Re: ATM Pyrex blank quality

["Bob May" <bobmay@nethere.com>]

You may consider that the received appearance of the pyrex is really bad but
you have to remember that you are going to be doing a lot of work on that
glass and most, if not all, of those marks that you find "poor" are not
going to affect anything.
The glass needs to have the top side ground a bit to remove the meniscus
that forms as that can crack off easy and make for nice cutting edges that
you will find even more objectionable.  Since the user of the glass is going
to be doing a lot of work grinding that surface, there is no need to make
the glass a nice ground flat surface unless you desire to spend another $15
or so on the glass.
As to hard points from annealing, that kind of thing really doesn't happen
at that stage although you can find devitrification on occasion from the
pouring of the glass and even that is not of much consequence as it is a
surface pheonomen that isn't affecting the glass inside.  Remember that
you're probably going to be grinding about 1/16th of an inch off of the
surface edges while making the mirror.
Anything in the 20" range for a mirror is going to be a cast piece unless
you are willing to accept a 1" thick piece and I'd suggest that you consider
having such a piece slumped to the approximate curve if at all possible to
maximize the thickness of the glass in the middle of the mirror.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net