Estimating Preservative Treatability of Wood from Its Air-Flow Properties

Several approaches to testing the preservative treatability of wood were evaluated using five western softwood species. Available void volumes were measured with an apparatus consisting of a specimen chamber, a mercury manometer, and a reservoir which included the volume of the manometer tube down to the normal level of evacuation (L). The reservoir was evacuated until the mercury in the left arm of the manometer tube reached level L; specimens were sealed in the chamber at atmospheric pressure, the stopcock to the evacuting system was closed, the stopcock to the chamber was opened, and the drop in mercury level during the test period was recorded. Results showed significant relationships between both creosote retention and logarithm of creosote retention and equivalent void volume parameters within species for coast Douglas-fir and western hemlock, but not for intermountain Douglas-fir, lodgepole pine, or western white spruce. Transverse measurements were more significant than longitudinal measurements; however, longitudinal equivalent void volumes values are adequate for segregating permeable and impermeable Douglas-fir and western hemlock. The very low available void volumes and fairly high creosote retentions of lodgepole pine suggest that most of the void space accessible to air is also accessible to creosote. The majority of the white spruce specimens had high equivalent void volumes but low treatibilities, possibly indicating that connecting channels in the wood’s total void space did not permit creosote to penetrate the cells at the pressure level tested. The procedure shows promise as a laboratory technique but not as an on-site test method. To take air-flow readings on full-size boards, a bench apparatus was constructed to evaluate two approaches. The first approach was to measure the equilibrium pressure drop across a tiny orifice when the exit air was forced to escape through a probe inserted in the end of a test board. The second approach was to charge a reservoir with 100 psi air pressure, then close the valve to the air supply, allowing the air to escape through a test specimen. Pressure fall was recorded at intervals for later comparison with creosote retention values. A portable device using this principle was also tested. Both air-flow procedures tested were effective in segregating permeable coastal Douglas-fir heartwood from refractory intermountain Douglas-fir heartwood. Correlations between values obtained using the protable device and creosote retentions were significant at the 0.01 level, though much poorer than with the bench apparatus.

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