Marcia, I found this thread after two 15" x 15" x 1/2" slabs cracking during bisque firing. I had used ^5 Laguna Bmix with Grog, rolling out the slabs to 1/2" while turning in all different directions. The slabs were completely dry (not cold to the touch) before I did a SLOW (15 hour) bisque firing to ^04 in a very loosely-packed kiln, with the 2 slabs on the cooler bottom portion of of the kiln, resting on full kiln shelves. One of the slabs cracked completely in half and the other cracked 75% across the surface. <sigh>
I didn't use coils or kiln posts to elevate the slabs off the shelves because I thought that might cause warping. I considered using silica or sand as "ball bearings" under the slabsm but decided against this after reading a cautionary essay by Vince Pitelka on the practice.
Do you still recommend using coils under a large slab? If so, how far apart should they be spaced? Pitelka talks about using short kiln posts in the corners, but all I can imagine is my large slab turning into a bowl as it sinks in the middle during the firing process.