Re: [ATM] Does exposed Pyrex lose polish over time?

[mdholm@telerama.com]

>Theory says yes but reality is no for glasses like pyrex.

Well, actually theory says no to most possible causes of glass surface
degradation.  First, it won't oxidize.  Most ordinary glasses, including Pyrex,
are about as oxidized as they are ever going to get.  Second, glass is pretty
unreactive to a whole lot of ordinary chemicals.

The most likely thing to get after glass surfaces is a caustic.  I mean caustic
in the chemist's sense of the word: alkaline, high pH.  Some of the windows on
my house are etched where rain water running off the brick walls falls onto the
glass.  It isn't the brick that causes trouble, but the mortar.  Mortar is made
with Portland cement and that is darned caustic stuff.  More so before it is
cured, but still fairly potent afterwards.

Some parts of the world have rather alkaline soil.  The deserts of the U.S.
have
alkaline soil in many places.  In those areas, dust in the air, combined with a
little moisture could go after glass fairly agressively.  (It will eat aluminum
even quicker.)

The most common alkaline houshold chemicals are oven cleaners, drain
uncloggers,
dishwasher detergents, laundry detergents, washing soda (sodium carbonate) and
borax.  By dishwasher detergents, I mean the kind used in mechanical
dishwashing
machines, not the kind you use in a sink full of hot water.  By comparison,
sodium bicarbonate (baking soda) though alkaline, is fairly weak.

Glass is reactive to water, but without the accelerating effect of alkali or
other agents (polishing compounds), the reaction rate is pretty darned slow and
Pyrex is slower than most.  There are specialty glasses for which this is not
true.  Some optical glasses are notoriously subject to staining and other
damage
by water.

Glass is fairly unreactive to most acids, even some rather strong ones.  Pyrex
is less reactive than most.  The one big exception is hydroflouric acid which
will eat right into glass, and close to anything else.  It is also horribly
toxic.  If you have significant HF exposure, you are dead already anyhow, so
the
condition of your mirror won't really matter to you.

Mark Holm
mdholm@telerama.com



Mark Holm
mdholm@telerama.com
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