Desiccant

Desiccants are useful for keeping small closed spaces dry, such as tool storage containers. Desiccants absorb water vapour, and then need drying out to work again, usually with heat. Some desiccants change colour when saturated.

DIY Desiccants

Silica Gel

Normally contains a wet/dry indicator
- Old silica gel uses pink/blue indicator colours
- New silica gel uses brown/yellow-brown indicator colours
Non-indicator silica gel is occasionally seen.
Easy to dry for re-use

Lime chips

Hydrated lime (bagged lime) is safe and absorbent
Quicklime absorbs more water, but is much more reactive, and can be a fire risk if it gets wet
Reactivation not an option (requires over 1000C)
No indicator
No guarantee that bought product will absorb, though it normally does
Very cheap

Sodium chloride

Also known as table salt, this is sometimes used as a cheap and safe desiccant due to its hygroscopic properties. More effective desiccants are available, but few are safe for humans to ingest, making this a good choice where infant access is likely.

Plaster of Paris

Also known as Gypsum, Anhydrous Calcium Sulphate, & 'Drierite' (a US trade name)

Magnesium Perchlorate

Calcium chloride

Magnesium sulphate

Epsom salts

Bentonite

aka Montmorillonite clay

Molecular sieves

Caustic soda

Reactivating

Desiccants must be dried when saturated so they can absorb water again. There is considerable variation in what this entails.

Sillica Gel

Silica gel is dried in an oven at 120-150C. Non-indicating silica may be dried at higher temps, though there seems little reason to do so.

Sticking to 120C is wisest to minimise risk of package damage.

Lime

Lime requires over 1000C to dry it, so reactivation is not a realistic proposal.

More Desiccants

Sulphuric Acid
Activated alumina
Benzophenone
Calcium hydride
Copper(II) sulfate
Lithium hydride
Magnesium
NaK, a sodium-potassium alloy
Phosphorus pentoxide
Potassium
Potassium carbonate
Sodium
Sodium chlorate
Sodium hydroxide
Sodium sulfate
sodium-benzophenone