by Steven J. Owens (unless otherwise attributed)
A lot of what makes working wood complicated is the grain and wood "movement".
Grain refers to the fact that wood isn't a homogenous substance, it has structure, and that structure affects how wood wants to split and cut, and how our tools interact with that.
Movement refers to the fact that wood expands and contracts as its moisture content changes, which typically happens over the course of the year as the ambient humidity changes. Wood always moves. And, unfortunately, wood movement is never uniform and even.
This all makes a heck of a lot more sense if you understand the fundamental structural nature of wood.
Wood is composed of fibers, which are mostly cellulose, and the fibers are embedded in a natural resin-like substance called lignin. The fibers are about twice as strong as the lignin. That's most of what you need to know about that part, it's how those fibers are arranged that complicates things.
Wood movement, and woodworking, would be simpler if the fibers in boards were all arranged in evenly spaced, rectangluar grids, with the fibers perfectly parallel to each other and to the sides of the board. But they're not.
These are the four biggest facts in understanding wood grain and wood movement:
Now we cut that tree down and cut off the ends, and now it's a log.
When a log "moves", aka expands or contracts due to changes in moisture in the wood, which is in turn due to changes in ambient humidity over the year, the entire circle of the growth ring will expand or contract.
But boards are rectangular in cross section and are cut out of that cylinder. This means that when you go from a cylindrical log, composed of fibers arranged in circles, to a rectangular board, the fibers in the board are always going to be arranged in some chunk of a circle, and in addition to the circular aspect, the spacing of the fibers and growth rings is always going to be uneven.
So you can see that a board's behavior will be determined by:
"Radial" and "tangential" are two words that woodworkers often use in talking about this stuff.
"Radial" means the dimension from the center of the log straight out.
"Tangential" in woodworking means perpendicular to the radial line, i.e. tangential to the circle of the growth rings. Kinda sorta, geometrically speaking; in geometry and math, the precise meaning of tangential is a straight line that touches a circle at exactly one point; they say that line is tangential to the circle.
"Longtitudinal" means running parallel to the long dimension of the log, but woodworkers don't really talk about longitudinal much, because longitudinal wood movement is tiny:
The amount of wood movement in these three different directions is different: as a rule of thumb, from freshly logged tree trunk to board it's around:
Some rules of thumb:
(* Relative humidity is a bit of a tricky thing. Long story short absolute humidity is, for a given volume of air, what percentage of that air is water vapor. But the temperature limits how high the absolute humidity can be at any given moment. Relative humidity is what percentage of that maximum-absolute-humidity-at-current-temp is water vapor. Since relative humidity dictates how water evaporates or condenses, that's mostly what we care about for human comfort, or for wood movement.)
"Seasoning" the wood (letting it dry naturally over time) or "kilning" it (using heat to dry it more rapidly) not only changes the moisture content (and size) of the wood, it also makes the wood absorb moisture more slowly (because the fibers are all contracted and hence smaller diameter and hence water vapor diffuses into the more slowly). So the board you get will probably not expand/contract as much as those rule of thumb amounts.
Wood terminology, like pretty much all human language, is all over the map, sloppy and nobody's in charge and nobody agrees on it all, but here's one popular set of terms, to make the rest of his discussion easier. If you have, say, an 8 foot long 2x6 it has 6 sides:
(For these examples, to keep the math simpler, we'll assume I'm talking about "true 2x6", meaning they actually are 2" and 6". This is also sometimes called "actual lumber", because, well, it's the actual size of the piece of wood.
The 2x6 board you buy at a home improvement store (in the US) is in reality neither 2 nor 6. It's a "nominal 2x6", nominal from Latin "nomen", for name, so it's a "2x6 in name only". This is also sometimes called "dimensional lumber", meaning it's sold in the final dimensions it's expected to be used in.
We'll talk about why all that is (in the US, at least) some other time.)
Leaving aside the longitudinal movement, which again at 0.01% is negligible:
Let's work that through:
The circumference of a circle is 2 times pi times the radius. So a 24" diameter log has a radius of 12" and a circumference of 2 x 3.142 x 12 = 75.4".
As any idiot can see, 75.4" is a lot bigger than 12", it's roughly 6.25 (2 x 3.142 = 6.284) times as big.
If the log's radius increases 1/2", in other words, goes from 12" radius to 12.5" radius, the circumference will increase 6.25 as much, from 75.4" to 78.54".
