Tracing the migration of liquid water and wood extractives in silver birch and Scots pine sawn timber during drying using a dye solution

Abstract

The formation of discolouration in wood can be connected to the migration and precipitation of water-soluble extractives during the drying process. The chemical precursors of wood discolouration develop during the capillary phase of drying, and the movement of liquid water is probably the most important transportation mechanism for the chemicals involved in discolouration. Liquid water flow in silver birch and Scots pine sawn timber during drying was investigated using a fluorescent dye, pyranine, to describe the migration and precipitation of water-soluble extractives. The trace of the dye solution differed between silver birch and Scots pine sawn timber, and also between Scots pine sapwood and heartwood sawn timber. In particular, in Scots pine sapwood timber, the dye precipitated in a layer below the surface where the evaporation took place. This is in accordance with earlier findings on the migration of wood extractives from the interior of boards to the surface layer, and from the location of the evaporation front. In Scots pine heartwood timber, no migration of the dye was found. In silver birch timber, the dye migrated mainly in the longitudinal direction in the interior of boards and did not form layers. The migration and precipitation of the dye can be used to illustrate the different segments in wood where discolouration can be expected with different wood species.