From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science,sci.space.tech
Subject: Re: LOX producing on Mars
Date: Sun, 30 Jun 1996 17:17:34 GMT

In article <4qsnvd$a8@lace.colorado.edu> fcrary@rintintin.Colorado.EDU (Frank Crary) writes:
>Glass you could make on the first mission, even if it were very
>small. But transparent glass is a different matter. You need
>nearly pure SiO_2...

Actually, even that isn't a full solution.  To get glass that is
transparent *and* can be worked easily and conveniently into desired
shapes, what you need is soda-lime glass:  SiO2 *plus* Na2O *plus* CaO. 
(Actually, you might be able to leave out the CaO -- it's necessary on
Earth because water attacks SiO2-Na2O glass, but on Mars that's less of an
issue.)  There are alternatives to Na2O, but you need *something* to make
the SiO2 more tractable, and as Frank points out, it has to be the right
something and has to avoid certain impurities.

Pure or near-pure SiO2 is extremely difficult to work with.  It softens,
grudgingly, at a very high temperature.  Even so it's sticky and viscous,
and if you try heating it further, it starts to evaporate instead of
softening more.  It keeps trying to crystallize instead of remaining a
glass.  And in general, it's not a material that you want to work with.
Glass did not become a useful material on Earth until the discovery of
soda-lime glass, which softens beautifully at much lower temperatures
and has no great tendency to crystallize.

There's a reason why fused-silica lab glassware, which is essentially pure
SiO2, is vastly more expensive than Pyrex (SiO2-B2O3) and is used only in
special applications.  (Soda-lime glass, although easier to work than
Pyrex, is too prone to breaking under thermal shock to be suitable for lab
use.)  Even in such applications, people will use 96%-silica glass -- made
by a complex and tricky process that involves working the glass in an impure
state and then leaching out the impurities -- rather than fused silica if 
they possibly can, because it's cheaper.
-- 
If we feared danger, mankind would never           |       Henry Spencer
go to space.                  --Ellison S. Onizuka |   henry@zoo.toronto.edu