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Cassar, M and Hutchings, J (2000) Relative Humidity and Temperature Pattern Book - A guide to understanding and using data on the museum environment. Museums & Galleries Commission. ISBN 0 948630 88 4

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In order to manage the environment, we should begin by trying to understand how objects respond to ambient relative humidity (RH) and temperature changes. Collections respond in various ways and at different speeds to the amount of moisture and heat in the air. Very damp conditions encourage mould and fungal growth. Absorption of moisture by objects made of plant or animal material (organic) makes them swell. As conditions dry out, this has the opposite effect, causing shrinkage. Organic objects may be made up of a combination of different materials, each of which will respond to moisture in a different way. The dimensional changes in different materials cause internal stresses, leading to cracking and failure of joints. Fragile objects with a layered structure, such as ivory or veneered furniture, behave in two ways: they can expand and contract at different rates lengthwise and widthwise. This has the same effect as if the object was made of different materials. Objects that are metallic or mineral in nature react in different ways to changes in RH and temperature. Excessive RH causes corrosion such as rust in iron, pyrite disease in fossils, or leaching in glass, which causes droplets to form on the surface. Museums must record changes in the environment within their buildings in order to gauge the level of environmental stress to which objects are subjected. This requires monitoring equipment capable of recording relative humidity and temperature over time. RH and temperature monitoring equipment is designed to respond to environmental changes in a similar way to the materials from which objects are made. The fact that a recording thermohygrograph registers RH changes through expansion and contraction of human hair, and temperature changes through the response of a bimetallic strip, is no accident. The result is that we are not only interested in the absolute values of data that are recorded. We are also interested in the rate or speed of change that occurs, and because different materials, whether objects or monitoring devices, respond to changes over time, we sometimes detect a slight hysteresis, or time delay between cause and effect - between a change in conditions and the response of the material to the change. The aim of these Guidelines is to help museums make the most of the environmental data they collect. Without understanding the information contained in the data, monitoring becomes a pointless task that does not benefit the collection. If fresh data is not analysed, its usefulness – like fresh food which becomes stale - decreases with time.

Item Type: Book
Uncontrolled Keywords: Museum; Guidelines; Environmental data interpretation
Subjects: English > Management and Case Studies
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Depositing User: Anna Samuelsson
Date Deposited: 23 Apr 2008 06:56
Last Modified: 14 Mar 2017 10:32

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