Determination of Lumen Size Distribution in Softwood By the Mercury Injection Method

A porous material will not absorb a non-wetting material, such as mercury; however, when pressure is applied, the liquid will be forced to penetrate the pores, and the pressure required to fill a given pore may be taken as a measure of the size of the pore. As the pressure is increased, the amount of liquid “absorbed” will increase monotonically at a rate proportional to the differential pore volume. Thus, a given pore-size distribution gives rise to a unique pressuring curve, and conversely, a given pressuring curve affords a unique determination of the pore size distribution–or, in this experiment, of the lumen-size distribution in a sample of wood. In this study, an eastern hemlock block was microtomed and extractives were removed. Measurements were made using the 5-7105 Aminco-Winslow Porosimeter in which the sample was evacuated and then injected with mercury at various pressures. This study revealed that the lumen-size distribution of eastern hemlock can be approximated by a modified Maxwellian distribution. Though data were gathered only from limited samples of one species, it is expected that most softwoods, because of their relatively uniform structure and tracheid size, will exhibit a similar pattern of lumen-size distribution. The method also provided information to the total volume of all lumens having certain radii. An equation was derived to represent the average lumen radius, and in eastern hemlock, the average lumen radius was calculated at 12.5 microns, but it is suspected that the true value is larger than this. The area of microscopically visible capillary surface and the number of tracheids in square centimeter of cross section were also estimated and equations derived for calculating these values. Values obtained were 3.36 x 103 square centimeters per gram for area of microscopically visible capillary surface, and 153,500 tracheids per square centimeter of wood in cross section. These figures also may be somewhat higher than they should be if in fact the determined average lumen radius is less than the true value.

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