The basics of identifying wood (ii): rings and pores

As mentioned in a  previous post, the rings form the most visible structure in a transverse surface, or cross-section through a tree, or branch. Can it be used to identify a piece of wood? Only vaguely in so much as some trees have very distinct rings, for example oak, ash, douglas fir, maple. Others have more obscure rings, e.g. birch, beech, ebony, purpleheart, poplar. This is very evident in tropical hardwoods where growth can occur all year round.

More important are pores.  Pores, or vessels, are the small circular holes visible on a cross-section of wood, and do not appear in softwoods. These vessels serve as the tree’s plumbing, transporting sap throughout the tree. The size and distribution of these pores helps determine the type of wood. So ask the question – does a cross-section of the wood show pores? YES – then it’s a hardwood, NO, then its a softwood. Some pores have contents, sometimes a result of transforming from sapwood to heartwood. One of these fillers are tyloses- which grow in open pores and can completely fill them. A good example is the difference between red and white oak. White oak contains tyloses, and does not absorb water the way red oak does, making it better for shipbuilding (check out this video to see why).

Hardwoods have three different types of pore arrangement:

• ring-porous: pores occur mainly in the earlywood, causing an abrupt transition to latewood which is very distinct. Latewood pores are more difficult to see. (oak, ash)
• semi-ring-porous: The pore transition from large to small diameter within a growth ring is gradual. (black walnut, hickory)
• diffuse-porous: The pores are uniform in size across the entire growth ring. (poplar, maple), often with no clear earlywood/latewood pore arrangement.

Here are some examples:

Three types of pore: ring-porous (oak), semi-ring porous (walnut), and diffuse-porous (hard maple)

And a close-up of what the pores would look like:

 

 

The basics of identifying wood (i)

The next few posts are just a basic overview of trying to identify wood, looking at some of the more common characteristics. So if we have an old piece of wood, how do we identify what it is?

The easiest way is to look for distinguishing characteristics, some of which will separate hardwoods from softwoods. These can include rings, pores, grain (long surface), rays, hardness, weight, smell, and colour. First some basics. All wood is designed to carry sap, and it starts life as sapwood. As a tree ages, the inner core of the tree requires more for structural support, and hence the sapwood is modified to become heartwood. An example of a cross-section from an elm tree is shown below. Notice the distinct difference between sapwood and heartwood (this isn’t always the case).

The photograph also shows the grow rings. Both softwood and hardwood have rings, although certain types of trees (e.g. tropical near the equator) lack apparent rings, because the growth cycle is all-year around. In temperate regions, wood is formed only during spring and summer, and the wood formed in the summer differs from that formed in the spring.

Springwood (or earlywood) is usually light in colour, light in weight, and contains numerous tubes to transport sap (i.e pores), and is only moderately strong. When summer takes over from spring, the tree changes the character of the wood it produces. Nutrients can now travel upwards through the springwood, but the tree needs more structural support. Summerwood (or latewood), is usually fibrous, with thick, strong walls, and small cavities – it is often darker in colour. The width of annular growth rings varies depending on water availability, sunshine etc. Fast growing trees may have thick rings, whilst slower growing trees have narrow rings. The other structure most apparent are rays – radial structures which occur perpendicular to the growth rings, used for the radial transport of sap.

The first thing to determine is of course is the wood a hardwood or softwood? A softwood is a coniferous or needle-leaved trees. It is called softwood because it is generally soft and easy to work, and it has a simple structure. A hardwood (deciduous) is a tree that has broad leaves, and is sometimes physically harder than softwoods and also denser. A notable exception is poplar. Here are some basic differences:

• Softwoods have rays that are very narrow, and barely visible.
• Softwoods contain resin (often giving them a characteristic turpentine smell).
• Hardwoods have visible pores.
• Hardwoods often develop large and characteristic rays.