Why Does Wood Warp (and what can be done about it)?

I’m in the midst of a long project which began with a tree felled in my backyard, which I then split into more manageable lumber. Since then, I’ve been further processing the lumber into planks, and the planks into boards and beams approximating the dimensions of the pieces I’ll need for the strycsitten I am making.

Today I was asked “Will the grain twist after felling?”

This is a great question! It touches on what wood is, how trees grow, how wood changes as it segues from living tree to usable material, and what we mean by “grain”.

Wood is made of long tubular fibers. These fibers make up the vascular system of the living tree, and also provide structure and strength (much as bones do in animals). There are two types of fibrous vessels: xylem (which carries water and minerals from the roots to the rest the tree) and phloem (which carries nutrient-rich water from the leaves to the rest of the tree). These vessels are bundled together, with a few more xylem vessels than phloem vessels in each bundle. Each year, the living tree adds more layers of bundles around its circumference. These rings of bundles are the familiar “growth rings” seen in the cross-section of logs. As the tree grows thicker, the vessels closest in towards the center of the tree cease operating, lose most of their water, and fill with tannins; this is the “heartwood” or duramen of the tree. The still-operating vessels further from the center are the water-engorged “sapwood” or alburnam of the tree. As you have guessed from my frequent use of the word ‘water’, it plays a critical role in what follows.

I often envision the vascular bundles as straws, but perhaps garden hoses might be more apt. The walls of the hoses hold the water in. Sever a hose, though, and the water begins to leak out through the cut end.

So when we fell a tree and cut the trunk into logs, and mill the logs into lumber, two things can happen if the wood isn’t properly prepared. One effect is that if the ends of the vessels aren’t sealed with wax, oil, or paint the severed ends (which we call the “end grain”) will lose moisture much faster than the sides. As the wood dries, the vessels shrink. So if the ends shrink faster than the middle, the ends of the lumber develop long cracks.

Photograph by the author.

But water doesn’t escape solely through the severed end-grain. The walls of the vessels also lose water (albeit more slowly). So the other undesirable thing which may happen is as the walls of the vessels lose water, the lumber may “cup”, “bow”, or “twist” (or any combination of the three). Recall the “growth rings” of the tree: as a log is divided into many boards, the orientation of the rings can differ depending on from where the board was cut and the direction relative to the center of the tree (called the “pith”).

Illustration provided by Core 77.

Plainsawn wood has the least waste, but the most variation in how the rings appear in the resulting boards. A board sliced right from the center has ring segments running almost perpendicular to the width of the board (just as in true Quartersawn). But the further out from the pith, the rings form long arcs almost parallel to the width.

The true Quartersawn is the most wasteful, but all of the ring segments are oriented the same. These segments are short and run evenly and nearly straight-through from wide face to wide face of the board.

Quarter & Riftsawn (as well as other variations) is a hybrid of Plainsawn and Quartersawn.

The problem with Plainsawn is in the relative drying speeds of the two wide faces of the board. The face from closer to the pith has more xylem and phloem vessels whose walls have been ruptured. This means that the “inside” face dries faster. As it shrinks, it causes the board to “cup”.

Image courtesy of the Highland Woodworker

A “cupped” board is one where one face has shrunk in relation to the other. Looking down the length of the board, the board will appear to have a shallow U-shaped cross section. (Please note that there are conditions –usually related to how a board was stored– in which the bark-side may be shrunken in comparison to thee pith-side, and the U will be in the other direction.)

“Bowing” is similar, but the differential shrinkage has caused the two short ends (the extremities of the lumber) to curl towards each other. Looking down the length of the board, thee two ends will appear higher than the middle.

Image courtesy of the Highland Woodworker

“Twisting” is caused by differential shrinkage and grain-direction running diagonally across a board face, with the effect of the two short ends turning in opposite directions.

All of these problems can be mitigated by carefully drying the boards. By “drying” the boards we mean controlling the evaporation of moisture until it reaches equilibrium with the moisture content of the air. This controlled drying begins by promptly sealing the end-grain and then “stack-and-stickering” the boards out of the elements. To “stack-and-sticker” means to stack the milled lumber or boards in layers separated by thin battens. The battens (very narrow, thin boards) are placed perpendicular to the boards being stacked. On top of the stack are placed weights to press down the top layer. This arrangement allows for air to flow around all of the boards’ faces while holding down the boards, preventing movement.

Illustration courtesy of RunTheMill.com

To get back the the original question, when we talk about woodgrain, we may be talking about two different things: how the fibers grew inside the tree; and, how the fibers are oriented in the board. So the answer to the question “Will the grain twist after felling?” is No and Yes. No, the grain is however it grew. But, Yes, the way the board was milled and how the board was dried can cause the grain to be in directions that cause challenges to the woodworker.

One more note on grain: trees do not always grow straight. Even when they do, they aren’t convenient rectangles ready for us to slice into smaller rectangles. Trees are more properly thought of as cones in shape, further complicated by changes in direction of growth caused by chasing the Sun and avoiding shade. The tree I harvested for the strycsitten project had an obvious spiral growth to it when it was standing. I knew from the outset this would mean the grain grew so that boards split from it using wedges (as I did) would have a slight twist along their length. I’ve been dealing with this through careful and tedious planing and sawing, to find rectangles within the odd shapes.

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