Change in fracturing and colouring of solid spruce and ash wood after thermal modification

Abstract

Effects of different thermal treatments (maximum treatment temperatures of 200, 210 and 220°C for 2.5 hours) on solid spruce (Picea abies L. Karst.) and ash (Fraxinus excelsior L.) were investigated in this study. The fracture behaviour in radial/longitudinal as well as in tangential/longitudinal and the change of the wood colour (CIEL*a*b* colour space) on all principal anatomical surfaces (cross sectional, radial and tangential) were analysed. The specific fracture energy and the maximum breaking load decreased almost significantly after all thermally treated samples in comparison to the untreated (standard dried) sample. The wood colour changed also significantly, in particular the lightness decreased with increasing intensity of thermal treatment on all investigated surfaces. A comparison of the percentage loss of the different fracture and colour values has shown a strong correlation between the maximum breaking load and the lightness after several thermal treatments, in both investigated crack propagation systems, on all measured surfaces and for both analysed species.

Keywords:

• CIEL*a*b* colour space
• fracture energy
• maximum breaking load
• thermally modified ash
• thermally modified spruce

Acknowledgements

The authors gratefully acknowledge the support of the Institute of Wood Technology and Renewable Resources (Boku Vienna), the Institute of Silviculture (Boku Vienna), the Austrian Research Promotion Agency (FFG), Aberger KG (Austria), Forstbetrieb Esterhazy (Austria), Forstbetrieb Stift Klosterneuburg (Austria), Metternich'sche Forstverwaltung Grafenegg (Austria) and Österreichische Bundesforste AG.