This post contains information on wood, the different species, its structure, different figures, and its use as a material for construction.
Wood SpeciesThere are over 100,000 different species of wood. Some species are full size trees and others are from small woody shrubs. In common commercial use there are dozens of species harvested in the United States and many more from around the world.
Hardwood and SoftwoodTrees are divided into two classes: Hardwoods and Softwoods. The hardwoods such as Oak, Poplar, and Maple have broad leaves. In general softwoods, such as Pine and Fir, originate from cone-bearing trees. Hardwoods come from trees that have their seeds contained in a seed-case.
Hardwood and softwood do not specifically refer to how hard the wood is. For example Balsa is a hardwood and yet is very soft.
The Structure of WoodWood can be thought of as a collection of drinking straws - where each straw is one of the fibers of the wood. These straws run along the axis of the tree, that is up the length of the tree or along its branches. These straws are what carry the sap of the tree which supplies nutrients to the tree between the leaves and the ground. The straws are the fibers of the wood and a view of them on edge is the what makes the grain of the wood.
Image from: http://awci.org.nz/look-whats-popped-up/
The grain direction is along the fibers of the wood:
Image from: http://www.ggi-myanmar.com/wood/
Grain DirectionAs a way to refer to the faces of a milled board woodworkers often use the terms Face Grain,
Side Grain, and End Grain. These are shown below:
FigureFigure refers to the appearance of wood on its face grain surface (what we think of as the face of the board). The figure can range from subtle to spectacular. Species which are sought after for the figure include, Maple (Curly, Birdseye), Lacewood, White Oak (Quartersawn) and many others.
Subtle Figure - Holly:
Highly Figured - Curly Maple:
Quilted, Birdseye Maple:
Quartersawn White Oak:
This image shows how Quartersawn boards are cut from a tree:
Heartwood / SapwoodSapwood is the living, outermost portion of a branch, while heartwood is the no longer active, inner wood, which often makes up the majority of a branches' cross-section.
This sapwood is where water and dissolved nutrients (minerals) are moved between the roots and the leaves of the tree. In the outer growth rings of a tree (those closes to the bark - away from the center) sap flows through the "straws" of the wood.
Often the sapwood is a different color than the heartwood. In most species, the sapwood is lighter than the heartwood.
An exception to the sapwood being lighter is Sycamore. Its sapwood is darker than the heartwood.
How Wood is Measured and Sold
Lumber is often priced by a quantity known as a board foot.
Board FootA board foot is a measure of volume. The proportions of the wood doesn't matter - only the space it occupies. A single board foot (1 BDFT or BF) is equivalent to a piece 12" x 12" x 1". That's 144 cubic inches of wood (12 x 12 x 1 = 144). A board that is 2' long, 3" wide, and 2" thick is also one board foot - that's the same volume (24 x 3 x 2 = 144 cubic inches). Two board feet (2 BDFT) could be, for example, 12" x 6" x 4".
4/4, 5/4, 6/4, 8/4, 12/4, 16/4Wood is normally sold using its thickness as a measure. You'll hear people discuss "four quarter", "eight quarter", etc. This refers to the thickness of the wood in the rough. For example 4/4 ("four quarter") is 4/4 of 1" thick, or 1". But that is not a usable dimension. The wood is in the rough, sawn right out of the tree, and not yet surfaced. That is, it has not been planed (run through a machine which flattens and smooths the face). Nor has it been jointed (run across a machine which flattens and straightens an edge). Once planing has been performed, the wood is thinner than 4/4. It is normal to assume that if you want a 1" finished size you'll need to buy 5/4. After planing it'll be a full 1" thick. 4/4 wood is usually about 3/4" thick after planing.
It is hard to come by wood thicker than 16/4. In fact for many species it is hard to even get 8/4 material.
Premium for Larger SizesOften wood is sold at a premium price for wider or longer boards. That is, even though the volume is greater for these boards, there is an additional charge per board foot for them. That's because it is harder to come by wider or longer boards. And therefore the sellers feel justified in upping the price.
Grain Direction in Wood ConstructionIt is imperative you respect grain direction when you assemble a project made from wood.
The important consideration is to refrain from a cross-grain glue up. When you glue wood together edge to edge the grain of the pieces should be running in the same direction.
Here's a clear example of what goes wrong when you glue wood without respect to the grain direction. This is one of the first pieces of furniture I made - a small Shaker bedside table. What a sloppy drawer fit! But it gets worse.
The table top has breadboard ends. These are meant to hide the end grain of the top when seen from the side. I naively simply glued them bread board edges on, which is a worst case situation of a cross grain glue-up.
The wood expands or contracts in width whereas the edge pieces do not expand in width. Something has to give and the top cracks.
The International Wood Collectors SocietyI am a lifetime member of the International Wood Collectors Society. This is a group of people who have a passion for working with wood, its identification, and collecting samples.
I have about 220 species in my collection. The standard sample size our members collect is 6" x 3" x 1/2".