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Influence of moisture content, temperature and air humidity during kiln drying on the lightness of Silver birch (Betula pendula Roth.)
Linnaeus University, Faculty of Technology, Department of Forestry and Wood Technology.
2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Discoloration of birch wood during kiln drying is a considerable problem for theindustry, since any deviation from the preferred bright whitish colour reduces theusability and value of the sawn timber. This study was conducted in an effort toincrease the understanding of how MC, temperature and air humidity influence silverbirch lightness. The air humidity was represented by the equilibrium moisturecontent (EMC). Within the investigated intervals of 20-70°C and 9-20% EMC, theCIELAB colorimetric responses, were measured at the surface and at depths of 1 and5 mm below the surface. The results showed that lightness reduction was mainlycontrolled by temperature when long as the local moisture content of wood (MC)exceeded the fibre saturation point (FSP). High temperature in combination withhigh EMC levels promote lightness reduction. Transformation of naturally occurringwood constituents into coloured substances takes place within hours and does notappear to be caused by process related precursors. This study showed that visuallydetectable darkening occurred in samples with a local MC exceeding the FSP whenexposed to drying temperatures higher than 40°C in combination with EMC levelsabove 15%.

Place, publisher, year, edition, pages
2013.
Keyword [en]
CIELAB, colour change, discoloration, wood drying
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:lnu:diva-29009OAI: oai:DiVA.org:lnu-29009DiVA: diva2:650950
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2016-01-22Bibliographically approved
In thesis
1. Colour Response in Drying of Nordic Hardwoods
Open this publication in new window or tab >>Colour Response in Drying of Nordic Hardwoods
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Colour and appearance of hardwood are of great importance for the interiorand furniture industry. The widespread use of transparent surface treatmentand a fashion that prescribe light colour on many species, means that deviationfrom the ideal have considerable impact on the industrial operations. Kilndrying is generally regarded as the process that has the greatest impact on thecolour of Nordic hardwood species. The lack of satisfactory explanation modelsfor many types of discoloration, however, complicates the control of the dryingprocess.This thesis is an attempt to increase the knowledge of which factors thatcontrol the appearance of some commonly found discolorations associated withdrying of beech, birch and oak. The main focus is on convection drying but alsothe influence of timber storage, pre-steaming and press drying has beeninvestigated for individual species. The studies have been conducted ascomparative studies based on design of experiments in which the colour wasdetermined using a colorimeter.Results show that reddish and dark discoloration of beech and birch duringconvective drying is mainly dependent on the temperature and time of exposurewhen the local moisture content exceeds the fibre saturation point. Theconversion of naturally occurring substances in birch into coloured compoundsis not due to active precursors created at high moisture content levels duringthe subsequent drying at low moisture content levels. Interior grey stain inbeech is caused by slow initial drying at low temperatures. Log storage in coldwinter and spring climate does not cause discoloration in beech. Birch becomeslighter when press-dried at high temperatures, resulting in a colour comparableto that of traditionally kiln dried wood. Steaming of oak before kiln dryingreduce the presence of brown discoloration, a general darkening of the woodoccurs at temperatures above 50°C.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2013. 78 p.
Series
Linnaeus University Dissertations, 148/2013
Keyword
beech, Betula pendula, birch, CIELAB, discolouration, drying, Fagus sylvatica, log storage, oak, press drying, wood colour, Quercus robur
National Category
Other Materials Engineering
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-29011 (URN)978-91-87427-52-7 (ISBN)
Public defence
2013-10-18, M1088, Linnéuniversitetet, Växjö, 10:00 (English)
Opponent
Supervisors
Available from: 2013-10-08 Created: 2013-09-24 Last updated: 2016-08-22Bibliographically approved

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Stenudd, Stefan

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