From: email@example.com (John Bercovitz)
Subject: Re: Silica gel
Date: 9 Oct 90 17:31:49 GMT
In article <6675@shamu.WV.TEK.COM> firstname.lastname@example.org (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
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.