From: Guy Brandenburg 
To:  atm{at}shore.net
Reply-To: Guy Brandenburg 


One of the folks in our mirror-making group had been working for quite a
while at trying to figure a candle-holder mirror that he had read about. It
polished nicely, and he got it to a very, very nice sphere for a while, but
after that everything just kept getting worse and worse. He cast pitch lap
after pitch lap, tried all sorts of strokes, had other people try strokes,
cast a dental-stone plug to put in the hole in the back, but nothing
helped. It just kept getting worse and worse. Finally, one day the center
of the mirror completely 'freaked out' and looked like somebody was taking
an aerial photograph of the Khumbu Icefall near Mount Everest (not that
I've ever been anywhere near there) - like lots and lots of sharp - pointed
mini-bergs had 'calved' off of the end of a glacier, making what I
understand is some of the most difficult terrain to be found anywhere on
our planet. Or else it looked like somebody was trying to do a seriously
mind-altering Op Art picture from the 1960's with all sorts of
criss-crossing lines and stuff. This was all seen on a Ronchigram.

At the 3rd Delmarva Mirror-Making Seminar, somebody told me what this
probably was: the glass itself losing its temper. They thought that the
glass had originally been tempered, and that the grinding and polishing
process had made that tempering get lost, and therefore you have that
incredible lack of homogeneity. They advised not messing with that type of
glass for that reason. I forget who gave me this advice.

So, if that is the correct analysis, let everybody know that trying to make
a mirror with one of those inexpensive candle-holders is not worth it. You
save a few bucks on the raw materials, but then you can't make a successful
mirror with it just about no matter what.

Guy Brandenburg

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