Silica Gel is a desiccant, i.e. a drying agent that absorbs and holds water vapour. Silica, or silicon dioxide (SiO2), is the same material found in quartz and resembles coarse white sand. The standard version is known as non-indicating silica gel because it remains physically unchanged as it absorbs moisture. Sachets remain dry and free flowing even when saturated.

Silica gel was patented in 1919 for use in gas mask canisters during World War I. During World War II, it was commonly used as a dehydrating agent to protect military and pharmaceutical supplies.
  Adsorbs up to one third of its own weight in water vapour
  Silicagel is non-toxic, non-flammable, stable and harmless in ordinary use
  Dry Silica Gel has an almost indefinite shelf life in airtight conditions
  Sachets conform to BS 2540 and FDA CFR21 for healthcare and food use
  Ideal desiccants for virtually all static drying applications
  Standard sachet made from durable DuPont Tyvek material
  Suitable for long distance transportation by land, sea and air
  Recommended 5grams per cubic foot, 170grams per cubic metre
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