What are the recommended standards of temperature and relative humidity for collection preservation?

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Category: Environments
Posted by Marcelle Scott(Questions:33:Answers28)
Answered On 18 January, 2014 7:29 am

A lot of the advice regarding caring for collections suggests that heritage items require certain conditions of temperature and relative humidity for their preservation. As we mentioned above it is true that extremes and regular fluctuations in relative humidity can lead to damage. Once it became possible to control the environment inside buildings by air-conditioning, it became necessary to determine a set of standard temperature and relative humidity parameters. Conservation scientists started working with mechanical engineers to develop guidelines for air-conditioned spaces in museums. The results were published as recommended environmental standards for museums, and soon became widely accepted. So much so that for the last twenty years or more, it has been generally recommended that museums maintain standard environmental conditions of:

20 °C +/-2 °C (that is, 18 – 22 °C) for temperature, and
50% +/-3% (that is, 47 – 53%) for relative humidity

These recommendations were taken up pretty quickly, and soon became a mantra for museum workers, appearing in texts and articles, and as conditions in object loan and travelling exhibition agreements.

Rarely mentioned, however, is that:

the standards were developed in the northern hemisphere and are more relevant to climates in that part of the world
the figures are based as much on the capacity of air-conditioning systems, as on what museum collections really need
the standards assume an air-conditioned building

More recent research into optimum conditions for collections has shown that even a 10% variation from the previously recommended 50% relative humidity (that is, 40 – 60%) does not necessarily place objects under stress. This and other research has shown that conditions similar to the climate objects have become accustomed to are just as favourable for preservation (and a lot more energy efficient) than trying to artificially control an environment within strict temperature and relative humidity parameters.

Australian researchers have for some time acknowledged that the above standard recommended conditions do not take into account the vast climatic variations within our continent and the greater Asia – Pacific region. This, along with the ever growing trend towards reducing energy consumption (for environmental and cost reasons), has led a push towards developing standards more relevant to our situation and requirements.

The Heritage Collections Council (HCC) commissioned research to develop environmental guidelines relevant to our region. While the results of the research have not strayed far from the above mentioned + or – 10% variation, they have taken into account climatic variations. The full report Guidelines for Environmental Control in Cultural Collections produced for the HCC by the Consortium for Heritage Collections and their Environment can be found on the AMOL website at http://sector.amol.org.au/collections/conservation/environmental_control/

The report recommends the following conditions:

22 – 28 °C and 55 – 70 % relative humidity, in hot humid climates
22 – 28 °C and 40 – 60 % relative humidity, in hot dry climates
18 – 24 °C and 45 – 70 % relative humidity, in temperate climates

Recommended standards are of course a useful tool in assisting us to manage collections, and to brief engineers of the requirements of an air-conditioning system. Many of us care for collections, however, housed in buildings that are not air-conditioned. Many of the buildings we occupy were not designed to be air-conditioned. In addition, as practical experience has shown us, many collections remain in excellent condition even in extreme storage and display environments. Unfortunately, we have also seen examples of aggressive deterioration of collections housed in poorly designed air-conditioned spaces.

By including the information about internationally agreed standards, we aim only to explain the background and purpose of these standards. This section is not meant to suggest that you must comply with these standards, or that they will be relevant to your collection. Rather, it aims to help you understand why temperature and relative humidity conditions are such a focus of much of the preservation literature, and in what ways they can affect your collection. In summarising, we hope you will primarily remember the following.

In general, relative humidity extremes are of more concern than temperature extremes.
Above 65% relative humidity means there is a risk of mould growth and metals are at risk of corrosion.
Below 35% relative humidity means there is a risk that materials will weaken and become brittle.
The recommended standards are a compromise, and need only be understood in terms of generalities.
A general understanding of the conditions inside your display and storage areas throughout the seasons will give you the information you need to make sensible decisions about managing your collection.

The following sections discuss what all this means to you and your collections.


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