Woodworking is widely considered an artform as it involves creating new wooden items by means of cutting, shaping, and crafting wood. While this notion is true, it is also safe to assert that woodworking, as an art form, is based on the science of wood. It is only by understanding the natural properties of wood that you can acquire the substance and technique needed to aptly undertake even the most demanding projects.
For instance, highly knowledgeable woodworkers know that dense timber species, such as Blood-wood and Mahogany, require precise fitting joints due to their brittle nature. On the other hand, resilient wood species, such as Cedar and Pine, are more compatible with less precise joints. Similarly, they also know that the season when wood is harvested goes a long way in determining its use. Case in point, stem cross-sections acquired during tree dormancy are best suited for projects that require the bark to be retained, such as making coasters. Inversely, wood for projects that do not require the bark, such as peeled poles, should be harvested during the growing season. Such pieces of information will greatly work towards sharpening your handiwork skills.
Thereby, this write-up seeks to help you gain a deeper insight of this woodworking material by taking a deep dive into the nature of wood. It will critically analyze the anatomy of a tree and how this structure plays into woodworking as a craft.
Anatomy of a Tree
Timber is harvested from trees. To better understand how this dynamic material behaves, it is integral to understand the process by which trees grow. There are three distinct parts involved in this growth process, namely:
This is the bottommost part of a tree. It anchors the tree, keeping it stable and erect. Its main purpose is to absorb nutrients, minerals and water from the soil, all of which are required for proper tree development and repair.
Also referred to as the bole or the stem, the trunk is the middle section of a tree. It offers the tree support, while also acting as the pathway through which nutrients, water and minerals pass to and fro the roots. For woodworkers, this is arguably the most essential part of a tree since it is the principal source of timber.
The topmost part of a tree is called the crown. It comprises branches stemming from the trunk and packs of leaves on each branch, forming a busty figure at the top. Herein, the leaves, through their stomata, take in carbon dioxide to be used in photosynthesis. The food produced during this procedure is the primary ingredient needed for steady tree germination and development.
These three parts are merely what meets the eye at first glance. However, it is the trunk’s internal structure and its various complexities that actually give wood meaning from a woodworker’s perspective.
A cross-section view of a tree trunk normally shows a series of layered circular bands commonly referred to as growth rings. These layers are also known as annual rings since they indicate the tree’s age. Notably, these layers are a result of interruptions in the tree’s growth caused by cyclical seasonal changes. Each growth ring represents the beginning and end of one season. In some cases, some growth rings are wider than others since they grew during a season with ample moisture and sunshine. Contrastingly, a narrower ring suggests that the growing season was hampered by unfavorable weather, insect attack or/and diseases.
A woodworker should always take note of a wood’s growth rings as they represent the material’s structural stability. In most cases, timber with unconventionally narrow or wide growth rings signifies a structurally weak material.
Earlywood and Latewood
Noteworthy, all growth rings are divided into early and latewood. As the names suggest, the earlywood sprouts first and the latewood grows later to create a single growth ring.
This is the part of growth rings that comes about during the beginning of the season. During this time, mostly in the spring, soil nutrients are in plenty as well as rain. As such, a growth ring starts emerging, made of new thin-walled cells that actively transport nutrients through the tree. Thanks to the nature of these cells, earlywood is porous and has a light tone.
This is the part of the growth rings that comes about during the later parts of the growing season. During this time, the weather is significantly colder and drier than the earlier stages of the season. Therefore, the growth rings are less involved in food transportation, causing them to have thicker cell walls and, as expected, large cells. Consequently, latewood is denser and darker than earlywood.
The degree of contrast that exists between earlywood and latewood is what woodworkers refer to as grains. An uneven grain is when this difference is considerably pronounced, while an even-grain is when the difference is subtle. Some wood species, such as Eastern Hemlock, have a fairly even grain given that the difference between latewood and earlywood is neither big nor small.