A rule of thumb is that for every 4% of moisture content change, the timber will grow or shrink by 1%. This is only a rule of thumb, the specific details of the wood species, etc, will affect things.
If you saw a 2x6 board from a log fully radial, that means the narrow edges of the board are going to be tangential to the circumference of the log, and the wide faces of the board are going to be paralle to the radius of the log. That, in turn, means that the wide faces are going to increase (percentage wise) in width less than the narrow edges are.
On the flip side, if you saw a board from a log fully tangential, the wide faces of the board are going to be perpendicular to the radius of the tree, and will increase (percentage wise) in width more than the narrow edges do.
This is where it gets more complicated, because unless you sawed that second 2x6 out of the outermost layer of a truly large log, those growth rings are going to be curved to some degree.
Obviously, the circles at the center of the tree trunk are smaller and tighter than the circles out near the edge of the tree trunk, which means they're more steeply curved, and that steep curvature affects how the wood changes shape as it "moves".
Also, because the fibers stay a consistent size but the circles don't, in the early years of the tree growing, the growth circles are added quickly, which makes the growth circles more widely spaced, which, agian, changes how the wood behaves as it expands and contracts.
Finally, the very nature of the early growing period produces thinner, weaker wood cells, often described as "spongy." See the section "Wood Layers", below.
Also, in an ideal world the fibers would all be straight the entire length, and level with respect to each other, and parallel to each other, and the face of a given board would all have continuous fibers from end to end. We don't live in an ideal world, so often the ends of fibers come up in the middle of a board, etc, and that affects how your cutting tools interact with the board. Look at some of Rex Krueger's videos about planing and grain for specific examples.
Note that I'm always using the word "fiber" here, but a lot of discussions of wood refer to "grain", meaning the specific arrangement and angles of the fiber in a given piece of wood.
As a quick side note, "riving" is a traditional technique for making boards by splitting rather than sawing. Because it splits between the grains, that means that the board does (mostly) have fibers that continue all the way from one end of the board to the other. This makes the board stronger. English longbows, for example, start life as a riven board, though they're then cut and sanded and etc, so the finished product is no longer solely end-to-end fiber.
Riving as a term mostly survives, in modern, hobby woodworking, in the form of the "riving knife" on table saws. This is a little knife that sticks up, just beyond the blade, to keep the two halves of the already-cut part of board slightly spread as you feed the rest of the board in. This reduces the likelihood of saw table "kickback". Saw table kickback is a very important safety topic you should learn about, but not in this document.
To further complicate life, even though my description above, about wood becoming distorted as it moves, generally assumes the fibers are longitudinally straight and parallel, in real life they sometimes aren't. Trees can grow crooked, or twisted. Trees can grow leaning at an angle. The individual fibers in the wood of a given tree can be in tension to one another; that tension can then be released when you saw the boards down smaller, and suddenly your straight board is now curved.
The only real answer to those complications is to:
Generally speaking, going from the center out, you have some big picture layers:
Pith: The very center of the tree trunk. Dead wood. Weak, thin-walled wood cells. Small/tight growth rings, often softer wood, often a different color. Weak, prone to cracking, prone to drying faster (and therefore move "movement" and splitting. Usually removed during milling.
Corewood or Juvenile wood are terms people sometimes use, reserving "pith" for the very small center part alone. Corewood/juvenile wood is the pith proper plus the first few years of rings (aka first few "annual rings").
Heartwood: Surrounds the pith. Dead wood. In the living tree, serves mainly a structural purpose. Strong. Because it's dead and no longer serving as sapwood, the hollows fill in with other stuff, "extractives" (whatever that means) and minerals. Because it's filled in, it's denser, harder and stronger than sapwood.
Sapwood: Surrounds the heartwood. Live wood. Transports water, minerals, plant sugars between roots and leaves. Often lighter in color than the heartwood.
Cambium: Surrounds the heartwood in a thin layer. Live wood. Where new wood grows. Cambrium growth speed varies across the year, faster at the beginning of the season (early wood, springwood), slower (and denser, and harder) as it warms up (later wood, summerwood). This is why wood has growth rings.
Bark: Surrounds the Cambium. Protective layer.
A good overview, with some good photos:
https://woodcarvingillustrated.com/anatomy-of-wood/
See also:
https://www.wood-database.com/wood-articles/hardwood-anatomy/
And
https://www.kaltimber.com/blog/how-to-understand-wood-movement