>I have students who have had to fire large flat plates/tiles that do not
>fit on one shelf in my kiln. If it strattles over a gap between the two
>half-shelves, it tends to warp or crack. Do I need full shelves (don't
>have) or can I push the two halves together tightly and make it work?
I have been working as a clay artist and teacher for many years. You
could be right about the reason for the cracks, but I doubt it. More
likely, the tile is cracking for other reasons. Most tile cracking
and warping relates to the drying and shrinkage. This stress often
does not show up until the firing. When pieces are dried correctly,
they generally fire without cracks.
These are a few web pages with some helpful hints to solve the drying
problems in the future.