Airtight
Sealed containers slow the loss of aroma and the creep of moisture into spices.
Also: sealed jar, oxygen exposure, tight lid, vacuum pack
Air carries away volatile oils and brings in oxygen, which oxidises oils and pigments, and moisture, which causes clumping and mould. An airtight jar, with a gasket or a well-fitting screw lid, keeps both out. Glass and steel do not absorb smells, whereas some plastics do and can hold scents after the spice is gone. Fill containers well, since half-empty jars hold more air. Resealable foil pouches and vacuum packs help for bulk spice. Hing in particular needs a tight tin to stop its odour spreading. A tight lid is the cheapest improvement most kitchens can make, and costs nothing.