This is one of the most fundamental tenets of woodcraft as grain patterns are used to describe or delineate wood species. More importantly, they influence multiple aspects of this craft. To give you an idea, it is prudent to sand wood surfaces along the grain. This is an important consideration as it ensures you minimize the number of visible scratches that appear on the surface after staining.
Sapwood vs. Heartwood
Growth rings are generally categorized into heartwood and sapwood. A cross-section of a tree is divided into multiple layers, ranging from the innermost heartwood to the outermost bark. In-between the heartwood and the bark are the sapwood and cambium. The former is the light section of the tree, which is encircled on the outward by a microscopically slim layer of living cells called the cambium. This is the only actively growing part of the tree given that its living cells contain protoplasm.
As the cambium grows, each of its layers are gradually added onto the sapwood, They slowly recede from the cambium, eventually clogging with gum and resins which turn them into dead heartwood cells. The large concentration of dead cells at the center of this layering forms the heartwood, a central dark pith.
As this layered growth process continues, the outermost layers gradually become dormant. It is from these dormant layers that the tree’s bark is formed.
Heartwood and sapwood are the only two layers used in woodwork projects. The choice between the two typically depends on their distinct attributes.
As mentioned earlier, sapwood is lighter and less dense than heartwood. In a living tree, this layer is tasked with transporting minerals and water to the crown. Therefore, it has a high starch content, making this layer highly attractive to wood damaging insects and rot-producing organisms, most notably fungi.
Additionally, since the xylem cells located in this layer are less densely populated, sapwood has a high cavity coverage, making it highly porous. This implies that sapwood has a higher absorption rate than heartwood when it comes to finishing.
It should also be noted that in addition to having a lighter tone, sapwood is also less structurally sound than heartwood.
This is the darkest and centermost section of a tree’s trunk. Before reaching this section. Living cells from the sapwood release their starch content to facilitate the tree’s growth. Once they get clogged with resins, they die off in the form of hollow, physiologically inert cells. They then get filled up with extractives and come together to form the heartwood.
Compared to sapwood, this dark pith is more durable. First, its high extractive content makes it highly resistant to wood-burrowing insects and rot-causing organisms. Secondly, its dense cell structure creates a formidable section reputed for its higher structural properties. Also noteworthy, its rich color gives off a more vibrant appeal than its lighter-toned counterpart.
On the flipside, heartwood’s dense cell structure makes it less porous than sapwood. Consequently, heartwood is less receptible to finishes.
In some species, such as oak and beech, you can also notice rays. These are the horizontal lines cutting across the heartwood and sapwood. They normally run radially outward from the heartwood and across the sapwood.
They are made from cell groups scattered throughout the wood and positioned in a horizontal axis. These cells not only store starch but also conduct sap horizontally through the tree.
Although these cells are mostly microscopic, some tree species have larger rays that are perceptible to the human eye. Whether these lines should be considered visual attributes of specific wood species or visual flaws is arbitrary.
Softwoods and Hardwoods
Wood is broadly categorized into softwoods and hardwoods. These terms, however, are somewhat imprecise. Not all hardwoods are hard and not all softwoods are soft. A typical case is that of Douglas-fir and basswood, wherein the latter is softer than the former despite it being a hardwood while Douglas-fir is a softwood.
One of the most reliable ways of telling these wood types apart is their tree leaves. Hardwood trees are deciduous, meaning they shed leaves annually. More importantly, deciduous trees have generally broad leaves. In contrast, softwood trees are evergreen conifers. They have tiny, needly leaves that do not shed annually. Additionally, the majority of conifers produce cones.
Softwoods vs. Hardwoods: Cell structure
Another more reliable means of differentiating between hardwoods and softwoods is by microscopically analyzing the structure of their cells. There are several notable differences between these two wood types when looked at through the close lens of a microscope. As detailed below, each wood type has a distinct cell structure.
Approximately 90% of all cells found in softwoods are tracheids. These are tubular fiber-like cells that are 100x longer than their diameters. Depending on the specific wood species, tracheids range anywhere between 2 and 6 mm in length. On average, a single cubic inch of coniferous softwood holds up to 4 million tracheids. They are considered the most primitive wood cell type as they gave way to other cells, including fibers and vessel members, by means of gradual evolution.
