Reaction wood and grain deviations

Two features can ruin a board that looks acceptable on the saw line: reaction wood, which makes boards bow and crook in the kiln, and spiral grain, which makes them twist.

Reaction wood

Trees that lean, or whose crowns are pushed by wind or slope, build special wood to straighten the stem.

• Compression wood forms in softwoods on the underside of the lean.
• Tension wood forms in hardwoods on the side opposite the lean (Penn State Extension, North America).

Both shrink abnormally along the grain.

On the log. The clearest sign on a log end is eccentric growth: an off-centre pith with wider rings on one side, the reaction wood sitting on the wider side. Severe zones are denser, harder and usually darker (Wood Review, Australia), but mild zones are hard to see even on a fresh surface. On the log deck, lean, sweep and crook are warning signs.

Compression wood has lower stiffness despite higher density; its across-grain shrinkage tends to be lower than normal, while its longitudinal shrinkage is much higher. Tension wood fibres tear out rather than cut clean when sawn, and it has a higher tendency to collapse in drying (Penn State Extension).

How big is the longitudinal shrinkage?

Normal wood shrinks very little along the grain. Sources differ on the reaction-wood multiple:

The Handbook figures give roughly 10 to 20 times normal by simple arithmetic (2 / 0.2 = 10, 2 / 0.1 = 20), consistent with the Australian upper figure. The sources are not fully reconciled, so quote a range, not a single multiple.

Worked example: what does it mean for a board?

Take a board 3.0 m (3,000 mm) long. Using the Handbook figures:

• Normal wood at 0.2 percent: 3000 x 0.002 = 6 mm
• Reaction wood at 2 percent: 3000 x 0.02 = 60 mm

If a board has reaction wood along one edge and normal wood along the other, the edges end up with very different lengths, and the board must bend. The difference of up to 54 mm over 3 m is an upper illustration for the extreme end of the figures; real boards are milder. This is why reaction wood leads to bow, crook and sometimes twist, and why distortion can continue after drying as locked-in stresses release.

Cross grain and spiral grain

Cross grain means fibres not parallel to the board axis. It is either:

• Diagonal, caused by sawing a tapered or crooked log parallel to the bark rather than the pith (a sawing issue), or
• Spiral, where fibres wind around the stem (a growth feature).

Spiral grain matters mainly because it causes twist on drying. In Swedish Norway spruce progeny trials (trees 28 or 29 years old, measured near breast height; Hallingbäck and co-authors, 2008, Europe) the mean grain angle under bark was 1.76 degrees, but about 7 percent of progeny trees had 4 degrees or more. Broad-sense heritability was 0.44 to 0.63, so seed-source selection can reduce the problem over generations. The grain angle rose over the first rings from the pith, peaked near ring 10 at about 3 degrees and then declined.

Worked example. In a supplier region that resembles this dataset, a stand of 400 sampled trees would contain about 400 x 0.07 = 28 trees at 4 degrees or more. A mean of 1.76 degrees therefore hides a tail of trees that can twist badly.

Checklist for reaction wood and spiral grain

• Read the log end: eccentric pith, wide rings on one side.
• Steer suspect logs away from long, thin, straight products.
• Segregate suspected reaction wood for gentler kiln schedules.
• Run twist trials on sample boards from each supplier region.

Key points

• Compression wood forms on the underside of the lean in softwoods; tension wood on the side opposite the lean in hardwoods.
• Longitudinal shrinkage of reaction wood is about 2 percent against 0.1 to 0.2 percent normal (Wood Handbook, North America), roughly 10 to 20 times; other sources quote lower multiples.
• Uneven shrinkage across a piece causes bow, crook and sometimes twist.
• Spiral grain causes twist; in the Swedish spruce data the mean was 1.76 degrees but about 7 percent of trees reached 4 degrees or more.
• Eccentric pith and lean, sweep or crook on the log deck are the warning signs.