Re: ATM 7" soda-lime tempered glass, Part Two, the Sequel!

[RDuvall@pixworks.com (Robert Duvall)]

At 9:50 PM 2/25/97, BRYAN LEENHEER wrote:
>My speculation on this project is thus...
>
>Tempered glass is a really bad choice for mirror blanks.

Absolutely true. You cannot use tempered glass. You can duplicate the
process you just did by taking a drill and starting to drill a small hole.
As soon as you break the surface (as you did with the grit), be prepared
for an explosion of tiny glass shards shooting out 15 feet or more. I first
learned this 20 years ago... it was not fun.

There is a tension between the outside and inside of the glass. This is
done by design and is what gives the glass extra strength against impact.
However, it also puts alot of residual stress on the glass, which is _not_
good for telescope mirrors. And it also cannot be cut or machined after
tempering.

DO NOT EVER USE TEMPERED GLASS for mirrors. The explosive power released
from the tension can cause serious injury.

I'm glad you were not injured.

Robert