I know you should be able to stack greenware for a bisque firing without incident -- but I'm always afraid to - mostly because I'm afraid that my cone 6 porcelain plates and platters will not stay flat. Generally speaking the shape doesn't change in my bisque firings (05 or 04) - but because I prefer to use fairly thin slabs to create my work, there is sometimes warping during the glaze firing.
Technically and intellecturally I know it is unlikely that cone 6 porcelain (Tuckers Bright White) will move in shape during a cone 04 or 05 firing...but still I hesitate. I would love to stack - to get a bigger load of bisque done at once. Would love to hear others experience with stacking (and when I say stacking, I just mean stacking two plates or platters - not a half dozen!)