Firebrick cracks as it is heated and cooled- it is to be expected. When the kiln is hot, all the cracks fill in from the expanding brick. The biggest firebrick slabs are made with ceramic fiber expansion joints to accommodate this natural occurrence. Not every brick or slab cracks right away, but it seems that all bricks in the kiln, especially the slabs, will crack eventually.
- Keep the stainless steel bands around the slabs and the sections tight.
- Have a solid floor kiln stand like recent L&L stands, or add a 14ga galvanized steel or aluminum/ stainless steel plate between an older hollow-center stand and the slab.
- With bad cracks, fill with NON-RCF ceramic fiber- not too much- just full, not stuffed!
- Use a top shelf in the load to block falling brick chips and to secure a very cracked lid for a little while.
- Re-gluing slab pieces with high temp cement and water may work well for a while too, or they may crack again right away.