1. adj. [Drilling Fluids]

Pertaining to a property of a substance that allows the substance to take up water from the surrounding atmosphere. Many materials used in drilling muds are hygroscopic, for example, high-purity grades of calcium chloride. Bentonite clay is also hygroscopic and absorbs water from the atmosphere. Care must be taken in packaging and handling such materials to avoid waste by premature hydration.

See: calcium chloride,  clay,  clay-water interaction,  humidity,  hydration,  prehydration