The most commonly known and used desiccant is silica gel which
is a form of silica dioxide (SiO2), a naturally occurring mineral.
It will work from below freezing to past the boiling point of water,
but performs best at room temperatures (70-90º F) and high
humidity (60-90%). Its performance begins to drop off over 100º
F, but will continue to work until approximately 220º F. It
will lower the relative humidity in a container to around 40% at
any temperature in its range until it is saturated. Silica gel will
absorb up to 40% of its weight in moisture. Some forms are approved
by the FDA for direct food use (check with your supplier to be sure).
It recharges easily (see below in the indicating silica gel text)
and does not swell in size as it adsorbs moisture.
INDICATING SILICA GEL
In the retail trade, the most commonly found form of silica gel
is indicating silica gel which is small white crystals looking much
like granulated sugar with small pink or blue colored crystals scattered
throughout. This is ordinary silica gel with the colored specks
being coated with cobalt chloride, a heavy metal salt. When the
gel has absorbed approximately eight percent of its weight in water
the colored crystals will turn from blue to pink making an easy
visual indicator of whether the gel has become saturated with moisture.
Because cobalt is a heavy metal, indicating silica gel is not food
safe and should be kept from spilling into anything edible.
The indicating silica gel will still adsorb up to 40% of its weight
in water vapor just like the non-indicating type will but once it
has gone past the 8% level and the crystals have turned pink there
is no way to tell how close it is to saturation. This isn't necessarily
a problem, you'll just have to treat like the other non-indicating
desiccants and either weigh it to determine adsorption or use a
humidity indicator card. These cards are made to show various humidity
ranges and can be had from many desiccant and packaging suppliers.
When saturated, both varieties of silica gel can be dried out and
used again. This is done by heating the crystals in an oven at a
temperature of no more than 300° F (149° C) for approximately
three hours or until the crystals turn blue. Dehydrating the desiccant
may also be accomplished by heating in a microwave oven. Using a
900 watt oven heat the crystals for three minute intervals until
the color change occurs. The exact amount of time necessary will
depend upon the oven wattage. Spreading the desiccant in a broad
pan in a shallow layer will speed the process. Heating to 325Â°
F (149Â° C) or more, or using a microwave oven over 900
watts can damage the gel and render it unable to adsorb moisture.
If your desiccant is packaged, particularly if packaged in Tyvek,
do not heat it above 250° F (121° C) or you could damage
the material. This leaves a fairly narrow temperature window since
silica gel will not begin to desorb moisture below 220° F (104°
C). It's a good idea to use a reliable oven thermometer to check
your oven temperature as the thermostats in home ovens are often
off by more than twenty five degrees. Start with the packets in
a cold oven and raise the temperature to 245° F (118° C),
keeping it there for twenty four hours. Spread the packets so they
are not touching and keep them at least 16 inches from any heating
elements or flames so that radiant heat does not damage the packaging.
Tyvek should not be microwaved.
Although not typically found for sale on the retail market, clay
desiccant is fairly common in commercial and industrial use. The
primary reason for this seems to be that it is inexpensive compared
to any other form of desiccant. Some mail order suppliers offer
it for retail sale.
The material is Montmorillonite clay, composed primarily of magnesium
aluminum silicate, a naturally occurring mineral. After mining it
is purified, reduced to granules and subjected to a controlled dehydration
process to increase its sorbent porosity. It recharges easily and
does not swell as it adsorbs water vapor. It works well at low and
room temperatures, but has a rather low ceiling temperature. At
120º F it will begin to desorb or shed the moisture it has
adsorbed. This is an important consideration for storage in hot
Subject to a degree of variability for being a natural material,
clay desiccant will adsorb approximately 25% of its weight in water
vapor at 77º F and 40% relative humidity.
Also known as "quicklime" or "unslaked lime", calcium oxide is
a slow, but strong adsorbent. It is efficient at low humidities
and can drop moisture vapor to below 10% relative humidity. Qucklime
is caustic and must be carefully handled, particularly with regards
to dust inhalation and exposure to skin and eyes. It expands as
it soaks up water vapor and this must be taken into account when
packaging it. It will adsorb up to about 28% of its weight in moisture,
but does it slowly over a period of several days rather than a matter
of hours like other desiccants. It is most effective when used in
high humidity environment where a very low humidity level is desired.
It will release a fair amount of heat if exposed to direct (liquid)
moisture or extreme humidities.
Calcium oxide can be recharged, but I do not have any details on
how to go about this other than roasting at fire temperatures.
For expedient use, quicklime can be manufactured from clean, pure
lime stone (calcium carbonate) or pickling lime (calcium hydroxide)
available in the canning sections of many grocery and hardware stores.
Also known as the mineral gypsum and commercially as Drierite,
calcium sulfate is another naturally occurring mineral. It is produced
by the controlled dehydration of gypsum (CaSO4). It is chemically
stable and does not readily release its adsorbed moisture. It has
a low adsorbency capacity, only approximately 10% of it weight.
It can be regenerated, but apparently not easily so.
For expedient use, gypsum is commonly used in household drywall
and Kearny mentions using this source in his Nuclear War Survival
Skills. This makes only a so-so desiccant and you'd be much better
off to use a more suitable choice but in an emergency it can get
the job done.