Colour ranks but does not measure mild thermal modification: optical, gravimetric, hygroscopic and structural responses of Avodire (Turraeanthus africanus) wood treated in dry air

Abstract Thermal-modification severity is often inferred from wood colour, assuming that colour change tracks mass loss, hygroscopicity and dimensional change, an assumption rarely tested in mild regimes or for tropical hardwoods. We tested whether the optical, gravimetric, hygroscopic and structural responses of Avodire share a common onset and form during dry-air treatment. Specimens from one lot of kiln-dried boards were treated at 160, 180, 200 and 220 °C for 3 h (í µí±› = 10 per group) and compared with a 105 °C drying control. CIELAB colour, anhydrous weight loss (AWL), dimensions, density and 14-day moisture uptake were measured on the same specimens; inference is restricted to this lot. Reddening, total colour difference and 14-day moisture content differed from the control from 160 °C; AWL did too, but only by water that 105 °C drying retains, and dry-matter loss was first detected at 180 °C. The domains separated partly by onset and mainly by magnitude and form. Drying-corrected Δí µí°¸ µí°¸ * í µí±Ží µí± rose from 3.4 to 31.1 while AWL reached only 2.85 %. Colour change was proportional to AWL, but the 180 °C group produced less colour per unit AWL (6.8, against 10.2–10.9). Lightness loss began at 180 °C, whereas Δí µí± * was non-monotonic. Treatment lowered moisture-uptake capacity and lengthened its timescale, while one Weibull shape (í µí»½ = 0.72) described all curves. A radial-shrinkage and densification anomaly occurred at 180 °C only. Colour can rank treatment severity within a calibrated process, but it is not a quantitative surrogate for mass loss, hygroscopic capacity or structural state.