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The Animator

Glazes Coming Out To Dark (Look Burnt)

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I recently made a gas kiln and when I did the glaze firing they all came out dark brownish/black. I don't really have an accurate way of measuring the temperature as the high temperature thermometer I have doesn't seem to be working(It registered around 1700 Fahrenheit during the bisque fire, but stayed at 73 Fahrenheit during the glaze firing)  . The glazes were cone 5 and the clay was supposed to be terracotta at 05 worm brown at cone 5-6, it came out dark brown/black as well. My guess is it fired to hot or the kiln environment caused it somehow. But I'm having trouble finding an answer online. Any advice is appreciated.  

 

 

*update: I did another glaze firing using witness cones, it was at the proper temperature and the glaze still came out dark brown/black instead of the blue's and greens they should have been.        

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neilestrick    1,381

First, put some cones in the kiln. You'll never get decent results without knowing how hot it's getting, and could end up doing a lot of damage to your kiln shelves or ruining your work. The thermocouple should only be used as a guide to measure rate of climb. Cones are the only thing that will be accurate in a gas kiln.

 

Any high iron clay will get darker the hotter it gets, and if you had a reducing atmosphere in the kiln then it will go really dark. Most brown/red clay bodies rated for cone 6/8 or lower are not intended for reduction. Their iron content is too high for that. Oxidation requires more iron to go dark, so in reduction it's too much iron.

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Thank you that is actually a big help. I did not realize they actually made pyrometric cones as physical objects. None of the pottery videos I watched ever talked about it. Thank you. 

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Marcia Selsor    1,301

I would think it is the clay also. TC going that hot will be really dark and turn the glazes dark as well.

Marcia

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@ Babs, that's what I'm thinking it might be. I did another glaze firing this time using witness cones and the glazes still came out dark brown/black instead of blue & green. Do you think I should remove the lid/keep  a fan running after I shut off the gas?   

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