In a message dated 97-04-30 07:30:59 EDT, bsutton@gladstone.uoregon.edu
(Barbara Sutton) writes:

<< Mine is not cracked, yet. Is there any preventive maintenance possible
 on these? Thanks,
 Cris. >>

Hmmm... remove it and pack it away in a dark, temperature controlled airtight
vault.

in other words, it's inevitable that it will crack. they all do when exposed
to sunlight for a length of time. 
The question is just when will they do it...

just keep the clear silicone handy....  :<)

chris