Sorption Hysteresis in Relation to Wood Thickness

In an attempt to establish the influence of wood thickness on sorption hysteresis, two groups of specimens of five thicknesses were brought into equilibrium with each of six different relative humidity conditions, one group by desorption from the green condition and the other by adsorption from the ovendry condition. Specimens were large-tooth aspen (Populus grandidentata Michx), 3 inches long by 2 inches wide by 0.8, 0.4, 0.2, 0.1, and 0.05 inch thick. The six relative humidities were 19, 31, 57, 71, 83, and 89 percent. There were three replicates of each of 60 variable combinations. A rubber-base compound was used to minimize moisture movement through the end grain. The weight of the end coating was accounted for in the calculation of moisture contents, which was done by ovendrying to constant weight at 212?F. The various relative humidity conditions were maintained in a closed desiccator by the use of` saturated salt solutions with an excess of solid phase. Results are presented in a table of moisture contents for desorption and adsorption specimens of the five thicknesses at six constant relative humidities. There is a definite hysteresis effect for each relative humidity condition regardless of the thickness of the specimens. The hysteresis ratios (adsorption divided by desorption moisture content) vary from 0.62 at 24 percent R.H. to 0.76 at 89 percent R.H. The sensitivity of the study was not sufficient to establish the exact influence of thickness of wood on sorption hysteresis. It was concluded that even under carefully controlled conditions, wood specimens of large sizes will still exhibit sorption hysteresis.

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