Incorrect relative humidity
Relative humidity (RH) is the amount of water in the air compared to how much that same amount
of air could possibly hold. It is represented as a percentage. So, if you know that a certain amount of air could
hold x amount of water, but is only holding half of that, you know that relative humidity is 50%. RH depends on
the temperature. At higher temperatures air can hold more water, at lower temperatures it can hold less.
In order to be kept in a stable condition, different materials need different amounts of water in the
air around them. If these levels are too high or low then the material will be affected, causing shrinking, swelling,
buckling, and collapsing in organic materials. A relative humidity of over 75% is said to be damp and causes mould
to grow on organic materials and metals to corrode.
These bicycles have rusted in damp storage
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Tins of cod taken from a coastal location which have been corroded due to high relative humidity (and possibly salt)
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Incorrect RH can be dealt with. If it is too low then the heat can be reduced to increase the RH (remember that RH depends on
the temperature). If it is too high then increasing the temperature will reduce RH. However, objects can also be protected
from incorrect RH in one of the following ways:
- keep objects in several layers of protection, eg put an object in a box, keep the box in a drawer, use a
drawer in a cabinet, keep the cabinet in a sealed room - this limits the rate of air exchange and protects from most agents
of deterioration
- avoid storing collections in/near places like basements, attics, windows or exterior walls where temperatures
and humidity tend to be extreme and can fluctuate
- keep objects away from sources of heat, like radiators
- improve insulation by repairing leaky roofs, draughty windows and doors - helps keep the temperature and moisture levels stable
- use buffers such as silica gel, wood, cotton, and paper
- install and maintain an effective air-conditioning system
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