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.
Glazes Coming Out To Dark (Look Burnt)
Posted 19 April 2017 - 09:24 AM
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.
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
Posted 19 April 2017 - 01:07 PM
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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