Silica gel desiccants are materials that adsorb water, and are typically used when a business wants to prevent damage to goods through excess moisture. Silicon dioxide is the basic ingredient of silica gel desiccants, but some types may also contain chemicals that change color with moisture adsorption. Typically, the silica is contained in sachets, but for some purposes, it is used in a loose form.
After World War I, a scientist named Walter A. Patrick patented silica gel. He had used it inside gas masks to trap dangerous gases during the war. As well as this application, silicon dioxide, which is the technical name for silica gel, had a natural propensity to soak up moisture from the environment. This feature is useful for people who want to store goods in a dry atmosphere, and for people who want to dry other objects.
Despite the name, silica gel desiccants are typically in granulated or bead forms, and not gels. These granules can be loose or can be enclosed in packets of breathable material. Packet sizes vary depending on the volume of air the silica is supposed to keep dry, and often a manufacturer encloses the item in a plastic bag to keep out external moisture. The silica can adsorb more water, at about 40 percent of original dry weight of desiccant, than another form of desiccant that uses clay.
Some silica gel desiccants incorporate an indicator, which is a chemical that displays a color change when certain conditions are met. In the case of silica gel, the color of the product changes when the gel granules contain as much moisture as possible. Some indicator chemicals are orange when the silica is dry, and turn to green with moisture. Others turn from orange to white. A blue silica, that changes color to pink when fully saturated, is also available.
An advantage of silica gel desiccants is that in their loose form the granules may be reusable. Drying processes like oven heat or microwave energy can release the moisture from the silica. Once the silica is dry, the user may then be able to use it in the same manner as before. Desiccants with indicator will also change back to the original color.
Most silica gels are not harmful to humans if ingested. The blue variety, however, uses cobalt chloride, a toxic chemical, as an indicator. Any product that may become damaged with moisture, ranging from shoes to electrical goods, may contain silica gel in the packaging. Drying processes, such as seed preservation, may also involve silica gel desiccants.