Boveda's 2-way humidity control technology automatically protects your instrument against warping, cracks and other damage caused by lack of humidification or too much humidification. Boveda is the only true 2-way humidity control that will add or remove moisture as needed to maintain a constant 45%-55% relative humidity level - the ideal climate for wooden instruments.
Each Boveda consists of a specially prepared saturated solution of pure water and natural salts. This saturated solution is contained within a water-vapor permeable "reverse osmosis" membrane. Boveda works on the scientific principle that certain salts mixed with water will naturally regulate humidity. Boveda's patents are on the methods invented to make this scientific principle portable and practical for the rigors of packaging applications. Boveda absorbs and emits only purified water vapor through its semipermeable membrane and does not damage material through direct contact.
More info here: https://bovedainc.com/wood-instruments-music/