From cells to defects: anatomy, sawing pattern and warp

Why the cell structure explains the board

The behaviour of a board was largely decided before the saw touched it: ring width, pith position, sapwood share and cell wall make-up all feed into shrinkage, warp and checking.

Why tangential exceeds radial

There is no single accepted explanation. Xue and co-authors (2018, China) group the proposals as:

• Ray restraint: rays run radially and may hold back radial shrinkage, though the study found this incomplete.
• Earlywood and latewood: latewood shrinks more than earlywood, which raises tangential shrinkage.
• Cell wall structure: a study of Scots pine tracheids (Gu and co-authors, 2001) found the radial wall of latewood cells about 25 % thicker than the tangential wall and a preferred orientation of the S2 fibrils, and suggested this might explain the anisotropy to a major extent.

For practical purposes the consequence is enough: the ring-following direction moves about 1.5 to 2.5 times as much.

Microfibril angle, juvenile wood and reaction wood

The angle of the cellulose microfibrils in the S2 layer is the most informative single wall parameter. Mature conifer wood has an S2 angle of 5 to 20 degrees; juvenile wood, the inner 10 to 15 rings, has the highest angles, especially in the first five rings at the base of the tree (Donaldson, New Zealand review). Higher angles go with more longitudinal shrinkage. Longitudinal shrinkage well above 0.1 to 0.2 % is a flag for juvenile or reaction wood.

A New Zealand study of radiata pine (Haslett and co-authors, 1991) shows the practical effect. Corewood (the first 10 rings) made up 44 % of log volume in logs over 400 mm but 99 % in logs of 150 to 199 mm. Twist rejection of 100 x 50 mm framing ranged from 18 % for the largest logs to 70 % for the smallest. These figures are from one stand and product, not general rates. Small logs contain the most corewood.

Sapwood, heartwood and rays

Heartwood extractives plug passageways, so heartwood generally dries more slowly than sapwood, although heartwood often starts drier (Lesson 2). Rays are planes of weakness; oaks, beech and sycamore check readily because of broad rays (Conners). Moisture moves along the grain 10 to 15 times faster than across it (Bergman), so board ends dry first and end checks form unless ends are coated.

Warp types and their causes

All warp comes from differential shrinkage (Wengert and Meyer). Bergman lists six types: bow, crook, twist, oval, diamond and cup.

• Cup: across the width, from the tangential versus radial difference. Flat-sawn lumber cups toward the bark side. Lumber cut near the pith is more cup-prone (Mitchell's model).
• Bow and crook: along the length, driven by longitudinal shrinkage differences from juvenile wood, reaction wood and sloping grain.
• Twist: linked to spiral grain; in 190 Norway spruce studs (Sweden), grain angle and ring curvature explained 73 % of the variation in twist.

Sawing pattern

Ring-angle limits differ by source: the Wood Handbook gives 0 to 45 degrees for plainsawn and 45 to 90 for quartersawn.

Worked example: choosing a pattern

You must supply 200 mm wide bur oak ending at 8 % MC. From Lesson 5 the flat-sawn board loses about 13 mm in width and the quarter-sawn about 6.5 mm, a difference of 6.5 mm. The trade-off: quarter-sawn dries more slowly and its edges are more crook-sensitive.

Checklist for log and board sorting

1. Note whether the pith is in the board; sort pith boards separately where the product allows.
2. Record log diameter; small logs carry more corewood.
3. Look for compression wood on the faces as a clue to bow and crook direction.
4. Coat the ends of valuable stock.

Key points

• Tangential shrinkage exceeds radial because of overlapping causes: ray restraint, earlywood and latewood, and the cell wall structure.
• High S2 microfibril angle in juvenile and reaction wood goes with high longitudinal shrinkage and warp.
• Small logs contain the most juvenile wood (radiata pine corewood: 44 % of volume in logs over 400 mm, 99 % in logs of 150 to 199 mm).
• Cup is mainly a sawing-pattern problem; bow, crook and twist come from longitudinal differences and grain.
• Flat-sawn boards move most in width; quarter-sawn boards move about half as much but dry more slowly.