Tracheids determine the texture of softwoods. Their functions include providing the tree with support and conducting sap from the root, up the trunk, and to the crown. For this reason, these cells are tubular and longitudinal to enable passage of sap vertically up the tree. During spring, when there is rapid growth of earlywood due to abundant rainfall, there is a lot of sap passing through tracheids. As a result, their walls are considerably thin with sizable cavities to allow for the unobstructed flow of sap through the tree. This cell structure leads to porous earlywood softwood.
Contrastingly, during the later stages of the season when the latewood sets in, there is reduced flow of sap due to diminishing water and mineral resources. As such, tracheid cells’ walls grow thicker and their cavities reduce in size. The result is denser timber.
In effect, this difference in tracheid cell structure in softwoods is more visible during staining. Ideally, once stain is applied on the softwood surface, the earlywood takes in more stain than latewood. Earlywood which is initially light-toned, becomes darker than latewood, which is initially dark-toned. This creates a reversal of the initial grain pattern as the wood’s dark parts become lighter than its initially lighter parts.
Softwoods’ cell structure also contains parenchyma cells, albeit in significantly smaller numbers than tracheids. These cells resemble bricks and are minute in size, about 0.15 mm in length. These cells play little to no role in determining the wood’s texture. However, they are instrumental in the storage and transportation of starch. Notably, horizontal parenchyma transport starch across a horizontal axis.
On the flipside, hardwoods’ cell structure comprises vessel members, parenchyma, and fibers. Vessels and fibers are always axially oriented, while parenchyma cells make up hardwood rays.
In hardwood anatomy, vessels are end to end tubular cells of indeterminate length. They are stacked vertically against each other and have partially or fully open pores. Their indeterminate length stems from the fact that these tubular cells are connected at the end to create tubular stacks that run all the way up the tree. From a cross-sectional view of a hardwood slab, vessels look like pores or tiny holes. By forming this sort of pipeline network in the tree, vessels are able to serve the same purpose as tracheids in softwoods; transporting sap.
Vessels determine the porosity of hardwoods. There are three main classifications of hardwood porosity. These include:
Herein, vessel elements come about in the early wood. As such, they form pores in a band or ring formation, ultimately creating an uneven grain. An example of hardwoods with this grain pattern include Europe ash and oak.
Diffuse-porous hardwood comes about when there is little to no difference between latewood and earlywood vessel distribution. The result is an evenly distributed pore arrangement whereby the pores are distributed evenly and are of equal size.
Also referred to as semi-ring porous, this is a scenario where there is slow transition from earlywood to latewood. In essence, while the pores do not form clearly distinct rows or bands, their size reduces gradually during the transition from earlywood to latewood.
These different cell structures bring about another visible difference between hardwoods and softwoods, with specific regards to their distinct staining patterns. In hardwoods, staining enhances the wood’s grain pattern given that the stain is absorbed rather evenly.
In hardwoods, particularly living trees, fibers are cells tasked with supporting the tree trunk. For this reason, they have very thick walls. Unlike vessels, fibers are not visible by the naked eye. However, you can distinguish them using the colored regions that form an end grain’s backdrop.
One of the few similarities between hardwoods and softwoods is the presence of parenchyma cells in both. Moreover, these cells store starch in both tree types. In the case of hardwoods, however, parenchyma cells are classified according to their resultant end grain patterns. These include:
The term “Apotracheal” is a combination of the Greek word “apo” and the English word tracheal. Tracheal refers to that which relates to a pie, while “apo” means separate from. Combined, these words mean a pipe structure separate from the pores. Ideally, when looking at the end grain, Apotracheal parenchyma cells form patterns of tube-like structures situated independently from the pores. Although parallel to each other, their arrangement is not dependent on the wood’s pore structure.
On the flipside, the word “para” is a Greek terminology for ‘beside’. As such, the phrase Paratracheal parenchyma implies a cell structure where the tube-like patterns are seen as straight lines bordering the pores.