To increase the durability of wooden furniture components subject to heavy everyday use several technologies have been developed to modify the density or surface hardness of softwoods but with limited success. This 2015 pdf booklet from Forest & Wood Products Australia Limited provides a summary of the research to date.
Per unit of weight, softwood is stronger than steel. Softwood derives its strength from a matrix of cellulose and hemicellulose molecules bound together with lignin. The tree produces this composite material to form (mostly) thin-walled, long tubular cells, shown in the diagram below. Softwood cells are 90 to 95 percent latewood or earlywood tracheids. The remainder are parenchyma, ray, resin and pith cells that primarily store and transit food. The tracheids' vertical orientation with the trees' trunk explains the bending strength of wood "parallel with the grain direction" and its susceptibility to splitting "perpendicular to the grain direction."
The chart shows that the density of common Canada and United States softwoods varies from 450 to 640 kg/m3. That some softwoods are denser than some hardwoods is often overlooked (red alder's density, for example, is only 460 kg/m3). Earlywood cells/tracheids (thin-walled, less dense, often lighter colour) and the latewood cells/tracheids (thick-walled, more dense, often darker colour) have a difference in density/colour that produces two features commonly termed "growth rings" on the trunk cross-section and "grain" on the radial and tangential sections (diagram below, left).
To counter the lower-impact resistance of softwoods, the radial surface/section with its high proportion of exposed dense latewood should be deployed for maximum benefit. This orientation produces "vertical grain" when the log is quarter-sawn or centre planks of a flat-sawn log are selected (diagrams below, right).
Conversion methods (above), typical 3D view of softwood grain direction (below)
At harvest "green" wood contains a large percentage of water in three forms - free water (in the cell cavity), water vapour and bound water (absorbed by the tracheid walls). For furniture applications all but six percent of moisture must be removed. The first stage of drying removes the bound water to the "fibre saturation point" and equates to approximately 30 percent moisture content (MC). All shrinkage when drying wood occurs between this point and the target six percent MC. A dry kiln (or a combination of air and kiln drying) lowers the MC to the required level. The production of furniture-grade wood takes considerable skill, time and patience - attributes often lacking in large-scale kiln-drying operations that process construction-grade softwood lumber.
To minimize the negative consequences of wood expansion/contraction designers and manufacturers must understand the hygroscopic nature of wood. After kilning, the wood's MC slowly changes until it is in equilibrium with the relative humidity (RH) of the air surrounding it. Wood at six percent MC (equivalent to 30 percent RH) is suitable for shipping to all regions in North America, except where there's very high humidity. Wood that acclimatizes to eight percent (equivalent to 43 percent RH) can be shipped to all regions, except those with very low humidity.
Without investing in expensive HVAC technology, SMEs will be challenged to maintain wood storage and shipping areas at the required RH in regions prone to high or low levels of humidity. SMEs should install and regularly monitor an electronic RH meter and consult a qualified HVAC engineer for advice if their facility RH regularly exceeds or drops below target levels.
There are two easy ways to control MC and minimize dimensional changes in components - use a species with low volumetric shrinkage (see chart ) and/or orientate the grain direction radially to best advantage. Wood shrinks or swells twice as much tangentially as radially (diagram below).
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