Trees have a variety of internal structures that affect the timber we get out of them. This post will highlight some of the main features in a growing tree and how that affects the finished timber that we use to build with.
Lets start with the different parts of a tree trunk. If you cut down a tree and look at cut section you will typically see the following parts.
We do not use the pith (centre of the tree) for woodworking as it is often very soft or spongy and has cracks around it. We also get rid of the dead and live bark, so we are left with heartwood and sapwood.
Sapwood is the part where sap flows up and down the tree and is vital to supporting the functions of a living tree. It contains a lot of moisture and is therefore more susceptible to cracking when dried. Sapwood is therefore less desirable than heartwood, although it is still used.
The heartwood is old "retired" sapwood and is drier and stronger than the sapwood. Sap does not run through heartwood and the pores plug up with organic material. Heartwood changes colour and composition and is usually darker and more beautiful than sapwood.
Each year the tree grows outwards and creates growth rings. During spring the growth is much more vigorous which creates larger cells with thinner walls and a lighter colour (called earlywood or springwood). After spring and early summer, the growth slows down. This creates smaller cells with thicker walls and a darker colour (called latewood or summerwood). If you look at the end of a piece of wood you can see the two different coloured sections that make up a yearly growth ring. The age of the tree can be determined by counting up these annual growth rings.
In areas near the equator where conditions for tree growth do not vary significantly between the different seasons these growth rings are not as obvious.
When timber is milled from a section of the tree, the grain pattern that is visible depends on the angle that the growth rings intersect the cut surface. There are three different ways to cut a log;- plain sawn (or flat sawn), rift sawn and quarter sawn. The most common (and cheapest) is plain sawn, with rift sawn being the most expensive.
Rift sawn timber will have parallel grains along all four sides of the timber (good for timber legs, as each side will look the same). Quarter sawn and plain sawn timber will have parallel grains on two faces and the cathedral type grain pattern on the other two faces.
There is a lot that can be learned by examining a piece of timber. Next time you see a chunk of wood, see if you can figure out how old the tree was, whether it was grown in a temperate or tropical climate and how the log was sliced.