Re: ATM About making a glass blank

["Dwight K. Elvey" <dwightk.elvey@amd.com>]

>From: "dan otto" <atmpob@yahoo.com>
---snip---
>
>On the other hand, I heard (probably on this list) that the Mt. Wilson 60"
>mirror was cast from wine bottles. I think bottle glass would work fine, but 
>you would want them all to be the same kind of glass, and not a mix of 
>different types because you wouldn't be able to get them to blend evenly when 
>melted, which could cause warping from thermal expansion and contraction.
>
>Mike

Hi
 One interesting thing to consider. Part of what makes Pyrex
work so well is that it isn't too completely mixed. This
makes a small grain effect that gives it the ability to
resist crack spreading. This is also what gives it the
thermal properties as well. If it were completely uniform
and mixed, it wouldn't work as well as it does.
 The graininess is also why for mirrors, it doesn't give
as smooth a surface as plate does.
 Still, in general, you need to stir the mix well if you get
glasses from a lot of sourses. Also, you don't want to
crunch it up too small. You'll get a lot of air boubles.
It takes quite a while to get these to come to the top of
the mix so that you can scrape it off or have a laddle
with a scimmer on it.
 Richard Schwartz has experimented some with alternate glass
sources.
Dwight