Canada's forests produce 77 percent softwood and 23 percent hardwood species. With this page on softwoods and a separate page about hardwoods, VCR is giving both "equal billing." Many designer/makers would question this, as they tend to specify hardwoods for furniture projects and consider all softwoods "inferior." This wood science resource page will attempt to reverse that attitude.

Cellular Structure

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."

Density and Grain Direction

The chart shows that the density of common Canadian 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).

Moisture Movement

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.

Shrinkage Direction

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).

More Information

Readers wanting more detailed information on Canadian and North American wood science can consult Understanding Wood and Identifying Wood by R. Bruce Hoadley.

For a complete list of VCR's wood science related resources, check the menu (above, left) and the sidebar (right) for external links.

VCR also provides resource pages on small-scale kilning, wood bending and hand tools.