Microstructure and Mechanism of Grain Raising in Wood

Preliminary research developed the experimental protocols for examining grain raising in different wood species. The species selected ranged from the lowest density wood species, such as quipo and balsa, through to the world’s densest wood species, such as ebony and lignum vitae (Table 1). One wood sample for each species was obtained from UBC’s xylarium (wood collection) and cut with a band saw to produce specimens measuring ~20 mm (wide) × 80 mm (long) × 20 mm (thick) with their growth rings oriented tangentially to their wide faces. Samples were selected at random and hand-sanded along the grain using 120 and 180 grit aluminium oxide abrasive papers (30 s each).

A sanded sample was placed on the x-y stage of a non-contact surface profilometer (Cotec Altisurf 500, Altimet, Alpespace, Grenoble, France). The elapsed time between sanding and surface analysis was approximately 5 min. An area measuring 16 × 4 mm² was scanned with a 300 μm probe using the following measurement parameters: spacing between measurement points, 7 × 7 μm²; scan speed = 1 mm/s; total scan time for each sample was 3 h 12 min. The software Papermap (Altimet, Alpespace, Grenoble, France) was used to level surfaces and remove form, calculate the roughness of sanded surfaces, and produce images of surfaces [15]. Following initial surface analysis, distilled water was applied as a droplet to the surface of each sanded sample using a micropipette (35 μL/cm²). Each droplet was drawn across the surface of the sample using a thin sheet of plastic. After 3 min the water was gently wiped off the sample using a cotton cloth. The sample was air dried in a conditioning room at 20 ± 1 °C and 65% ± 5% r.h. (relative humidity) for 16 h and its surface roughness was re-measured, as above. These procedures were repeated until the initial (dry) and final (dry/wet/dry) roughness of all samples were measured. Grain raising was calculated as the difference in average surface roughness (S_a) of samples before and after wetting and drying. The basic density of all wood species was assessed on separate irregularly-shaped wood blocks cut from parent samples. The wood blocks were oven-dried at 105 °C overnight, placed in a desiccator over silica gel for 10 min, and then weighed on an analytical balance. Samples were then vacuum-impregnated with distilled water and left to soak in water for one week. The volume of the water-saturated samples was measured using an Archimedean volume-displacement method. Basic density was calculated as oven dry weight (kg)/green volume (m³).

A sub-set of the 34 wood species examined above were used to further explore the grain raising of different wood species, and also the influence of surfacing methods on grain raising. The species selected were: Balsa, western red cedar, black cherry, sugar maple, and lignum vitae. Air-dried wood samples of different sizes were obtained from UBC’s xylarium or purchased commercially. Forty samples were examined (eight for each of the five species). Samples were sanded (as above) or planed using a disposable stainless steel microtome blade (Type S35, Feather Safety Razor Co., Osaka, Japan) attached to a blade holder (Type 130A, Feather Safety Razor Co., Osaka, Japan) and clamped to the stage of a microtome (Spencer Lens Co., Buffalo, NY, USA). A fresh blade was then inserted into the blade holder and each specimen was replaned to produce a clean surface. Sanded or planed samples were placed on the x-y stage of a profilometer and the roughness of the samples before and after wetting and redrying was measured as above. The software Papermap was used to calculate the roughness parameters of five regions (0.2 × 0.2 mm²) within each scanned area. Following the initial surface analysis, a droplet of distilled water was applied to the surface of each of the sanded and planed samples using a micropipette, as above. Each sample was air-dried and its surface roughness was re-measured, as above. These procedures were repeated until the dry and wet roughness of all samples was measured (5 species × 8 samples × 2 surface types × 5 replicate roughness measurements = 400 measurements in total). Analysis of variance was used to examine the effect of fixed (species and surface preparation) and random factors on grain raising, calculated as the difference in roughness of samples before and after wetting and drying. Statistical computation was performed using Genstat (version 18.2, VSN International, Hemel Hempstead, UK). Before the final analysis, diagnostic checks were performed to determine whether data conformed to the underlying assumptions of analysis of variance, i.e., normality with constant variance. These assumptions were confirmed using residual plots produced by Genstat. Significant results (p < 0.05) are plotted on graphs and error bars (p < 0.05) on graphs, which were derived from Fisher’s least significant difference (LSD) test [16], can be used to compare differences between individual means. Sanded and planed samples were also examined using scanning electron microscopy to better understand the microstructural features responsible for grain raising. Samples were attached to separate aluminium stubs using nylon nail polish and coated with a 10 nm layer of gold to make them electrically conductive. A scanning electron microscope fitted with a high brightness lanthanum hexaboride electron source (Cambridge Stereoscan 360, Leica Microsystems, Wetzlar, Germany) or a variable pressure scanning electron microscope (S-2600N, Hitachi Ltd., Tokyo, Japan) was used to obtain images of the tangential surfaces of samples after sanding or planing, and again after wetting and drying [17]. Loose material present at sanded or planed surfaces, both before and after wetting, was removed using transparent tape, and the tape was mounted on aluminium stubs and imaged using a scanning electron microscope, as above.

Marra commented that the “abrasive action of the grits on wood surfaces is difficult to visualize” [10]. We faced the same difficulty when trying to interpret images of sanded wood surfaces before and after wetting and drying. Furthermore, models of the ploughing action of abrasives on metals were not helpful because wood is softer than most metals, and is porous, unlike metals [18]. However, physical models of wood have been developed in the past to aid understanding of its complex three-dimensional microstructure [19]. We made a similar physical model and simulated the abrasive action of mineral grits on wood using a serrated metal tool. The model was made simply to visualize the abrasive action of mineral grits on the microstructure of wood fibres. The model was made from an 8 × 4 array plastic tubes each measuring 257.2 mm (length) × 7.1 mm (diam.) Plastic tubes were bonded together using a fast-setting epoxy adhesive. The model was fixed to a horizontal benchtop and a serrated metal tool (https://www.kuglers.com/kitchen/cuisipro-julienne-peeler) was pressed down on the surface of the model and drawn across the surface of the uppermost layer of plastic tubes. Photographs of the surface of the model were taken using a digital single-lens reflex (DSLR) camera (Canon 5D Mark II-Canon Inc., Tokyo, Japan) mounted on a tripod.

Twelve maple veneer faced panels purchased from a commercial retailer (P.J. White, Vancouver, BC, Canada) were each cut into four samples measuring 150 mm × 280 mm. Samples were levelled with an 80 grit aluminum oxide belt and then finish-sanded with 120, 120/150, or 120/150/180 grit aluminium oxide abrasive belts using a wide-belt sander (Unisand K 1350 M3, SCM Group, Rimini, Italy) with a feed speed of 7 m/s. The roughness of sanded samples was measured using profilometry, as above. Water was then applied to an area (as above) on the surface of the samples and they were then conditioned overnight at 20 ± 1 °C and 65% ± 5% r.h. The roughness of the samples was re-measured using profilometry. Grain raising was calculated as the difference in roughness of sanded samples before and after wetting and drying. Analysis of variance for a randomized block design was used to examine the effect of grit size on grain raising of samples [20]. Wood samples sanded with different abrasive papers, before and after wetting and drying, were also examined as above using a field emission scanning electron microscope (Hitachi S-4700, Hitachi Ltd., Tokyo, Japan).