Silica gel was Re: Excercising vintage items

[Pete Turnbull]

On Oct 26 2004, 12:55, John Foust wrote:
> At 12:12 PM 10/26/2004, Paul Koning wrote:
> >If it's calcium chloride you want, try snow melting "salt" -- that's
> >often CaCl.
> >Right.  Silicagel is one of those.  I don't think calcium chloride
is,
> >though -- but I'm not positive.
>
> Yes, but a mixture of NaCl and CaCl will happily suck up water
> from the air until turns into a rich, gooey, metal-eating mess.

That's the CaCl in it, and there's usually not much.  Calcium chloride
is not merely hygroscopic but deliquescent.  It's "use once" -- you
can't really dry it off again, effectively.

If you don't mind a liquid, concentrated sulphuric acid is a more
active dessicant than either silca gel or calcium chloride ;-)
 Phosphorus pentoxide is good too, and doesn't get wet and sticky.
 Pricey, though!

> A silicate solution would be less reactive, I think, and if
> it dried, it's almost a protectant.  "Water glass" was once a
> common way to extend the shelf life of eggs, as it prevented
> evaporation through the shell.

For a different reason, though.  There's a slow double-decomposition
reaction between the calcium carbonate in the shell and the sodium
silicate (water glass) which leaves a layer of relatively impermeable
calcium silicate on the eggshell.  Sodium silicate isn't a drying
agent.

The drying action of silica gel (silicon dioxide) is due to adsorbtion,
a purely physical (and easily reversible) process.  Water molecules
stick to the surface of the silica.  The granules are extremely porous
on a microscopic scale so they can have quite a lot of water adhering
to them.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York