Sounds like it should be under warranty, unless they're claiming it was
broken by something not covered by the warranty.

That does sound expensive -- 3" dia x 1/8" thick extreme heat resistant
glass disc is under $50 at www.mcmaster.com

--
is Joshua Putnam
http://www.phred.org/~josh/
Braze your own bicycle frames. See
http://www.phred.org/~josh/build/build.html