Longitudinal Water Permeability of Western Hemlock. I. Steady-State Permeability
Keywords:Tsuga heterophylla, sapwood, heartwood, wetwood, pit aspiration, transmission electron microscopy, scanning electron microscopy
Average initial permeability to water of sapwood was found to be 9.6 X 10-10 cm2, that of wetwood from heartwood was 6.64 X 10-10 cm2, and that of normal heartwood was 4.4 X 10-12 cm2. All the specimens were never-dried, approximately 0.95 cm in diameter and 2 cm long, and were embedded in a lucite tube using epoxy resin as binder.
Using polyethylene glycol 1000 as an embedding agent, 23% of sapwood pits, 42% of pits in wetwood from heartwood, and 84% of pits in normal heartwood were found to be aspirated. Scanning electron microscopy revealed that the normal heartwood of freeze-dried heartwood was heavily incrusted, but that of wetwood was relatively free of incrustation. High water permeability of wet heartwood was attributed to a low level of pit aspiration and freedom from incrustation.
Both sapwood and wetwood exhibited deterioration of permeability with time. In sapwood the cause was considered to be time-dependent pit aspiration because of hydrostatic pressure differentials during testing, but in wetwood the deterioration was attributed to extractives transported by water and deposited on pit membranes to form an impermeable coat of film.
A further proposal is that formation of wet pockets during drying of western hemlock lumber is caused by formation of an impermeable zone from the incrustation of pits by extractives during the migration of water, which traps the moisture in lumber.
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