A. Teischinger
1), M.L. Zukal
1), T. Meints
1), C. Hansmann
2), R. Stingl
1)1) Institute of Wood Science and Technology – Department of Material Science and Process Engineering – University of Natural Resources and Life Science, Konrad
Lorenzstrasse 24, 3430 Tulln, Austria, alfred.teischinger@boku.ac.at
2) Competence Centre for Wood Composites and Wood Chemistry, Wood K plus, 4040 Linz, Austria
Keywords: Wood colour characteristics, CIE L*a*b*-colour system, colour intensification (German: anfeuern)
ABSTRACT
Wood as a material absorbs and reflects the visible light and this physical interaction produces the typical wood colour within a range of almost white, various yellowish, reddish and brownish hues to almost black. The colour characteristics depend on the chemical components of wood that interact with the light which has been analyzed in many studies. Lots of scientific research has been performed on the discolouration of wood surfaces caused by light irradiation and technological processes such as wood drying, steaming or modification. There is hardly any reproducible digital data on the wood colour of the various wood species as in most wood handbooks the colour of the various wood species still is described as “almost white”,
“bright yellow”, “reddish-brown”, “dark brown” etc.
In our current research we set a focus on a spectrophotometric characterization of various wood species, especially hardwood species by applying the CIE-L*a*b*-colour-system on 17 domestic wood species.
According to an extensive literature review and our own experience, colourimetry is a good method for obtaining objective information on the colour of wood. A special focus was put on the analysis of colour enhancement (German: “anfeuern”) by wetting (clear lacquering) a wood surface. It could be shown how the various wood species react to wetting by a colour enhancement analyzed by the CIE-L*a*b*-colour-system. These data also support the understanding of the mechanisms of the colour enhancement as a physical phenomenon of light reflection and refraction on a “dry” and rough surface and a wet or clear coated glazed surface.
Especially the huge variety of hardwood species benefits from the colour enhancement for a more intensive colour appearance which can be capitalized in the furniture and flooring industry. Understanding this colour enhancement also supports innovative natural colour engineering of specific wood species. characteristics depend on the chemical components of wood that interact with the light which has been analyzed in many studies such as Hon et al.
(2001). Lots of scientific research has been performed on the discolouration of wood surfaces caused by light irradiation and technological processes such as wood drying, steaming or modification (Tolvaj, et al. 1995, Hon et al. 2001, Müller et al. 2003, Mitsui 2004, Tolvaj et al. 2005). Oltean et al.
(2008) tried to establish a classification of wood surface discolouration due to indoor UV light irradiation by analyzing the dynamics of discolouration and building three groups of wood species with strong, medium and low discolouration.
Another approach to characterize the effect of light on lacquered surfaces as analyzed by Luljka (1997), Forsthuber et al. (2011) and Forsthuber et al.
(2012).
Wood colour research was initiated in Japan by Onodera (cit. in Sullivan 1966), who related visual phenomena to spectrophotometric measures of the Y tristimulus value, luminance. An extensive review on the historic development of wood colour measurement is provided by Sullivan (1966).
But still today there are hardly any reproducible digital data on the wood colour of the various wood species themselves and in most wood handbooks colour terms of the various wood species are still terms such as “almost white”, “bright yellow”, pale straw-buff”, “reddisch-brown”, “darkbrown”
etc. (e.g. Wagenführ 1996) rather than distinctive numeric values. Nishino et al. (1998) measured the colour of 97 wood species from French Guiana with a colourimeter according to the CIELAB system. In our current research we set a focus on a spectrophotometric characterization of various wood species, especially hardwood species by applying the the CIE-L*a*b*-colour system
The 5th Conference on Hardwood Research and Utilisation in Europe 2012
on 24 domestic wood species. According to an extensive literature review and our own experience, colourimetry is a good method for obtaining objective information on the colour of wood. A special focus within this study was put on the analysis of colour enhancement (“anfeuern” in german) by wetting a wood surface (fig 1).
Any wetting (and clear coating) of wood surfaces leads to an intensification and enhancement of the colour. Meichsner et al. (2011) describe the different colour appearance of a dry (uncoated) and a wet (clear coated) wood surface by the microscopic roughness of the respective surface and their reflection characteristics. Due to the laws of reflection less light is reflected from a laquered surface as a part of the diffuse light is “captured” within the laquered (or wet) layer of the surface and is transformed into heat. Therefore a laquered surface exhibits a higher chroma and a lower ligthness.
Ash (Fraxinus excelsior L.)
Maple (Acer pseudoplatanus L.)
Beech (Fagus sylvatica L.)
Common Oak (Quercus)
Dutch elm (Ulmus ssp)
Black locust (Robinia pseudoacacia L.)
Common walnut (Juglans regia L.)
Pear (Pyrus communis L.)
Figure 1: Selection of domestic hardwood species: The upper part shows a sanded surface with no further treatment. The lower part shows colour intensification (German: anfeuern)
due to a clear laquer treatment
MATERIAL AND METHODS
17 domestic hardwood specimens were taken from 20 (in a few cases from 17) different wood specimen collections, where the wood specimen were manufactured from technically kiln dried boards. The specimens with a size of about 100 x 200 mm and a thickness of about 12 mm were conditioned at 20° C and 65% r.H. which resulted in an EMC of about 10 to 13%. Before colour measurement the surfaces were sanded with a sanding paper of a grid of 400 as the last sanding step.
The basis for colour designation lies in a measure of the energy distribution of light reflected from a surface. The spectrometer accomplishes this by selecting any particular wavelength in the light sources and measuring the radiant flux of the selected wavelength that is reflected from a standard surface. This is compared arithmetically to the flux of the same wavelength that is reflected from the test material.
Colour was measured with a spectrophotometer (Phyma CODE 400) at the range of wavelengths between 400 and 700 nm. The light source was defined by a standard illuminant D65 with an observer of 2°. The diameter of the field of view was 20 mm and the analysis of the colour data was due to the CIE L*a*b* colour space (ENISO116664). The CIELAB colour space can also be used for computing the colour difference (∆E) by computing the Euclidean distance between two points representing the colour stimuli in the colour space (Urland 1999).
Colour measurement was firstly performed on dry untreated surfaces.
Afterwards the specimen was put into a pot of distilled water for about 15 seconds and the wetted surface was measured again.
The 5th Conference on Hardwood Research and Utilisation in Europe 2012
RESULTS AND DISCUSSION
The results of the colour measurement of the untreated native surface and the wetted surface are summarized in table 1.
Table 1: L*a*b* / L*C*h –colour values of native hardwood species and intensifying colour respectively colour differences (mean values of the specific sample set)
White willow SA
due wet surface Colour difference
1) Abbr. according to ÖNORM B 3012
The colour difference ∆E as a measure of the change of colour stimuli due to the wetting of the wood surfaces is shown in fig. 2.
8,92
0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 45,00
White poplar (Populus alba L.) Ash (Fraxinus excelsior L.) Maple (Acer pseudoplatanus L.) Common birch (Betula pendula R.) Hornbeam (Carpinus betulus L.) Lime (Tilia platyphyllos Scop., cordatal Mill.) Sweet chestnut (Castanea sativa Mill.)
Figure 2: Colour difference ∆E* (indicator for accentuated colours) between colour of native hardwood species and intensifying colour, due to wet surface
CONCLUSIONS
For the most prominent domestic wood species a colour characterization could be provided based on the CIE-L*a*b* colour designation system. The lightness L* ranges between 57.4 (poplar, followed by maple) and 88.7 (walnut). The most intensive colour is represented by black locust whereas hornbeam is the most palish wood species.