Silica gel is silicon dioxide (SiO2), an amorphous form of silica which is manufactured from sodium silicate & sulfuric acid. It is a naturally occurring mineral that is purified & processed into beaded or granular form. Like clay desiccant, silica gel is also non-corrosive & chemically inert.
Much like a sponge, silica gel's interconnected pores form a vast surface area that will attract & hold water by adsorption and capillary condensation, allowing silica gel to adsorb about 40% of its weight in water vapor at 100% humidity.
As a desiccant, silica gel has an average pore size of 24 angstroms (24Å). It will pull in moisture at temperatures up to 220°F (104°C). As temperatures increase above 100°F (38°C), the rate of moisture pick-up will slow down, but the silica gel will still work. Silica gel performs best at room temperatures between 70°F to 90°F (21°C to 32°C) & at high humidity between 60% to 90% relative humidity.
Indicating silica gel is a 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 dry & turns from blue to purple to pink as it becomes saturated with moisture.