 19 April 2013 - 06:15 PM



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.
Diane Puckett
Dry Ridge Pottery

#8 timbo_heff

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Posted 22 April 2013 - 12:15 PM

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

#9 Diane Puckett

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Posted 23 April 2013 - 07:38 AM

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.
Diane Puckett
Dry Ridge Pottery

#10 neilestrick

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Posted 23 April 2013 - 09:31 AM

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


Firing schedules can greatly alter the matteness of a glaze.
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#11 Diane Puckett

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Posted 23 April 2013 - 06:10 PM


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.
Diane Puckett
Dry Ridge Pottery

#12 neilestrick

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Posted 23 April 2013 - 08:43 PM

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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#13 Diane Puckett

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Posted 23 April 2013 - 09:23 PM

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.
Diane Puckett
Dry Ridge Pottery

#14 neilestrick

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Posted 24 April 2013 - 09:07 AM

Just eliminate the hold before you go adjusting the thermocouples more. One change at a time.
Neil Estrick
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