Homemade Desiccant Packets
If you have purchased, salvaged or made your own bulk desiccant you will need to make your own packets with measured amounts of desiccant. How much you need depends on the size of the container and the type of desiccant used.
Filter paper, cheese cloth, thin loosely woven cotton sheet, nylon pantyhose material, and fine fabric screen are all permeable to moisture but it is tight enough not to let the desiccant grains out. Edges can be glued and or sown together.
Using Desiccants
Use the charts below to determined how much of the particular desiccant you will need for the size of the storage containers you wish to keep dry. Desiccants begin to adsorb their moisture as soon as they are exposed so you want to keep out in the open air for any longer than fifteen minutes or so.
Types Of Desiccants
Silica Gel
The most commonly used desiccant is silica gel, 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.
Indicating Silica Gel
This is ordinary silica gel with the colored specks being coated with 0.3 – 0.4 % of cobalt chloride. 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. The indicating silica gel will still adsorb up to 40% of its weight in water vapor 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. When saturated, both varieties of silica gel can be dried out and used again by heating the crystals in an oven at a temperature of no more than 300° F for approximately three hours or until the crystals turn blue. Spreading the desiccant in a broad pan in a shallow layer will speed the process. Heating to 325° F can damage the gel and render it unable to absorb moisture. Spread the packets so they are not touching and keep them at least 16 inches from any heating elements or flames.
Clay Desiccant
The material is Montmorillonite clay, composed primarily of magnesium aluminum silicate, a naturally occurring mineral. 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 areas. 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. When temperatures rise above 120° F, Bentonite Clay releases the moisture it has absorbed back into the air. As a result, this desiccant can be reactivated to be reused many times.
Calcium Oxide
Quicklime is a slow, but strong adsorbent, 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 humilities. Calcium oxide can be recharged by roasting at fire temperatures.
Calcium Sulfate
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. 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 but the low cost of calcium sulfate must be weighed against its equally low adsorptive capacity; it adsorbs only up to 10% of its weight in water vapor but in an emergency it can get the job done. Gypsum or Drierite regeneration: spread about 1 inch deep in shallow pans and heated in a oven at 450° F for 2 hours to achieve complete dehydration The temperature at which Drierite desiccants are regenerated is crucial in restoring it to its original condition. Absorbed moisture is water of hydration and is chemically bound to the calcium sulfate and temperatures in the range of 400° - 450° F are required to break these bonds and release absorbed moisture. Lower temperatures, regardless of heating time, will not regenerate. Do not to overheat Drierite Desiccants as high temperatures can alter the crystal structure and render the desiccants permanently inactive.
Dry Wood
Dry a piece of wood in the oven until it is bone dry then put it in your container and seal it. The wood will suck up available moisture, up to 14% of its weight in moisture, depending on species. Low density, non-resinous woods with coarse, open grains work the best. I could not find temperature it will begin to "desorb" but it might be fairly low. Some empirical experimentation would be in order before relying heavily on it.
Desiccant Needed to Adsorb 6 Grams of Water Vapor at 40% relative humidity at 77º F.
Silica Gel 15 grams
Indicating Silica Gel 15 grams
Montmorillonite Clay 24 grams
Calcium Oxide (quicklime) 21.5 grams
Calcium Sulfate (gypsum, Drierite) 60 grams
Wood 43 grams
To maximize surface area for optimal adsorption, desiccants are granular or powder forms.
Requirement Chart
Container Size
Gallons Cubic Ft. Cubic In. Grams of Silica Gel Desiccant
1.1 or 0.14 or 237 Grams needed = 5 *
2.1 or 0.28 or 476 Grams needed = 10
3.2 or 0.42 or 714 Grams needed = 15
6.2 or 0.83 or 1,428 Grams needed = 30
12.5 or 1.67 or 2,856 Grams needed = 45
18.7 or 2.5 or 4,284 Grams needed = 60
25 or 3.33 or 5,712 Grams needed = 90
37.4 or 5 or 8,568 Grams needed = 135
50 or 6.66 or 11,424 Grams needed = 180
*Silica Gel 15 gr = Montmorillonite Clay 8 gr = Calcium Oxide 7.2 gr = Calcium Sulfate 20 gr = Wood 14.3 gr
Humidity Indicator Card.
These cards are made to show various humidity ranges and can be had from many desiccant and packaging suppliers. They will help you find the actual humidity in your storage.
Where Do I Find Desiccants?
Indicating silica gel: is sold in one and five pound cans for flower drying.
Silica gel: A number of companies in the US market use 100% silica gel crystals in their cat litter; include Clorox (Fresh Steps Crystals), Nestle-Purina (Tidy Cats Crystals)and Amazing Cat Litter are all 100% silica gel crystals granules
Montmorillonite clay: in shipped containers coming in from overseas, bags as large as a pound.
Gypsum: from drywall and Plaster of Paris, reprocessed to make your own desiccants.
Calcium oxide (quicklime): produced from limestone (calcium carbonate) or slaked or pickling lime (calcium hydroxide) by roasting to drive off the adsorbed water and or carbon dioxide. Note: calcium oxide is caustic and hazardous, handle with care.
The most applicable standard for home food storage defines a unit of desiccant as the amount of desiccant that will adsorb at least 6 grams of water vapor at 40% relative humidity at 77F (25C). thicknesses are best kept to 1.5 inches or less
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