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Olstad, T and Haugen, A and Nilsen, T (2001) Polychrome wooden ecclesiastical art - Climate and dimensional changes. NIKU Publikasjoner (110). NIKU. ISBN 1502-4903

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The project Measuring dimensional changes in painted wood was part of NIKU's strategic institute programme for the period 1996-2001.This article concerns dimensional changes in painted wood which result from changes in the relative humidity and temperature, and is confined to some of the problems of damage affecting painted wood. The article provides a background showing why it is important to keep painted wooden church art in a suitable climate, and provides a brief description of wood as a material and of the relationship between wood and humidity. The project is based on the preservation conditions for painted wooden works of art found in Norwegian churches. The project uses the results of earlier climate projects carried out by the Directorate for Cultural Heritage and NIKU, and other relevant research in this area is considered. The main question in this project is the size and rate of climatic changes which the painted objects can withstand before shrinking and swelling in the material lead to damage. We have chosen to use a definition of damage which is based on research carried out by The Smithsonian Center for Materials Research and Education (SCMRE), and which states that damage can occur when the structure is loaded beyond the yield point. The practical part of the project was carried out by the Norwegian Building Research Institute (NBI) in Trondheim, as part of a co-operative project between NIKU and NBI. The objective was "to develop a measurement method for registering surface movements and to use this to measure how large and how rapid the dimensional changes are in painted wood in the event of the rapid climatic changes”. According to NBI, this is a new optical measurement principle for surface movements in wood, which has been developed, tested and utilised in this project. Measurements of dimensional changes were carried out on a test piece (pine 1050 x 215 x 43 mm) which was exposed to different climatic stresses in a closed and controlled system (the climate room). The test piece was surface treated on the two largest surfaces in the same way as was standard for materials in the Middle Ages. All measurements were carried out on the same test piece and in the same climate chambers. The requirements for the measurement method were that it should be able to register surface movements over short distances (millimetres) and that it should affect the humidity and temperature conditions in the test piece as little as possible. A measurement scale in the form of pairs of holes was drilled into the layer of primer on the test piece. Each pair of holes defines a measurement line. The total measurement area of around 115 mm is called the macro area, while the distance between each pair of holes (measurement point), around 1 mm, is called the micro area. The measurements were carried out by scanning the measurement scale on the test piece at given intervals. While the test piece lay undisturbed in the climate room, the climate changes were carried out in pre-determined series as climate cycles. The climate cycles were based on the climatic changes which we know from experience, occur when churches are heated. The tests were carried out over three periods, with several climate cycles in each one. The results of the measurements are discussed and considered in the light of the materials research carried out at The Smithsonian. The project concludes that the size of the measurement area (micro-/macro level) is significant for registered dimensional changes and response times following changes in temperature or relative humidity. Climate changes appear to result in very large movements at micro level and movements which cause damage in painted wood occur at this level, long before they can be registered at macro level. The materials react very quickly and within just a few minutes of changes in temperature or relative humidity. The temperature, as well as the relative humidity, appears to be an important factor for dimensional changes in woodwork at micro level. The results of this project are somewhat unexpected. They are directly connected with the wood anatomy of the test piece and its surface treatment, and must be confirmed through further tests before they can be applied.

Item Type: Book
Uncontrolled Keywords: Church; Painted wood; Wooden objects; Climatic stress; Indoor climate
Subjects: English > Damage functions > Mechanical damage
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Depositing User: Anna Samuelsson
Date Deposited: 13 Mar 2008 07:43
Last Modified: 13 Mar 2017 16:34

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