Growth ringsOne ring is one growing season: pale, fast earlywood in spring, dense dark latewood in late summer. Count them and you have the tree's age.
Every board is a slice of weather.
A tree writes down each year it lives: wide rings for wet summers, tight ones for drought. This is a field guide to reading the grain, choosing a species, and knowing why a table moves in winter.
Read the grainAnatomy
What you are looking at when you look at grain
Wood grows outward in layers. Cut a log and the layers show as rings. Cut a board and the same layers show as lines, arches and the occasional knot.
Cathedral figureWhen a board is sawn tangent to the rings, the cut slices each cone-shaped layer at a slant, and the lines bend into arches.
KnotsA knot is the base of a branch that the trunk grew around. It is harder, darker, and it does not move with the grain around it.
- Hardwood or softwood
- The words are botanical, not about hardness. Hardwoods come from broadleaf trees, softwoods from conifers. Balsa is a hardwood.
- Heartwood and sapwood
- The dark core of a trunk is heartwood, no longer carrying water. The pale outer band is sapwood. Many species are darkest in the middle.
- Plain-sawn or quarter-sawn
- Plain-sawn boards show cathedral arches and move more. Quarter-sawn boards show straight lines and stay flatter.
Species
Eight boards from the sample rack
Hardness is the Janka test: the force in pounds needed to push a steel ball half its width into the wood. Higher is harder to dent, and harder to work. Figures are rounded averages.
White oak
Quercus alba
1,360 lbf
Black walnut
Juglans nigra
1,010 lbf
Black cherry
Prunus serotina
950 lbf
Hard maple
Acer saccharum
1,450 lbf
White pine
Pinus strobus
380 lbf
Mahogany
Swietenia macrophylla
800 lbf
Gaboon ebony
Diospyros crassiflora
3,080 lbf
Yellow birch
Betula alleghaniensis
1,260 lbf
Log to board
Five steps between a forest and a shelf
Most of what goes wrong with wood goes wrong in the second and third steps, long before it is a table.
Fell and log
A tree is cut and bucked into logs. Fresh wood is mostly water, sometimes more than half its weight.
Saw
The sawyer chooses how to cut the log. Plain-sawn gets the most boards. Quarter-sawn gets the most stable ones.
Dry
Boards are air-dried or kiln-dried until the wood holds only 6 to 12 percent moisture, the level indoor air will keep it at.
Mill
Dry boards are flattened and squared. This is when hidden stress shows: a board that was dried too fast will cup or bow.
Finish
Oil soaks in and darkens the grain. Wax and lacquer sit on top. None of them stop wood from taking on and losing moisture.
Movement
How much will it move?
Wood swells when it takes on moisture and shrinks when it loses it, mostly across the grain and most of all along the growth rings. Pick a species, a board width, and how many points of moisture content the air will change.
0 mm
An estimate, not an engineering number. It assumes the wood moves in proportion to its tangential shrinkage from green to oven-dry (taken from USDA Wood Handbook averages), spread evenly over the roughly 28 points of moisture content between fully green and the fibre saturation point. Real boards depend on how they were cut.
Fire
The last thing wood does
Firewood burns best when it has been seasoned, split and stacked off the ground for six months to two years, until it holds under about 20 percent moisture. Wet wood spends its heat boiling off water, and smokes.