So often when the word “desiccant” is used, people automatically think of the three main types of desiccants: Indicating Silica Gel, Molecular Sieve, and Clay desiccant. Those who have heard of other desiccants, like Calcium Oxide, Montmorillonite Clay and Calcium Sulfide, may or may not have a clear understanding of those desiccant types. Realizing how important proper discernment is where the use of desiccants is concerned

Montmorillonite Clay

Montmorillonite clay is a naturally occurring adsorbent created by the controlled drying of magnesium aluminum silicate of the sub-bentonite type. This clay will successfully regenerate for repeated use at very low temperatures without substantial deterioration or swelling. However, this property causes clay to give up moisture readily back into the container as temperatures rise.
Clay is a good basic desiccant that works satisfactorily below 120°F (approximately 50°C). Above 120° F, there is a possibility that the clay will give up moisture rather than pulling it in, so anticipated storage and transportation conditions should be considered. The upside to clay is that it is normally the least expensive desiccant per pound.
Clay is highly effective within normal temperature and relative humidity ranges. Its appearance is that of small gray pellets. Care should be taken to be sure that any low level impurities in the clay are not incompatible with the packaged product.

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.