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.