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Colour Response in Drying of Nordic Hardwoods
Linnaeus University, Faculty of Technology, Department of Forestry and Wood Technology.
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 [en]
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: urn:nbn:se:lnu:diva-29011ISBN: 978-91-87427-52-7 (print)OAI: oai:DiVA.org:lnu-29011DiVA: diva2:650975
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
List of papers
1. Colour changes in birch and beech during kilndrying
Open this publication in new window or tab >>Colour changes in birch and beech during kilndrying
2001 (English)In: Proceedings of the 7th International IUFRO Wood DryingConference, 2001, 300-305 p.Conference paper, Published paper (Refereed)
Abstract [en]

The aim of this study was to investigate the colour responses of Silver Birch (Betula pendula Roth.) and Beech(Fagus sylvatica L.) during conventional kiln-drying at industrial used conditions. The response of dryingtemperature, relative humidity, initial moisture content and board thickness on wood colour was investigated in two24 factorial designed experiments. The method was used to investigate and rank the effects of the variables and allpossible combinations of the variables with a minimum of tests. A total number of 224 clear green sapwood samplesdivided in to 32 groups were kiln-dried below 20% in a climate chamber, followed by room climate drying toapproximately 8%. Colour measurements were conducted on dry planed samples using a photoelectric colorimeterand results were expressed in the CIE L* (lightness) C* (chroma) h (hue) colour space. A comparison was also madewith 32 samples dried in room climate from green to 8%, differences were calculated and expressed as CIE (ΔE*ab)effects. The factorial experiments used high and low level for investigating the variables: temperature (60/30C),relative humidity (82/62%), initial moisture content (green/30%) and wood thickness (16/10 mm).Experimental results showed that drying temperature is the most important factor for the colour responses in theinvestigated intervals for both birch and beech. The second most important factor, for both species, was thickness ofthe wood. Relative humidity was found to be the third most important factor. Both species reacted in a similar way toincreasing temperature, thicker dimensions and high relative humidity. The wood colour became darker, moresaturated and redder compared to the reference material. The colour response effects were larger on birch comparedto beech.

National Category
Other Materials Engineering
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-29003 (URN)
Conference
7th International IUFRO Wood Drying Conference, Tsukuba, Japan, July 9–13
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2013-10-08Bibliographically approved
2. Colour response of Silver birch to press drying
Open this publication in new window or tab >>Colour response of Silver birch to press drying
2003 (English)In: Proceedings of the 8th International IUFRO Wood DryingConference, 2003, 449-454 p.Conference paper, Published paper (Refereed)
Abstract [en]

Press drying is a rapid drying technique capable of reducing deformation, but also known to cause discolourationin some wood species. The colour of Silver birch is of great importance to the furniture and flooring industry, wherethe blond colour is highly appreciated. This study investigates the colour response in the core of 36 mm Silver birchsamples to the process variables: plate temperature, plate pressure and air pressure and to the material variable initialmoisture content. Short samples were press dried in a electrically heated laboratory press to approximately 10%moisture content and the interior colour of dry wood was registered according to the CIE L*C*h colour system. Theresults show that plate temperature is the most influential process variable controlling the wood colour. Hightemperature results in brighter more yellowish colour and reduced drying time. Other process and material variablesalso have some influence on the final colour but their contributions are smaller. Press drying of green wood at 170Cin normal atmospheric air pressure gave a wood colour comparable to that of conventionally kiln dried birch wood,only slightly darker and more reddish. The study indicates that press drying of Silver birch is possible from a colourperspective by careful selection of process parameters.

National Category
Other Materials Engineering
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-29004 (URN)
Conference
8th International IUFRO Wood Drying Conference, Brasov, Romania, August 24–29
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2013-10-08Bibliographically approved
3. Color response in silver birch during kiln-drying
Open this publication in new window or tab >>Color response in silver birch during kiln-drying
2004 (English)In: Forest products journal, ISSN 0015-7473, Vol. 54, no 6, 31-36 p.Article in journal (Refereed) Published
Abstract [en]

Color change in silver birch caused by kiln-drying is a major down-grading factor and has substantial economical impact on high value furniture and flooring production. This study investigates the color responses to process parameters at different stages in the drying process. Samples were dried in a climate chamber and laboratory kilns and the color of dry wood was registered according to the CIE L*C*h color system. The results show that during the capillary drying phase (moisture content > 55 %) time is more important than temperature. Swift initial drying even at elevated temperatures increases the lightness and decreases the saturation. The major color changes occur later on, during the diffusive drying phase from approximately 30 to 20 percent moisture content. Here a combination of temperature, time, and initial moisture content is decisive for the final wood color. Swift drying in both stages even at moderately elevated temperatures results in a highly desirable brighter color.

Place, publisher, year, edition, pages
Forest Products Society, 2004
National Category
Other Materials Engineering
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-29006 (URN)
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2017-12-06Bibliographically approved
4. Influence of Pre-heating on Brown Discoloration whenDrying Oak Heartwood
Open this publication in new window or tab >>Influence of Pre-heating on Brown Discoloration whenDrying Oak Heartwood
2005 (English)In: Proceedings of the 9th InternationalIUFRO Wood Drying Conference, 2005, 127-130 p.Conference paper, Published paper (Refereed)
Abstract [en]

General loss of lightness and brown discoloration of oak heartwood are major problems to the European woodindustry. In this investigation has the effect of pre-heating timber to 100°C prior to conventional kiln drying at 15, 35and 50°C been studied. Pre-heating significantly reduced the level of local brown discolouring streaks and spots aswell as slightly increasing the general lightness. Drying temperature has a major impact on the development of localbrown discoloration as higher temperature cause more discoloration. The thermal effect on general lightness is lessdistinct and a turning point seams to exist in the 40-50°C range above which the lightness is distinctly reduced.

National Category
Other Materials Engineering
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-29007 (URN)
Conference
9th International IUFRO Wood Drying Conference, Nanjing, China, August 21–26.
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2013-10-08Bibliographically approved
5.
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6. Influence of moisture content, temperature and air humidity during kiln drying on the lightness of Silver birch (Betula pendula Roth.)
Open this publication in new window or tab >>Influence of moisture content, temperature and air humidity during kiln drying on the lightness of Silver birch (Betula pendula Roth.)
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%.

Keyword
CIELAB, colour change, discoloration, wood drying
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:lnu:diva-29009 (URN)
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2016-01-22Bibliographically approved

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