Maintaining acceptable levels of temperature and relative humidity
in mueum display cases is very important. This will
help slow the rate of deterioration for the items on display just as it does for
those in storage. The guidelines provided in Basic Preservation Considerations
should be followed in determining acceptable levels.
What Are Macro- and
Micro-environments?
Acceptable levels of temperature and
relative humidity are controlled within a macro-environment, a
micro-environment, or a combination of both. The definitions of these terms seem
to vary within standard museum preservation practice. In general, the term
macro-environment refers to the conditions within a large space, such as the
entire display area, whereas micro-environment refers to the isolated conditions
within a smaller enclosed space, such as a display case.
What Are Active and Passive
Systems?
The conditions in the macro- or
micro-environment can be achieved by what preservation professionals refer to as
an active or a passive system. These definitions also vary in the preservation
field. As the terms are used here, active systems usually employ equipment such
as furnaces, boilers, air conditioners, dehumidifiers, and humidifiers. Passive
systems, on the other hand, usually rely on the natural buffering capacity of
materials such as paper, cloth, wood, and silica gel.
How Are Conditions
Maintained?
In practice, a combination of macro- and
micro-environments and active and passive systems is often utilized to maintain
acceptable conditions. The temperature and relative humidity of the entire
display area—the macro-
environment—are maintained by the
building-wide heating and air conditioning equipment—an active system. For
highly sensitive materials, such as metals that corrode in high humidity or wood
that shrinks in low humidity, a humidity- buffering material such as silica
gel—a passive system—is used in a display case—a micro-environment—to adjust
conditions and maintain them at the special levels needed by especially
sensitive materials.
If you expect to loan items to a
museum in another geographic area where the conditions are very different from
those to which your items are acclimated, you may need to use a microenvironment
with a passive system. The same is true when borrowing items that you need to
protect while they are in your care. Do not hesitate to consult a preservation
professional for guidance in this.
Monitoring
The temperature and relative humidity in the
display area should be monitored just as in the storage area. The instruments
described in Basic Preservation Considerations can be used. Some of these are
available in small sizes that work well in display cases.
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