The Animator Posted April 19, 2017 Report Share Posted April 19, 2017 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. Link to comment Share on other sites More sharing options...
neilestrick Posted April 19, 2017 Report Share Posted April 19, 2017 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. Link to comment Share on other sites More sharing options...
The Animator Posted April 19, 2017 Author Report Share Posted April 19, 2017 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. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted April 19, 2017 Report Share Posted April 19, 2017 I would think it is the clay also. TC going that hot will be really dark and turn the glazes dark as well. Marcia Link to comment Share on other sites More sharing options...
Babs Posted April 20, 2017 Report Share Posted April 20, 2017 inadvertent reduction also?? Link to comment Share on other sites More sharing options...
The Animator Posted May 15, 2017 Author Report Share Posted May 15, 2017 @ 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? Link to comment Share on other sites More sharing options...
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