What is Wood?

It’s common knowledge that wood comes from trees. What may not be so apparent is the structure of the wood itself, and the individual components that make up any given piece of lumber. Unlike a mostly homogenous piece of foamboard, MDF, or other man-made material, wood is an organic material, and has many distinct characteristics which will be helpful to learn.

Eric Meier

 

Hardwoods and Softwoods

An immediate and broad distinction that can be made between types of trees (and wood) is the label of hardwood or softwood. This is somewhat of a misnomer, as the label is actually just a separation between angiosperms (flowering plants such as maple, oak, or rosewood), and conifers (cone-bearing trees such as pine, spruce, or fir).

Hardwoods (angiosperms) have broad-leaved foliage, and tend to be deciduous—that is, they lose their leaves in the autumn. (However, many tropical hardwood species exist which are evergreen—they maintain their leaves year-round.) Additionally, hardwood trees tend to have a branched or divided trunk, referred to as a dendritic form.

This White Oak tree—with a branching form, and leaves that drop in the autumn—is characteristic of most angiosperms. Their high density and rich heartwood colors make hardwoods well-suited for furniture and interior woodwork.  

 

Softwoods (conifers) tend to have needle or scale-like foliage, though in some uncommon instances, they can have rather broad, flat leaves, such as Kauri (Agathis australis). Most softwood trees are evergreen, however, some conifers such as larch or cypress lose their foliage in the autumn, (hence the common name “bald-cypress”).

Softwoods tend to have a single, dominant, straight trunk with smaller side branches, referred to as an excurrent form—this cone-shaped growth form helps trees in temperate climates shed snow. Again, there are several conifers that are an exception to this growth form, such as Cedar of Lebanon (Cedrus libani).

 

This spruce tree is a good representation of a conifer with evergreen, needle-like foliage and a single, dominant trunk. Their long, straight trunks and lightweight timber make softwoods well-suited for structural building purposes.

The confusion in labels arises in that the wood of angiosperms is not always hard (a glaring example is Balsa, which is technically classified as a hardwood), while the wood of conifers is not necessarily always soft, (an example of a relatively hard softwood would be Yew). However, as a rule of thumb, hardwoods are of course generally harder than softwoods, and the label is still useful to distinguish between two broad groups of trees and certain characteristics of their wood.

Sapwood and Heartwood

As the cambium forms new wood cells, they develop into different sizes, shapes, and orientations to perform a variety of tasks, including food storage, sap conduction, trunk strength, etc. When a tree is young, certain cells within the wood are alive and capable of conducting sap or storing nutrients, and the wood is referred to as sapwood.

After a period of years, (the number can greatly vary between species of trees), the tree no longer needs the entire trunk to conduct sap, and the cells in the central part of the stem—beginning at the core, or pith—begin to die. This dead wood which forms at the center of the trunk is thus called heartwood.

The transition from sapwood to heartwood is accompanied by the accumulation and buildup of various deposits and substances, commonly referred to as extractives. Most notably, these extractives are responsible for giving the heartwood its characteristic color: the jet-black color of Ebony (Diospyros spp.), the ruby-red of Bloodwood (Brosimum rubescens), or the chocolate-brown of Black Walnut (Juglans nigra)—all owe their vivid hues to their respective heartwood extractives. (Without extractives, the sapwood of nearly all species of wood is a pale color, usually ranging from white to a straw-yellow or grey color.)

But heartwood extractives are responsible for more than just color; extractives increase (to varying degrees) the heartwood’s resistance to rot and decay, as well as giving it added stability and hardness. (Sapwood has virtually no resistance to decay—which, being essentially alive in the tree, is not needed.) From a biological standpoint, it’s easy to see the benefits that heartwood brings to the tree as it grows taller and broader—and incidentally, many of these same benefits translate into advantages for woodworkers as well.

However, it should be noted that the transition area from sapwood to heartwood, commonly referred to as the sapwood demarcation, can vary from gradual to very abrupt: this can be important in wood projects where decay resistance is needed. A clear line of demarcation helps prevent the inadvertent inclusion of sapwood, and minimizes the risk of subsequent rotting or structural damage.

Grain Appearance

Although quartersawn and flatsawn surfaces are named after their original method of sawing, in practice, the terms typically just refer to the angle of the growth rings on a piece of processed lumber, with anything between 45° and 90° being referred to as quartersawn, and anything between 0° and 45° generally being flatsawn, regardless of how the log was actually milled.

There’s sometimes an intermediate angle commonly called riftsawn or bastard grain, which corresponds with growth rings angled between 30° to 60°, (with 45° being the average). Although it is called riftsawn, sawyers will rarely if ever specifically saw up a log in order to get such an angle; usually the name merely serves as a convenient term to describe wood that is not perfectly quartersawn.

Additionally, the term face grain usually denotes the most predominant/widest wood plane on any given piece of lumber (excluding the endgrain), and does not refer to any specific cut. By observing the angle of the growth rings—as when looking at a stack of boards where only the endgrain is visible—a reasonably accurate prediction of the appearance of the face of the board can be made. Likewise, in many instances where only the face of a board is visible, the endgrain may be extrapolated by “reading” the grain pattern. Each grain cut has varying strengths and weakness, and is used in different applications.



 



 

 

  

 

 

 

 

 

 



 

 

 

 

 

 

 

 

 

    Note the appearance of each of the faces of these three boards, as well as their corresponding endgrain surfaces beneath. On the left, Beli (Julbernardia pellegriniana) is almost perfectly quartersawn, resulting in a straight, narrowly-spaced, and uniform grain pattern. In the middle, Ponderosa Pine (Pinus ponderosa) is flatsawn, resulting in a characteristic “cathedral” grain pattern. On the right, Western Hemock (Tsuga heterophylla) has a section on the left that is flatsawn, grading down to riftsawn, as reflected on the face of the board, which appears flatsawn on the wild portion on the left, and closer to quartersawn on the straighter and more uniform portion on the right.

Quartersawn boards are very uniform in appearance and are good for long runs of flooring where the boards need to be butted end-to-end with minimal disruption in appearance. Quartersawing also produces the stablest boards with the least tendency to cup or warp with changes in humidity, which is very useful in many applications, such as for the rails and stiles of raised panel doors. However, because of the extra handling involved with processing the log, and the higher waste factor, quartersawn lumber tends to be more expensive than flatsawn lumber.

Most would agree that flatsawn boards—with their characteristic dome-shaped cathedral grain—tend to yield the most visually striking patterns, (and it should come as no surprise that many veneers are also rotary-cut from logs to reproduce this appearance). Flatsawn boards are also available in wider dimensions than quartersawn stock, and are well-suited to applications such as raised or floating panels, or other areas where width or appearance are important.

Riftsawn wood lies somewhere between these two aforementioned grades. It has a uniform appearance that is very similar to quartersawn wood—and it’s nearly as stable too. On large square posts, such as those used for table legs, riftsawn wood has the added benefit of appearing roughly the same on all four sides, (since the rings on each of the surfaces are all at approximately 45° angles to the face), whereas quartersawn squares would have two sides that display flatsawn grain, and vice versa.

 

Eric Meier