There is nothing that I know of that will glue a broken kiln shelf with
sufficient strength that you'd want to use in the kiln again. However, don't
throw away the broken pieces. They can be used as smaller shelves when
stacking the kiln with different height pieces. I find half shelves, 1/4
shelves, etc extremely useful to maximize the firing content of our kilns.
Broken shelves because of their unique shapes can be the perfect fit in some