Desiccant is a product that removes moisture from the atmosphere. It adsorbs water particles, essentially binding them to itself. In some circumstances a desiccant may 'breathe' back moisture due to variations in humidity level. Because of this, it is best to use desiccant in a closed container, such as a gun safe or flexible packaging.
Clay, silica gel, and molecular sieve are 3 common kinds of desiccant.
Clay desiccant is commonly used in industrial applications and can absorb up to 28% of its weight in moisture.
Silica gel desiccant is a porous form of silica dioxide. Silica gel is the most common form of desiccant used in food storage or consumer applications. It can absorb up to 40% of its weight in water. It is often used due to its ability to be regenerated (via heat, usually several hours at 200-250 degrees) and reused. On occasion, silica gel may burst due to absorbed water or heat.
Molecular sieve is a more powerful form of desiccant, often used in industrial or scientific applications. They capture water in tiny pores which are measured in Angstroms or nano-meters.