Drierite is the brand name and registered trademark of a variety of desiccant

Desiccant

A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....

s made from anhydrous

Anhydrous

As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...

 calcium sulfate

Calcium sulfate

Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

 (gypsum). Some varieties contain 3% cobalt(II) chloride

Cobalt(II) chloride

Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...

, a moisture-sensitive color indicator that shows when that material's activity has been depleted. Drierite is manufactured in the USA by the W.A. Hammond Drierite Co., Ltd., founded by William Archie Hammond and headquartered in Xenia, Ohio

Xenia, Ohio

Xenia is a city in and the county seat of Greene County, Ohio, United States. The municipality is located in southwestern Ohio 21 miles from Dayton and is part of the Dayton Metropolitan Statistical Area...

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After drierite has been exposed to moisture, it can be regenerated by baking at 210°C for an hour.