Environmental Engineering Reference
In-Depth Information
FIGURE 7.9
Effect of moisture content on the colour of artefacts.
7.3.5.4 Design Criteria
In an average library or museum, less stringent design criteria are usually
provided than for archives because the value of the topics and collections
does not justify the higher initial and operating costs. Low-efficiency air
filters are often provided. Relative humidity is held below 55%. Room tem-
peratures are held within the 20-21.5°C range. Archival libraries and muse-
ums should have 85% or better air filtration, a relative humidity of 35% for
topics and temperatures of 16°C in topic stacks and 20°C in reading rooms.
7.3.5.5 Building Contents
The reaction of museum contents and collections to room conditions should
be carefully considered and critically examined. For example, paper used
in topics and manuscripts prior to the 18th century is very stable and is not
significantly affected by the room environment. For archival preservation,
this paper should be stored at very low temperatures. It is estimated that for
each 5°C dry bulb the room temperature is lowered, the life of the paper will
double and that any humidity reduction will also lengthen the life of paper.
7.3.5.6 Effect of Ambient Atmosphere
The temperature and, particularly, the relative humidity of the air have a
marked influence on the appearance, behaviour and general quality of
hygroscopic materials such as paper, textiles, wood and leather because the
moisture content of these substances comes into equilibrium with the mois-
ture content of the surrounding air. The object humidity is usually defined
as the relative humidity of the thin film of air in close contact with the
surface of an object and at a temperature cooler or warmer than the ambient
dry bulb. If artefacts are permitted to cool overnight, the next day they will
be enveloped by layers of air having progressively higher relative humidity.
This may range from the ambient of 45% to 60% to 97% immediately next
