Abstract: The present study was a first step of a systematic comparative study on the surface colour of 83 wood species from different geographical regions. The colour measurements were monitored by CIE L*a*b* and CIE L*h*c* colour coordinated systems. Moreover, the exposure to light of a solar type in the conditions of accelerated ageing of 24 selected wood species made it possible to evaluate the stability of their natural colour. Generally the colour of wood from Europe and North America was lighter than the colour of the tropical wood form Africa and South America. The European woods showed wide variations in the stability of their natural colour while the African and darkest South American woods had highest stability. It was suggested that colour stability of wood was mainly attributable to the diverse chemical nature specific for each species.
INTRODUCTION
The colour of wood is very important for its identification, aesthetic and commercial values. The variation of environmental factors such as solar radiation, humidity, pollution gases, temperature etc., affects the wood surface and its colour; its protection is of constant concern (Janin et al., 2001). The UV region of solar radiation causes the most damage to the wood surface, which is an excellent absorbing material (Hon and Feist, 1992). The photochemistry of this process is very complex and it is determined by the chemical composition of wood (Janin, 1987). Among the principal macromolecular components of wood (lignin, cellulose and hemicelluloses) it was found that, only the lignin has chromophores that absorb light energy and thus contributes 80-95% in the colour change by complex reactions forming free radicals species (Merlin and Deglise, 2001; Moore and Owen, 2000; Pastore et al., 2004). However, the phenolic extractives in general affect the wood colour and colour change to the greatest extent (Nzokou and Kamdem, 2006).
If natural wood is exposed to solar radiation, the exposure leads initially to a fast colour change due to absorption of all wavelengths of electromagnetic radiation and in the further stages to large chemical changes and breakdown of the surface layer (Ayadi et al., 2003; Deka and Petric, 2008). On exposure to light, some woods become bleached or grey, others turn yellow, red-orange, or brown colour, depending on the effect of their extractives composition (Hon, 2001; Kamdem and Grelier, 2002).
There is a large body of literature concerning the colour of wood. The work on fir tree (Abies grandis) is significant because this wood is lacking coloured extractives and can be considered as a reference for the lightest-coloured wood in colour studies (Dirckx, 1988; Dirckx et al., 1992; Merlin and Deglise, 2001). European oak is another extreme frequently studied tree because of its slow but distinct colour change due to its high extractives content (Mazet et al., 1993; Merlin and Deglise, 2001). The comparison of the photo degradation of the heartwood and sapwood of oak has made it possible to understand the role of extractives in the process of colour change (Scalbert and Monties, 1987).
Although, the literature concerning the wood colour is immense it is difficult to find comparable data obtained with the same method for species from different geographical areas. The objectives of the present work were to produce colour data for 83 wood species from different geographical regions by the same method and to add it to the available data in this respect. Furthermore, from these wood species 24 species were selected to observe the wood colour change as function of their extractives content at artificial ageing conditions of irradiation with UV light of 365 nm for 800 h.
MATERIALS AND METHODS
Wood extraction: Extraction of wood samples was carried out with the accelerated extracting system ASE 200 of Dionex Co. After defatting with petroleum ether for 24 h, each wood sample was extracted with a solution of ethanol-water (70/30 v/v) at 100°C and 100 bars in two 5 min stages. Extracts were dried with a vacuum rotary evaporator at 40°C and the yield was determined.
Colour measurement and ageing system: The colour measurements (Bourgois et al., 1991) were carried out in the CIE- L*a*b* colour space using a spectrophotometer (Spectro-penlange) with a standard illuminant D65 and 10° circular illumination. The colour coordinates are calculated from the reflection data on the sample surface. In this colour range, a colour is defined by its Cartesian chromatic coordinates: lightness L* which varies from 0 (black) to 100 (white) and a* and b* coordinates which define the chromatic plane; negative values of a* indicate green while positive values indicate red, while the negative values of b* indicate blue and positive values indicate yellow. The difference in chromaticity is defined as ΔC = [(Δa*) 2+ (Δb*) 2]1/2. The CIE L*a*b* system produces a fair correlation with the visual perceptions (Chrisment, 2000). Concerning the colour development during irradiation, it is useful to present the data in the CIE L*C*h* system which is more uniform (closer to the psycho-sensory assessment). The colour development can be derived simply from the CIE L*a*b* system by changing the Cartesian coordinates to cylindrical ones in the chromatic plane [a*, b*]. The chroma (concentration or degree of colour saturation) is defined then by C* = (a*2 +b*2)½ and the hue (saturation angle) h = arctan (b*/a*). The mean values of chromatic coordinates were calculated from ten measurements at different points on the radial surface of the sample (30x30 x10 mm, LxTxR).
