Element content and pH value in American black cherry (Prunus serotina) with regard to colour changes during heartwood formation and hot water treatment

Element content and pH value in wood tissues of veneer grade logs of P. serotina Ehrh. were investigated with regard to wood colour variations, measured in the CIEL*a*b* system. The average pH value of heartwood tissue was about pH 4.0 and medium colour parameters of veneer sheets were determined at L* = 73, a* = 9.8, and b* = 23.5. Optical emission spectroscopy (ICP-OES) analyses showed differences in the element contents between two regional forest sites coming from Pennsylvania and West Virginia, USA, respectively. The latter is mainly characterised by higher variations of micro-element content in the transition zone (influencing heartwood formation) and also pH value of wood tissue, which contributes to higher variations in colour response of industrially produced veneer sheets. Investigations under industrial conditions underline the correlation between length and intensity of heat treatment in veneer production and colour development: with increasing duration and temperature of hot water treatment, veneer surfaces become darker and wood colour is intensified (ΔL = 3.6, Δa = 2.1, comparing 12 and 72 h of hot water treatment at 60°C). However, no equalisation of wood colour was achieved by modifying the treatment conditions. Artificial radiation by UV–visible light, quickly and extensively darkens and intensifies wood colour (ΔL = 16, Δa = 3.5, and Δb = 4.0 after 15 h of artificial radiation), but variations in wood colour deriving from different treatment conditions during veneer production, were not reduced.