Caring for Cultural Material 1

Caring for Cultural Material 2

Damage and Decay

Managing Collections

Managing People

Handling, Transportation, Storage and Display

Glossary

Index

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Damage and Decay
In this volume:
– Light and Ultraviolet Radiation
arrow Humidity and Temperature
– Biological Pests
– Dust and Pollutants
– Common Deterioration Processes
– Acknowledgments
Humidity and Temperature
In this chapter:
– Objectives
– Introduction
– What is relative humidity?
arrow How does relative humidity change?
– Why worry about relative humidity and temperature?
– Can the damage be prevented?
– Identifying the source of the problem
– What can be done to minimise damage?
– Modifying the conditions in buildings
– A word of caution about travelling exhibitions and loans
– In an ideal world …
– Australia’s climatic zones
  MORE ABOUT RELATIVE HUMIDITY AND TEMPERATURE
– Measuring relative humidity
– Dehumidifiers
– Humidifiers and steam generators
– The use of silica gel
– For further reading
– Self-evaluation quiz
– Answers to self-evaluation quiz

 

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Humidity and Temperature

How does relative humidity change?

Relative humidity is a measure of the amount of water-vapour contained in air at a particular temperature. The capacity of air to hold water-vapour varies according to the air temperature.

However, although raising the temperature increases the capacity of air to hold water, there is not always water available to move into the air to fill that increased capacity. So changes in temperature often lead to quite significant alterations to the relative humidity.

For example, in an empty, sealed box containing a fixed amount of water-vapour, raising the temperature will lower the relative humidity. This is because the capacity of the air to contain water has increased but the actual amount of water has remained the same. Using our previous example of air at 200C with 8.5g/m3 of water-vapour, if the temperature is raised to 300C and no additional water is available:

at 200C RH = 8.5/17 x 100% BUT at 300C RH = 8.5/30 x 100%
  = 50%   =28.3%

The reverse is also true. If the temperature in a sealed box continuing a fixed amount of water-vapour is lowered, the relative humidity will increase. The capacity of the air to hold water has decreased but the amount of water has remained the same.

This is an important concept, because display cases and sealed storage areas in some ways behave like sealed boxes; and the relative humidity can vary because the temperature varies.

This principle can also be applied to some extent to museums, galleries and libraries. But they are generally far more complex.

They have doors which admit moisture-containing air from the outside, where the temperature and relative humidity is usually different from the inside conditions.

People come into these areas. They raise the temperature, especially when they are in large groups; and they take in and give out moisture as they breathe.

Museums, galleries and libraries contain objects which take up water and give out water, according to the temperature of the surrounding air.

There may also be heating and cooling devices and/or air-conditioning in the building.

 

 

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