The Structure of Wood

A good woodworker does not need to be an expert in botany or even dendrology (the study of trees). It does help, though, to have a basic knowledge of the internal structure of a tree.

If you are a keen woodworker, this allows you to select the best material for the job at hand. Such knowledge also permits you to spot potential defects which can show up and spoil the finished product if not picked up on.

Drying Wood

An important part of the design process is knowing how wood behaves as it is worked and as it dries. This can vary depending on the part of the tree from which it originates.

This cutaway section shows the principal elements that are common to most commercially available softwoods and temperate hardwoods…

cross section of tree

The Layers of a Tree Trunk

The outer layer is the bark of the tree. This protects the tree as it grows.

Just beneath the bark is the bast. This is a vital part of the tree’s make-up. It conducts the sugars manufactured in the leaves down toward the base of the tree for storage. The narrow cambium alongside it is responsible for creating new growth each year.

The outer layers are attractive to pests in search of the food products they contain. Most of the seasoned timber supplied for woodworking will have had them removed at some stage.

In the sapwood layer, each year’s new growth is laid down. The cells conduct the sap upwards from the root system. In some species the sapwood is distinct and pale in color. In others, there is no visible barrier between sapwood and heartwood.

Sapwood has the same strength as heartwood but is more vulnerable to attack by furniture beetle. Generally, it should be cut away. It can develop rather unsightly grayish-blue stains and is porous. This makes good glued joints and smooth finishes difficult to obtain.

The heartwood makes up the vast bulk of the tree. It is the most usable part. The heartwood cells are not alive but they give the tree all the strength it needs. Sapwood becomes heartwood as the tree grows. This is more durable and often darker. The new growth each year forms another layer and the rings enable the tree’s age to be easily determined.

Species from temperate climates where there is a distinct growing season every year have clearly visible rings. This gives an attractive striped grain figure. When a board is cut radially – through the growth rings to the center – the rays will be revealed in cross-section. These rays run horizontally through the tree. They store food deposits as the tree increases in girth. They are particularly visible in oak but not so clear in other species.

The pith is at the core of the tree trunk. It is the oldest part of the tree but not the strongest. It is the growth laid down when the tree is immature. It’s softer and rather less durable than the true heartwood. The pith can flake out when being worked. It is also prone to decay and uneven shrinkage as the wood dries. Defects can arise at the center of the pith. It is often removed when the log is processed.

Wood Grain

 Some species of timber are known as straight-grained. This means that the growth rings are tight and concentric around the heart of the tree.

The American white oak boards pictured below show the attractive striped grain figure common to woods with distinct growth rings. They have been cut tangentially – that is, across the face of the growth rings – to reveal the cone-shaped formation.

white oak boards

Knots

knots in wood

A knot is simply the remains of a branch where it grew from the tree trunk. It should not always be regarded as an outright defect. Small knots can actually create interesting and unique effects in an otherwise bland piece of timber.

However, they do cause certain problems when working the wood. You should position joints and fine detail away from the knots.

A dead knot is encased in a ring of bark where a dead branch has been absorbed by the growing tree. You can see above the difference between a dead knot and a live knot. Dead knots are to be avoided. It’s likely to shrink and fall out. In softwoods in particular, they can be very resinous which causes problems when attempting to apply a finish.

Defects

Short-grain or cross-grain defects still occur. This is a result of uneven growth or the grain being deformed around a knot. Very short or close grain is nearly impossible to work successfully with hand tools without it tearing. It drastically weakens the material. It should be sidestepped for constructional members and sections of wood close to joints.

Certain species – especially some tropical hardwoods – can always be expected to have difficult grain. It grows naturally in spiral or interlocking patterns.

Reaction Wood

This occurs where uneven growth in the trunk puts the fibers under extreme pressure. It can be identified by tight bands of growth rings that are much darker than their surroundings.

These areas are much weaker, more difficult to work and subject to abnormal movement and shrinkage. Large areas of reaction wood should be cut out when you are selecting stock.

Shakes and Checks

defects in wood

Checks are small fissures in the outer surface and are not a serious issue.

More problematic are shakes. This arise within the tree due to decay or high stresses, sometimes even due to old age.

Star-shaped heart shakes can develop in the center of a tree that has passed maturity.

Ring shakes form between the growth rings. They can be caused by the shock to the tree of being felled. A board with these defects should be roundly rejected. It is unsound and will deteriorate further.

Conclusion

This basic knowledge of the structure of wood will certainly help you when you use your miter saw or other tools to work with wood and create new projects.

It’s not necessary to become an expert on the science but a good overview will serve you well in your woodworking career.

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