Pith, reaction wood, shake, wane and resin

This lesson covers features that come from the centre and shape of the tree: pith and juvenile wood, reaction wood, shake, wane and resin. Several show up only after sawing or drying, so they cause claims as well as downgrades.

Pith and juvenile wood

The pith is the soft central core, surrounded by juvenile wood. Conners (University of Kentucky) defines juvenile wood as the first 10 to 15 growth rings, which shrink more along the grain than mature wood. Boards containing the pith ("boxed heart") tend to crack radially in drying (general trade knowledge, not checked in the portal's sources), and many specifications limit or exclude pith. In the DIN 4074-1 table in lesson 5 pith is permitted in S7 and S10 but not in S13, except in squared timber wider than 120 mm.

Boards from near the pith are the most likely to bow, crook and twist, and fast-grown plantation wood has a larger juvenile core. The boundary depends on the property (in lodgepole pine about 15 years for microfibril angle, 31 for density), so a single ring number is a convention.

Reaction wood

Leaning or wind-pushed trees build special wood to straighten the stem. Softwoods form compression wood on the underside of the lean; hardwoods form tension wood on the side opposite the lean (Penn State Extension, North America).

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 on the wider side. Severe zones are denser and usually darker; mild zones are hard to see even on a fresh surface. Lean, sweep and crook are warning signs.

On the board. Compression wood has lower stiffness despite higher density. Tension wood fibres are partly torn out rather than cut clean, it can look uneven when stained, and it is more likely to collapse in drying.

Shrinkage along the grain. The US Wood Handbook gives 0.1 to 0.2 percent along the grain from green to oven-dry for most species, and says reaction wood and juvenile wood can shrink about 2 percent. By arithmetic that is 2 / 0.2 = 10 to 2 / 0.1 = 20 times normal, in line with an Australian trade source (up to 20 times). Penn State gives lower multiples (up to six times for compression wood, three to five for tension wood), and no source resolves the difference.

Worked example. On a 4 m board, 0.1 to 0.2 percent is 4 to 8 mm green to oven-dry, while 2 percent is 80 mm. A kiln-dried board loses less than the oven-dry figure, but a board that is part normal wood and part reaction wood bends because the parts shorten by different amounts. That is the mechanism of bow and crook. Johansson (2003) cut studs into sticks and found that sticks with severe compression wood shrank significantly more along the grain between 18 and 8 percent MC than sticks with none or mild compression wood.

Shake

Shake is a separation along the grain. Ring shake follows the growth rings and looks like a partial or complete circle on a log end; star shake runs radially from the pith. It sometimes shows only after sawing or drying. The portal's figures are for British oak (Price, 2015): about 21 percent of high forest oak trees were estimated to be affected, and one highly shake-prone site had 73 percent affected, with a likely financial loss of about 21 percent on the sale parcel. They must not be transferred to conifers. DIN 4074-1 allows no ring shake in any of its three classes. Shake is not visible on a sawn face until the board is cut, so log-end inspection matters.

Wane

Wane is missing wood at an edge or corner where the log's rounded surface or bark remains. It appears when the log is small, tapered or crooked relative to the sawing pattern, and optimisers reduce it through offset and skew. DIN 4074-1 allows wane up to 1/3, 1/3 and 1/4 for S7, S10 and S13, while a UK supplier summary gives wane under 3 mm for oak grade QF3: a fraction and a size, not comparable.

Resin

Resin pockets are cavities holding resin, found mainly in softwoods such as spruce, pine, larch and Douglas fir. They matter for painted and visible surfaces (general trade knowledge, not sourced in the defect article). Swedish Wood says spruce usually has pitch pockets and pine rarely does. Teknos adds that bonding problems arise in contact with resin and increase with resin content, and that very resinous wood can discolour, especially under light coatings.

Key points

• Pith and juvenile wood (first 10 to 15 rings by one definition) shrink more along the grain and raise the risk of bow, crook, twist and cracking.
• Compression wood forms on the underside of the lean in softwoods; tension wood on the opposite side in hardwoods. Eccentric pith with wide rings on one side is the log-end clue.
• Reaction wood can shrink about 2% along the grain against 0.1 to 0.2% normally; sources disagree on the multiple.
• Shake may show only after sawing or drying; the incidence figures are for British oak, not conifers.
• Sort resin-rich boards away from glued or lightly coated uses.