Just a couple of broad strokes.
In high temp wares, clay is often your enemy. Yes. I know, it sounds stupid, but if you think of clay as a 'green strength' binder that holds together the more durable materials until they solidify at higher temperatures, you're getting my point. Try to keep the clay content as low as practicable. All they clays I used , whether Fire Clay , ball clay or Kaolin are all high Alumina clays. In some trials I tried high iron, terracotta type clays. In low %'s , some made reasonable non-ferrous pots but most just melted at stoneware temps.
And