Wood

An introduction to the anatomy and chemistry of wood

To understand how wood behaves, it helps to have some understanding of the structure of the living tree.

Wood in a living tree is composed of cells—with cell walls made of cellulose—which transport food and waste products through the tree.

As the tree grows, new cell layers are added to its outer circumference, forming seasonal—generally annual—growth rings.

Eventually the older cells in the inner part of the tree produce lignin in the cell walls and die.

This creates an area of wood in the centre of the tree stem which is comparatively dry—it still has bound water—and is free of sap. This is known as heartwood.

The outer, moister portion of the wood is called sapwood.

The differences between the types of cells and their relationship to each other determine the characteristics of particular species of trees, such as their colour, grain and strength.

Many trees also contain other chemicals such as resins and oils which affect the nature and appearance of their wood.

To help with identification, timbers are divided into two distinct groups, which are based on their botanical order:

• softwoods are derived from conifers—gymnosperms. Softwoods have a more uniform structure than hardwoods, but are not necessarily softer; balsa wood, the softest timber, is actually a hardwood; and

• hardwoods are derived from dicotyledons or broad-leaved trees—angiosperms.

There are significant structural differences between softwoods and hardwoods. These allow them to be distinguished from each other by microscopic examination. Every timber species has a characteristic arrangement of cells and tissues which enables it to be specifically identified. The tree structure determines the properties of the resultant timber-for example, strength, degree of shrinkage, durability, resistance to biological attack, porosity and moisture permeability.

The susceptibility of wood to damage from a number of factors depends on the chemical composition of the wood—that is, the percentages of the various components, such as cellulose, lignin and resins. This composition varies, depending on the original species of tree, the part of the tree used and the seasoning process.

When trees are cut for timber, the moisture which was present in the living wood dries out, until the wood reaches its equilibrium moisture content; this is called seasoning.

The equilibrium moisture content—EMC—of a particular piece of wood varies according to the relative humidity of its environment.

If the relative humidity increases wood will absorb water and its EMC will rise. The absorption of water causes wood to swell. If the relative humidity decreases, the wood's EMC drops and it shrinks.

If wood is seasoned too quickly, the sapwood dries and shrinks faster than the inner heartwood—this causes the sapwood to crack.

How much the wood shrinks when it dries depends also on where, within the tree, the wood comes from.

Wood perpendicular to the grain shrinks substantially more, that is, it shrinks across the grain rather than down the length of the grain.

Wood warps when it swells and shrinks at different rates.