Wood Science – Hardwoods
Woodworkers overwhelmingly prefer to work with hardwoods. Softwoods have their place in construction and paper making along with some specialty applications, but for fine furniture, turnery, carving and all the other uses we find for wood, hardwoods are the wood of choice.
The hardwoods are generally where you’ll find the great figure that woodworkers love so much.
While they have a different function for a growing tree, medullary rays provide a striking focal point when lumber is quarter cut. Red oak is one of the species that has very pronounced rays.
Ring Porous Woods
Red oak is a ring porous wood. The earlywood growth each year produces a band of large, hollow pores to transport sap, before putting on denser latewood pores.
Large Earlywood Pores
This slice of red oak is 1/8" thick, yet it’s possible to see light through the larger diameter earlywood pores of this ring porous wood.
Larger volume of pigment stains get trapped in the deeper earlywood pores of ring porous woods like this red oak. (Photo by Shutterstock)
Diffuse Porous Woods
Maple is a diffuse porous hardwood, making its end grain more even than most woods. Maple earlywood and latewood are more similar to each other than a ring porous hardwood.
Many types of woods put on a layer of larger pores early in the growing season, then add denser, stronger pores if the growing conditions are optimal. Teak, shown here growing in a Mys-Teak plantation in Costa Rica, are a great example of how faster growth equals stronger wood. (Photo by Mys-Teak)
There are four types of cells found in hardwoods. Vessels (or pores) are thin-walled cells of varying diameter that have evolved specifically for sap conduction. Fibre cells are thick-walled and provide structural support. Ray cells extend horizontally from the pith and are involved in sap movement and sugar storage. Finally, tracheids are thin-walled cells orientated vertically that help move food around the tree. Pores are the focus of this column, as they help determine the texture, density and even the strength of a piece of wood. The other cell types will have their own story to tell in future columns.
Pores: where woodworking magic happens
Pores can vary in size and distribution within a given growth ring. When large pores occur only in the earlywood of a growth ring, these hardwoods are called ring porous. There is a distinct separation between the large-pored earlywood and latewood with its small diameter pores. Oaks, elms and ashes are typical ring porous hardwoods, and the earlywood pores are visible to the naked eye and give these woods a distinctive look and coarse texture. If you carve ring porous hardwoods or work them with hand tools, you will notice a change in resistance between the earlywood and denser latewood.
Ring porous woods are excellent candidates for staining. Pigmented stains will accumulate within the large pores, deepening the colour of the earlywood and accentuating the grain of the wood. If you want a glass-smooth finish on these woods, you will have to use a pore filler to create a level surface to finish. It’s important to realize that the cell structure of a hardwood will determine what finish you can use successfully.
At the opposite end of the spectrum are the diffuse porous hardwoods. The pores in these woods are relatively small and evenly distributed throughout the growth rings. Cherry, basswood and yellow poplar are good examples of diffuse porous hardwoods. The small pore sizes within the growth rings give these woods a fine texture. Basswood is a great carving wood because the texture of the wood allows for fine details to be carved wherever needed to suit the design. In addition, with its low density, basswood makes for easy and effortless carving. Diffuse porous hardwoods are also harder to stain as the small pores don’t allow the stain’s pigments to accumulate and colour the wood.
Finally, there is an in-between category called semi-ring porous. In these woods, there is a gradual decrease in pore size from earlywood to latewood. The change is subtle with no distinction between the early and latewood. These woods have a medium texture; walnut and mahogany are typical examples. Like the ring porous woods, you can stain them and a pore filler is recommended if a glass-smooth finish is desired.
Sapwood / hardwood transition
When the sapwood changes to heartwood as the tree ages, there are changes in the pores. In some species, tyloses form within the heartwood pores. These are bubble- or dome-shaped structures that block the pores. Red oak does not have tyloses, while white oak does. You can’t build a barrel from red oak, as the contents of the barrel will drain out the open pores. On the other hand, white oak barrels, with tyloses blocking the vessels, will not leak that expensive wine onto the floor.
Pore size can also affect the strength of a given piece of wood. This is most evident in the ring porous woods with their large pores. The large pores will create areas of weakness parallel to the grain direction. As a result, these woods can be easier to split into firewood.
An interesting use of the weakness of ring porous woods is basket making. Some Indigenous peoples have been making baskets from black ash for centuries. When a green, freshly cut log is pounded, the weaker pores in the early wood collapse. This allows thin pieces of wood to be peeled off the log, similar to an onion. The larger pieces are cut into thin strips that can then be woven into baskets.
Growth rate is important for strength
Growth rates can also help determine the strength of ring porous hardwoods. Ring porous hardwood trees will always produce large, weak earlywood pores at the start of the growth season, and only then start adding stronger latewood layers if growing conditions are optimal. Slow growing ring porous hardwoods are weaker because their annual growth rings are predominantly earlywood, with large, thin-walled pores. The opposite effect is seen in fast growing ring porous hardwoods. Here, the latewood, with small pores and an increased number of fibre cells dominating the growth rings, yields a tougher, stronger wood. I would expect fast growing teak from a plantation to be stronger than teak harvested from the forest. This doesn’t always apply to diffuse and semi ring porous trees, as there are other factors in play.
Cellular structure is what creates the incredibly diverse world of hardwoods. When planning your next project, take this into consideration. Do you like the look of stained wood? Then look to ring porous woods for candidates. Do you like to carve? Diffuse pored hardwoods are your best bet, as they yield consistently to a sharp edge. When choosing the wood for your next project, take the time to look at the end grain to determine its pore structure and the stories it tells.