Glass in the kiln

What I have learned over the years, is the unpredictability of firing in a kiln.  Here are some pics of a piece that I started on this past summer.  I made a 20" round "blank", about 1/4" thick with a  combination of glass powders and frit.  The initial firing was perfect but when it was fired on the mold, ..... well you can see what happened.  I knew the damage had happened in the annealing stage due to the razor sharp edges.  Smooth edges would indicate that the cracking happened in the beginning of the firing and the glass still heated.  Really, that was a good find.  What I determined was that my very old, big kiln relays were just tired and needed to be replaced.  They shut down completely in the annealing stage which caused the glass to cool too quickly and thermal shock.  So onto replacing the relays  breaking all the pieces, adding more frit and re-firing again into a 20" round.  Never give up!