How wood works

Paul Epp
Paul Epp

I think that I shall never see,a poem as lovely as a tree… — Joyce Kilmer.

Actually, a tree is simply an apparatus that facilitates photosynthesis. It puts the little factories, the leaves or the needles, up there out of the way of other things where they can do their job.

The tree itself is the plumbing that supplies those factories with water and its dissolved nutrients, drawing it up out of the ground, and the minimal structure that provides the maximum amount of operation with the minimum amount of material.

Wood is simply a by-product of this obsession of the tree with turning sunlight into oxygen and carbohydrates. This image of plumbing is, I think, quite useful in understanding the nature of the material that we know as wood.

It is a material composed of little tubular cells, attached end-to-end and parallel to each other and oriented along the (mostly vertical) length of the tree and its branches. These little hollow cells are made of cellulose and are bound together with a material known as lignum.

A second type of cell, which is much fewer in number, is oriented at right angles to the primary ones and allow for lateral fluid movement. These are known as rays and are more-or less pronounced, depending on the species, but are often invisible.

Trees grow from their outer perimeter and their tips, adding new layers of material. The active and lively layer is known as the cambium and is just under the protective bark of the tree.

The health of this tissue determines the health of the tree. As the tree grows, the woody tissue converts from active plumbing to the role of more dormant scaffolding structure. It goes from being sapwood (sap being the liquid that moves up and down) to heartwood.

The wood that we value is usually the heart (wood) of the matter. If we intend to use this material for the construction of things, there are a number of important considerations.

One is that wood is, in its natural state, quite wet. It is essentially plumbing, after all.

Once the tree is no longer alive, this moisture will seek equilibrium with its surrounding environment. It dries out. As it dries, the cellulose cells shrink in diameter. Not in length, only in diameter.

To complicate things, the shrinkage is uneven; with approximately twice the shrinkage occurring along the diameter of the tree as occurs along a path radial to the centre of the trunk, although this varies with the species of tree.

This change in the size (and weight) of the material usually needs to be predicted and accommodated by the wood worker. Once the wood’s moisture is in a state of equilibrium with its environment, the change in size stops and the material is stable.

If the drying out occurs too rapidly, the material will likely split. It is usually worth remembering that as the relative humidity of the environment changes with the seasons, the dimensions of the wood will also change. In Canada, this usually means that wood shrinks in the winter and swells in the summer, making doors and drawers stick or loosen and unsecured panels warp.

If solid wood isn’t allowed to change its dimensions, it may split (or burst things) instead. The splitting that occurs during drying is a vulnerability that wood retains.

In addition to the stresses of drying, this splitting can occur through other means. When we split wood for the fireplace we soon learn that wood only splits along its (original) length. The little plumbing tubes are driven apart.

To deform the cells across their circumference requires a completely different magnitude of effort. We use this characteristic as an essential component of our strategy when we work wood.

A different type of consideration is that trees grow the most in the spring, when they are fresh from their winter repose. Later, in the summer, the growth slows down and changes in character.

The early (spring) wood is usually lighter in colour and thicker in volume. The later (summer) wood is darker and occupies less space.

This contrast between the early and late wood provides a record of the age of the tree and the wood with a visual differentiation that gives it what we call figure. This varies hugely between wood species, with pine, for instance, having little visible figure, with ash having a great deal.

Colour variation and the range of less usual structures of the cells contribute to this visual distinctiveness. It is also worth noting that the dimensions of the wood that is available will correspond to their source, the trunks of trees.

Natural, un-jointed, solid wood in sheets four-feet by eight feet, for instance, is not feasible. This is information that is very important to those of use who design for the use of wood.

What I’ve written here is a highly simplified description of a much more complex topic. Even so, I’m often surprised by how it is often not taken into consideration.

You might also like