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From: (John Bercovitz)
Newsgroups: rec.guns
Subject: Re: Silica gel
Keywords: storage
Date: 9 Oct 90 17:31:49 GMT

In article <6675@shamu.WV.TEK.COM> (Jim Purtilo) writes:
>Anyone out there have experience in protecting firearms from humidity
>using silica gel?   I specifically refer to storage in cabinet, drawer,
>gun safe, etc, where one routinely accesses the weapon (i.e. this note
>is not a followup to discussion about what to do about moisture inside
>the PVC tubes 'yall have stashed in your backyards).

I've been using Drierite from the W.A. Hammond Company, Xenia, Ohio
for the past fifteen years (it seems to last a long time).  It is
available at chemical supply houses.  It is cheap.  Drierite is calcium
sulfate (CaSO4).  I use indicating Drierite.  Indicating Drierite has
cobalt chloride in it.  When the Drierite starts to get loaded with
humidity, the cobalt chloride changes color - from blue to pink.
Since CaSO4 sounded suspicious to me, I put some on a bare, clean
piece of steel.  It did not cause rust.  But I wouldn't pack anything in
it anyhow - I suspect it could cause contact corrosion if things get
damp enough.

The mechanism is that CaSO4 likes to hydrate.  The hemi-hydrate
equilibrates with air at 6% RH; the di-hydrate at 20%.  After that,
the cobalt chloride turns pink.  Increasing temperature has weak
deleterious effects on capacity and RH.  When you compare them on
an equal basis, I like the CaSO4 better than silica gel because of the
increased capacity and lower RH.

To regenerate Drierite, spread it thin (monolayer) on some aluminum
foil in your oven; heat to 400-450 Fahrenheit for one hour.  It stinks
a little; you may wish to open your kitchen windows.  The reason for
the monolayer is to prevent loss of the cobalt chloride.  I don't know
why this is so, but I can tell you from experience that it is.  You
don't have to spread it thin if you put it in a tube (column) and
regenerate by blowing hot air through the column; the idea is to keep
the cobalt chloride exposed to air.

To prevent rust, you need to keep the relative humidity below 40%; I
prefer to keep it below 30% - a safer figure.  Our local humidity is
about 50%.  Local temperatures are mild and stable.

I use an open one pound jar in each of three cabinets which have
metal toys in them.  All cabinets have a volume of about 20 cubic
feet.  Two of the cabinets are leaky and one is sealed.  I have to
regenerate the Drierite in the leaky cabinets every few weeks; every
year does it in the sealed cabinet.  The stuff is very effective.  In
the sealed cabinet, the exposed lead of ammunition turns white in a
few months due to the lack of a protective layer of moisture.  I've
never had any rust, but then I'm also very careful (to the point of
paranoia?) about oiling and about removing fingerprints.  I store toys
muzzle down and tilted back to keep oil from going into wood.

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