Mineral dehumidifiers can be tinged with color indicators to show when they are saturated.
Several dehydrated minerals can be used as dehumidifiers. Manufacturers provide chemical dehumidifiers for equipment, crawl spaces and basements. These minerals have a natural affinity for attracting water and will absorb a certain amount of water vapor. This reduces the likelihood of corrosion, mold and fungus.
Types
Mineral dehumidifiers or dessicants come in many forms. Simple table salt (sodium chloride) will absorb water and is used for preserving ("salting away") different types of foods. Another common mineral dessicant is silica gel. Calcium sulfate and calcium chloride are also extremely hygroscopic, meaning they will pull water out of the air. These minerals are commonly used in industry and trade as dehumidifiers.
Silica Gel
Silica gel is a form of sodium silicate that is made synthetically to have a highly porous structure. This gives silica gel a high surface area that has an affinity for water vapor molecules. Silica gel is used for innumerable purposes in which small contained areas are meant to have a low humidity. Silica gels are packaged in enclosures that allow moisture to pass into the silica gel. Silica gel often contains dye indicators that change color when they are saturated with water. Silica gels can be regenerated by placing them in a warm oven until their color changes.
Calcium Salts
Calcium sulfate, also known as anhydrite or gypsum, has a high affinity for water vapor and is used as a dessicant. Calcium chloride and calcium oxide also have high affinities for water. Powdered or granulated calcium salts have higher surface areas and are more effective as dessicants. Drierite is a commercial dessicant that is made from anhydrous (dehydrated) calcium sulfate. Cobalt chloride is added as a color indicator. When Drierite is saturated, the cobalt chloride changes color from blue to pink.
Considerations
The best mineral dessicants are non-toxic and do not have significant vapors or scents. While table salt can be used as a dessicant, once it becomes saturated, it liquefies and loses surface area. To recharge table salt, it has to be dried and then re-granulated by grinding or pulverizing. Calcium sulfate and silica gel retain their solid form because they are not soluble in water. They can be recharged by heating and do not have to be reprocessed to regain original surface area.
Other Minerals
Montmorillonite clay is used as a dehumidifying mineral. It contains magnesium aluminum silicate and is particularly suited for drying areas with low temperatures. Activated alumina has a high surface area because it is porous and is an efficient mineral dehumidifier.
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