Relative Humidity (RH) is the amount of water vapour held in the air relative to its temperature.
Organic materials contain and hold moisture in their cells. If the RH fluctuates constantly, the cells absorb and release water causing a constant expansion and contraction of their cellular structure. This leads to weakening and break-up. When conditions are constantly damp (over 60%) mould growth can become a serious problem for organic materials. If conditions are too dry (< 40%) some objects become brittle and can crack or crumble.
In inorganic materials, the presence of salts lead to damage. If a porous or soft ceramic contains soluble salts these will solidify and expand when they dry out, then shrink and dissolve again when wet. If the cycle repeats many times the expanding salts can force the pot to flake and split apart. For metals, the combination of oxygen and water with the salts causes corrosion via an electrolytic cell process. This reaction can be slowed down or stopped altogether by removing the moisture.
The aim throughout the museum is to provide as stable an RH as possible. Since the rebuild in 2009 the museum consists of two different environment areas. The historic part of the building, which contains most of the Western Art Galleries, has underfloor heating and plumbed-in radiators. In these areas the conditions are modified by humidifiers controlled by humidistats. In the new part of the building, created in 2009, gallery and store conditions are controlled by a heating, ventilation and air conditioning (HVAC) system providing air pre-conditioned to acceptable levels of moisture and temperature. Some objects need to be kept in very dry conditions whilst others require damper surroundings. Within individual showcases, specific conditions (drier for metals and moister for books and paper) are achieved by the addition of a passive conditioning agent which is itself pre-conditioned to the appropriate level.