Ann. For. Sci. 47 (1990) 255-268
DOI: 10.1051/forest:19900306

La qualité de l'aspect des placages de chênes : mesures de couleur et critères d'appréciation des professionnels français et italiens

JF Mazet and G Janin

INRA, station de recherches sur la qualité des bois, centre de recherches forestières, Champenoux, 54280 Seichamps, France

Résumé - L'étude des différentes qualités possibles de l'aspect des placages de chêne (Quercus petraea (Matt) Liebl et Quercus robur L) et des critères qui permettent habituellement aux professionnels de les apprécier a été menée par une enquête (présentation de 53 paires de placage) auprès de 90 professionnels du bois français et italiens, répartis en 71 ateliers ou entreprises. L'objectivité de la démarche a été assurée par le traitement statistique des résultats (analyse factorielle des correspondances et tests du χ²) et par la caractérisation de l'aspect de tous les échantillons présentés par des mesures effectuées dans le système CIELAB de quantification des couleurs. Il ressort des résultats que la différence de couleur (mesurable par ΔE*) entre 2 placages favorise de manière nette la décision de choix. La clarté des placages (mesurable par la luminance L*) est une caractéristique appréciée d'une majorité de professionnels. Le caractère uniforme ou non des placages est également apparu important et bien décrit par la mesure de sL*/mL* (coefficient de variation de la clarté L*). Enfin, la variabilité naturelle des tonalités de couleur (mesurables par l'angle de couleur moyen h*) des placages européens de chêne est largement suffisante pour générer des choix différents, spécifiques de certains groupes professionnels.

Abstract - The quality of appearance of oak veneers: colour measurements and visual appreciation of French and Italian professionals. (Oak logs (Quercus robur L and Quercus petraea (Matt) Liebl) are selected for the wood veneers' industry on the basis of their size, length, circumference, width and regularity of annual rings. After slicing, there are numerous quality indicators used to characterise the appearance of oakwood veneers (colour, figurations of wood, etc). The industrial classification of these different qualities is very complex, normally requiring the evaluation of several criteria. These criteria are descended from empirical and traditional use. The aim of this work was to clearly define these criteria in order to find the best parameters to measure them, using the method of colour measurement CIELAB (1976), developed by the CIE (Commission Internationale de l'Eclairage). For this purpose, an inquiry was carried out, presenting to 90 French and Italian wood professionals a collection of samples of various industrial oak veneers (20 x 22 cm), associated randomly in 53 pairs (fig 1). Each sample had been measured at 20 points using a spectrophotometer (Colorquest Hunterlab), which gives colour values in the CIELAB system. Thus, L* (the lightness), h* (the hue angle) and sL*/mL* (standard deviation for L*, gained from the 20 measures) were obtained. For each pair, an opinion on quality was requested from each professional (preference for sample A, preference for sample B, no preference). It was not revealed to the professionals that the samples had been previously measured for their appearance. Multivariate analysis (Reciprocal Averaging) and χ² tests, which were used for statistical treatment of data, clearly show that a great difference of colour between 2 veneer samples (measured by the total colour difference ΔE*), is an important factor in the choice of the professionals (fig 2). The first axis of the Reciprocal Averaging (fig 4), is connected with the uniformity of colour of the samples. The second axis shows that natural colour tone variability of European oak veneers is responsible for specific choices depending on different professional groups (fig 4). The standard deviation of the lightness L*, and the hue angle h* appear to be related to these 2 professional criteria which are shown on the first 2 to axis of the analysis (figs 5 and 6). The lightness of oak veneers (closely associated with L*, the lightness) is a very much appreciated characteristic for the majority of professionals (fig 11). In conclusion, it seems that it is possible to obtain some measured parameters which can objectively describe the qualities of the appearance of oak veneers.

Key words: oak / veneers / multivartate analysis / colour / colour measurement / criteria of quality

Mots clés : Quercus petraea / Quercus robur / bois de placage / analyse des correspondances multiples / couleur / mesure de qualité / critère de qualité