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Some more info on desiccants

Silica Gel Silica gel is silicon dioxide (SiO2).

Silica Gel
Silica gel is silicon dioxide (SiO2). It is a naturally occurring mineral that is purified and processed into either granular or beaded form. As a desiccant, it has an average pore size of 24 angstroms and has a strong affinity for moisture molecules. The silica gel will pull in moisture at temperatures up to 220°F (105°C). As temperature goes above 100°F, the rate of moisture pickup will slow down but the silica gel will still work.
Silica gel performs best at room temperatures (70° to 90°F) and high humidity (60 to 90% RH) and will drop the relative humidity in a container down to around 40% RH. In the United States, silica gel is commonly used in food and pharmaceutical applications as only silica gel has been approved by the FDA for direct contact with these items.
As with clay, silica gel, with its wide range of pore sizes, has the capability of adsorbing compounds other than water. The relative order of adsorbability is: water, ammonia, alcohols, aromatics, diolefins, olefins and paraffins. When the potential for multicomponent adsorption is present, expect the more strongly adsorbed compounds, such as water, to displace the more weakly held ones.

Indicating Silica Gel
Indicating silica gel is a silica gel bead or granule that has been washed with a concentration of cobalt chloride ( a heavy metal salt). The cobalt chloride is a deep blue color when it is dry and turns from blue to purple to pink as it becomes saturated with moisture. Typically, the color changes as the desiccant goes past 8% moisture levels (by weight) and indicates it is time to replace the desiccant.
The most typical use for an indicating silica gel is for a moisture sensitive product that will be inspected regularly as it gives a quick visual indication of how well it is doing. Because of the addition of cobalt chloride, indicating silica gel should not be used in contact with products for consumption such as food or pharmaceuticals.
The Dri-Box canister is packed with an indicating silica gel that is regenerable. Instead of replacing the desiccant inside, heating the canister will reverse the adsorbing action and allow the material to be reused.

Calcium Sulfate
Calcium sulfate (better known commercially as Drierite™) is created by the controlled dehydration of gypsum. It is a general purpose desiccant geared mainly toward laboratory use. It is chemically stable, non-disintegrating, nontoxic, non-corrosive and does not release its adsorbed water easily when exposed to higher ambient temperatures.
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. Calcium sulfate also has regeneration characteristics that tend to limit its useful life. Although available, it is not normally sold in package form.

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