I'd tried extended bisque fire hold (~1500°F) for red, dark brown and black clays I was trying out (a few years ago), where the idea was to thoroughly "burn out" the stuff that will  burn out*. The hold did seem to help with the black clay, however, the main problem I was having with the red clay, fizzy bubbling, was most likely associated with too much heatwork. I'd set up a powered kiln vent at about the same time. Any road, my observation (notations were helpful here) was that rando