Re: ATM BVC Weirdness part II

[Leftfieldstar@aol.com]

In a message dated 8/19/01 1:52:56 PM Eastern Daylight Time, 
jlerch1@tampabay.rr.com writes:

<< What confuses me is I thought the main advantage to BVC mirrors was there
 near zero thermal coefficient of expansion.
 
 I inspected the mirror rather closely, and the only unusual thing I noticed
 as the form of the very small bubbles (1/6" or less) at what appeared to be
 several distinct layers thru the mirror, I'm not sure how these mirrors are
 created, but it appears they fuse several layers together to form the
 mirror, are bubbles to be expected?
 
 I didn't notice any signs of delaminating, nor could I figure out a way to
 test for internal stress inside the mirror (IE crossed polarizes) >>

James,
  If the laminations are of material with intentionally dissimilar CE, it may 
explain the zonal changes over time. The laminated layers could possibly be 
acting much like a bi-metallic spring and the accelerated effects of forced 
cooling of the mirror by washing is contracting the outer laminations faster 
than the internal laminations at a rate that shows as abnormal figure. How 
many layers are there? From what I understand about the firing process for 
BVC, it is not a vitrification process, but a fusing method and bubbles occur 
frequently at the plate interfaces. The bubbles may be entrapped air or 
products of outgassing. The fact that there was a previous problem of 
delamination, as posted in the archives, means the firing temperatures are 
too low for ceramic vitrification. Contamination of the plates surfaces may 
leave non-volatile residue that upsets the CE futher and may even be zonal. A 
transmitted light analysis using polarized light or interferometry would be 
interesting. In your spare time you should do something about that! 8^)  If 
the fusing is incomplete, the CE rate of change may be different also. If you 
have a laser source available, try passing the beam thru the glass and 
observe the internal relective qualities. You may be able to observe beam 
splitting at the interfaces. Funny stuff!!

Dominic DiLeo