understanding wood
As a material, wood is all around us; we see it everyday.
It’s all too easy to look at it without ‘seeing’ it though, giving new meaning to the old saw about the forest and the trees.
Almost everyone knows wood is made from trees, but do they know how trees are made?
We all recognize that a tree grows up; it’s less obvious that it grows out as well.
Each year, every tree adds 2 layers or rings outwardly, called early growth or spring wood and late growth or summer wood.
Early is light and wide, late is dark and dense. Any board, when viewed from its end, will show growth rings which have also come to be known as ‘grain’.
what is grain?
The photograph above represents what the end of a board would look like if we could see it with microscopic vision. It’s a magnified view of ‘end grain’ in wood.
The cells that form trees fall into 2 major categories: tubes that are hollow and tubes that are solid. The hollow tubes allow the tree to draw nutrients up from the ground as sap and the solid tubes give the mass its structure.
Woods that are known to be ‘grainy’, like oak, have their hollow tubes concentrated in one growth ring, usually the early growth. These woods are called ring porous.
Woods that are known for ‘tight grain’, like maple, are diffuse porous; that is, their hollow tubes are spread randomly throughout late and early growth.
Grain, then, is another term for the wood’s pores as they appear on the surface of the board. How the board was sawn out of the tree and what species of wood it is will greatly affect what its grain looks like.
Here’s a quick visual of just how porous wood is:
so what?
If there is one takeaway from this primer on wood and wood science, it’s this:
wood moves.
It doesn’t ‘breathe’ – that’s a misnomer – but it is constantly taking on and giving off moisture through its pores. As it takes on moisture mostly in the summer, lumber expands across its width and in thickness but not in length. As it gives off moisture, mainly in winter when the heat goes on, it shrinks in reverse.
it’s unstable.
Wood is hygroscopic. This means the attraction between dry wood and water is so strong it is impossible to prevent moisture gain, or loss if the environment is dryer than the wood. Wood is a lot like a sponge. It’s in a constant state of imbalance due to its cell structure and how it grew. In a humid environment, wood will absorb moisture from the air and expand. When the air is dry, it will give up moisture it holds and shrink.
This is a fundamental quirk of wood and needs to be recognized and accommodated rather than controlled. There is no finish that permanently blocks its pores nor environment so perfectly controlled as to eliminate variation in humidity.
accommodation of wood
An unusual concept.
Even so, understanding wood’s inherent instability is critical to designing for its use. A 12″ wide, 3/4″ thick maple board, for example, could expand and contract across its width by 1/8″ during a yearly cycle and perhaps 1/32″ in its thickness. While that might not seem like a lot, such a gap or crack can cause problems if it was not expected. There’s also the potential for warping and cupping as a plank tries to ‘return’ to its former place within the tree. The ‘sap side’ of a plank, that side which was closest to the outside of the tree has more ‘lively’ pores than the heart side. That is, as pores are layered over by succeeding cell growth, they tend to collapse and become less moisture responsive. This can set up an imbalance in a board between one side and the other which is usually evidenced by cupping or warping. A solid wood panel must be contained by a frame to restrict its potential warpage but allow for its expansion. (The alternative is veneered plywood, which is another kind of accommodation and part of the next lesson.)
Besides the appearance of grain – ring porous tends to look coarser and diffuse porous tends to have more subtle grain patterns – there’s a difference to the touch, too, as ring porous woods feel a little rougher than diffuse porous. By the way, neither is inherently ‘bad’ or ‘good’. Understanding their qualities enables a designer to make an informed choice in pairing wood species to a project.
Ironically, water, the source of a tree’s sustenance becomes the enemy of wood in its next life as furniture or cabinetry. A little water can cause discoloration or distortion and a lot of water can destroy a piece. Aside from enhancing the color of wood grain and lending an attractive sheen to the surface, the primary purpose of finishing is to enable wood to resist damage from both sudden and prolonged exposure to moisture. No finish, though, is going to seal wood off hermetically from moisture.
More on finishing in a subsequent lesson.
One of the keys to effective furniture and cabinet design is making choices driven by knowledge rather than relying upon assumptions or misinformation. I’ve structured this mini-course to empower you with practical information about wood as a material. The tools I’ve provided here are basic – there’s always more. If you have questions or would like to build upon what you’ve learned here, please do contact me.
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