A stoneware glaze firing- reaching 1300 C in a gas kiln takes around 10 hours. You need to watch and adjust the settings frequently to get the right temperature rise to avoid cracking due to thermal shock. Reduction has to start at the right temperature -usually around 1000 C to get the best results in glazes and clay effects. The kiln may need a soak near the top temperature to allow the whole kiln temperature to even out. ( A soak is really just holding at temperature for 30 minutes to an hour. It is surprising how you can have a cool spot in a kiln where glazes don’t mature- even 20 degrees makes a big difference!)
Glazes react to temperature, the clay and the atmosphere in the kiln. They are different if applied thickly or thinly. Layering one glaze over another can create interesting effects. With glazing it sometimes seems there are unlimited variables.
Below is the same glaze- Janet deBoos 136 on a white clay and a buff clay!
Below a teadust glaze on 2 different clays – I was surprised at such strong variation
This is usually a lovely golden brown but was in a cool section of the kiln and did not mature.