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Storage Tips:
Desiccant and the Storage of Chemicals
by Fire Erowid
May 2003
Citation:  Erowid, Fire. "Storage Tips: Desiccant and the Storage of Chemicals". Erowid Extracts. May 2003; 4:10.
Most chemicals that are in powder, crystal, or tablet form are best stored as dry as possible, whether for short or long-term storage. Unfortunately, many factors can make dry storage difficult. Dark and cool locations that are otherwise ideal for chemical storage tend to be damp. As containers are opened and closed or as seals age and leak, humidity can creep into a container, increasing the speed of degradation of materials stored inside. Many plastic containers are made from material that is permeable to water vapor over time. As temperatures go down, humidity trapped in a container may reach its dew point and precipitate out, potentially ruining the material.

One solution to this storage problem is the use of desiccant. A desiccant can be any material that is hygroscopic--meaning it will absorb water from the air. Desiccants can be purchased as loose powder, pellets or small sealed pouches of various sizes. These pouches--often made of uncoated Tyvek--contain the powder or pellets while still allowing air flow.

There are a number of desiccant products designed for different purposes. Some people use desiccant to keep delicate electronics dry. Gun collectors use it to keep their weapons from rusting. Pharmaceutical companies include desiccant packs inside medicine bottles, and shoe companies use it to keep leather dry. Home supply stores or craft stores often sell it for drying flowers.

Certain products are marketed specifi cally for the purpose of drying foods for long-term storage. These have the advantage of being approved for use with items that will be ingested. The most common commercially available desiccants are silica gel and clay.

silica gel
montmorillonite clay
calcium oxide (quicklime)
calcium sulfate (gypsum)
molecular sieve
activated carbon
dry sawdust or dry wood
Silica gel can absorb water from below freezing to past the boiling point, but functions best at room temperature. There are some varieties of silica gel that are approved by the FDA for use with food.

Indicating silica gel, which is normal silica gel with some of its granules coated with cobalt chloride, changes color as it becomes saturated with water. Cobalt chloride is a heavy metal that is NOT food-safe and should not come into contact with anything that will be ingested. Indicator silica gel is more expensive, but is very useful for determining when the desiccant has reached its saturation point and lost its effectiveness. It is often available at chemical supply stores.

Once they have absorbed humidity, both types of silica gel can be re-activated (re-dried for future use) by spreading them in the bottom of a baking dish and drying them in an oven at 220-250 F for about three hours. Lower temperatures will not dry the gel and higher temperatures can damage it. Silica gel packaged in Tyvek packets can be dried the same way.

Clay desiccants are less common than silica gel, but can also be less expensive. They work well at lower temperatures, but begin to release water at 120 F. This can be a problem for items stored in hot areas, but also makes it easier to re-activate them in the oven.

Calcium oxide is a caustic material, and inhalation or exposure to the eyes or skin should be avoided. Unlike silica gel or clay desiccant, calcium oxide expands as it absorbs water. It is slower than other desiccants, but can achieve lower humidity. When exposed to water or high humidity, it can release quite a bit of heat.

Calcium sulfate, also known as gypsum, is sold as "Drierite" commercially. It is relatively common and is available in an indicator variety, but is less efficient than most other commercial desiccants. Calcium sulfate can be re-activated by heating in an oven for one hour at 210-425 F.

There are a number of ways that desiccants can be used to help keep stored materials dry. The simplest is to put food-safe desiccant packs alongside the stored material inside a sealed container.

A second method is to put a layer of desiccant or desiccant packs in the bottom of a sealable container such as a glass canning jar or plastic box. The material to be stored is set into a separate and smaller sealed container. This smaller container is set on top of the desiccant in the fi rst container, which is then sealed.

A third method is to make a do-it-yourself desiccant pack. A small glass jar can be half fi lled with desiccant pellets and then the top of the jar "sealed" with a piece of airpermeable fabric or two layers of coffee filter secured with string or a rubber band. This jar containing desiccant is then placed into a larger sealable container with the material or chemical being stored.

Other dry materials such as wood can also be used as a makeshift desiccant. A small piece of wood, dried in the oven until it becomes bone dry, can be placed in a container where it will suck moisture out of the air. While less effective than commercial desiccants, this method is also less expensive and can be done from materials found at home.

Most desiccant is fairly inexpensive, can be reused for years, and helps maintain the quality of materials stored over long periods of time. The key is to keep any non-foodsafe desiccant separated from materials that might be ingested, while still allowing the desiccant access to the air that surrounds the stored materials.