Accelerated ageing of the wood was carried out in a SEPAP chamber (MPC, France) equipped with standard pressure mercury vapour lamps with a light flow of 5 mW cm-2 at 365 nm. Samples, rotating at constant speed and distance from the sources, were exposed during 800 h at 50°C. The variation of chromatic coordinates during irradiation was determined as:
Δα *(t) = α*after irradiation at time t-α* before irradiation
Where:
α* = L*; a*; b*; c* or h
The total colour change ΔE* in the CIE L*a*b* system:
ΔE* = [ΔL*2 + Δa*2 + Δb*2]1/2
where, ΔL*, Δa* and Δb* are changes between the initial and final value of L*; a* and b* values, respectively (Brock et al., 2000).
RESULTS AND DISCUSSION
Natural colour diversity of wood species: The colour measurements carried out according to the CIE L*a*b* and the CIE L*C*h* systems of different wood species from different geographical origin are given in Table 1. Comparison between species of different geographical regions in their range of lightness indicated the following order in decrease of lightness from the European (51.1-84.5), North American (59.9-79.5), Asian (48.1-55.1), South American (36.3-81.2) to African (30.5-72.3) species. As expected the temperate woods from Europe and North America were lighter (i.e., they had higher L* values) than most of the species from Africa, South America and Asia, which are tropical woods richer in extractives (Khristova, 1996). An important correlation between the values of lightness L and the hue angle h was observed (Tolvaj and Nemeth, 2008). The more light-coloured (high L values) were the wood species the more yellow they were (h around 90°). However, Peltogyne wood (North America) had a peculiar behaviour in revealing a dark violet colour characterised by low L* value (36.3) and a very narrow colour angle (low value of b*-3.4 and high value of a*-11.3) at corresponding lowest hue angle of 16.5°.
Photo degradation under solar light: The total change in the wood colour (ΔE*800) for the 24 species selected from the species studied after 800 h of UVA irradiation at 365 nm is presented in Table 2. All species had their lightness after radiation decreased to a different degree depending on their original lightness. Generally it was observed that the change of colour during photo ageing was more pronounced in wood species of lighter colour i.e. with originally high L* values of above 55. The low and the negative values of ΔL* indicated loss of lightness, which resulted in darkening of the wood. European species studied revealed a greater decrease in colour and photo stability, in addition to the known colour changes from season to season (perenniality). The dark African and darkest Southern American woods changed colourless under irradiation. The most intensive degree of discolouration observed was 23.9 and the lowest was 0.96 (Table 2), which is below the limiting value of discolouration that can be distinguished by naked eye of ΔE = 3 (Pastore et al., 2004). A peculiar behaviour of Pterocarpus soyauxii wood was that despite its dark colour (L* = 43.8, Table 1) it underwent the most intensive total colour modifications (ΔE*800 = 23.9, L800* 13.6, a800*-19.6, b800* -0.73, Table 2). This could be due to the considerable presence of extractives (R = 13.0%) of low photochemical stability.
| Table 1: | Colour characteristics of wood species from different geographical regions |
| L*, brightness a*, b* colour coordinates C, chroma h, hue angle | |
Furthermore it was observed that the wood species with the exception of African Pterocarpus soyauxii (ΔE*800 = 23.9, R = 13.0%) and Millettia laurentii (ΔE*800 = 14.1, R = 14.4%) species, most affected by solar radiation contained less extractives (R). The opposite was true for the species with low extractives content that had low ΔE*800 and were less affected by irradiation, except for African Lophira alata (R = 3.4%) and Asian Shoria curtusii (R = 4.6%) and Intsia bakerie (R = 4.0%) species (Table 2). It is well known that different wood from different species contain extractives of diverse chemical nature and quantity (Fengel and Wegner, 1984). Thus it is not surprising that the irradiation with UVA at 365 nm during 800 h had affected in a different way the wood colour of the different species studied.
| Table 2: | Extractives content and colour variation after artificial ageing of selected wood species from different geographical origin |
| L* , L*800 brightness in CIE L*, a*, b* system for untreated and aged species*, b* and a*800, b*800 colour coordinates in CIE L*, a*, b* system for untreated and aged species h hue angle in CIE L*, a*, b* system DE*800 total change in colour after ageing R extractives content | |
CONCLUSIONS
European wood species with light colour showed a wide variation both in colour and photo stability. African and darkest South American woods, which were dark from the very beginning, did change colour less upon irradiation. The observed differences in the photo stability of the different wood species seems dependent not on the extractives quantity but on their diverse chemical nature. The effect of extractives nature on colour stability should be studied for each species individually.