Not sure if this question should be here or in glaze chemistry. I usually use Amaco Velvets on my stoneware greenware, then bisque and glaze fire. When there are small touch ups needed after bisque, I can usually do the touch up and go ahead with glaze, however if the touch up is large, such as when a large space needs a whole new coat of u.g., then I usually re bisque because the glaze absorbs different;y on the new application.
Recently I had a whole kiln load to touch up so I did a hardening on after bisque but before glaze firing at cone 022. However I forgot to take out the electric kiln's bungs. Many colors went VERY UGLY ! I am not sure if the reason is oxygen starvation of just not reaching a mature temperature??