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What is “green” lumber? The American Lumber Standards Committee classifies green wood as having 20% or higher moisture content, and dry lumber as 19% or less. Board moisture is measured in terms of weight, not volume.
Kiln-drying is done in a gas, electric or solar-powered oven. Kilns are expensive to operate, but they offer a precisely controllable drying environment. Some mills may be inclined to speed up the process to save money. However, rapid drying can lead to a multitude of defects, such as case-hardening (See here).
From a woodworker’s point of view, air-dried lumber is a lot cheaper, but it is less common. Most lumber, including everything you’ll find at a home center, is kiln dried, because it is ready for market in a shorter time. The kiln is also a more controllable method than air-drying, especially with large volumes of lumber.
Moisture & Wood Movement
Regardless of whether boards are air-or kiln-dried once they are cut from a log, lumber will continue to seek what is known as equilibrium moisture content (EMC): it will absorb moisture or dry out until its moisture content matches the relative humidity in the surrounding air. A kiln-dried board will never absorb as much moisture as it initially had when it was green, but its sponge-like qualities cannot be stopped, even when a wood finish is applied.
The amount of moisture a board contains at the lumberyard is measured in percentages, which range from 6% to more than 20%. Framing lumber should be less than 18% moisture when purchased (about 14% is ideal), while stock destined for furniture or casework should be down around 6 to 8%. Moisture percentages are measured in terms of water weight vs. wood weight, not according to volume.
The only accurate way to check this is with a moisture meter (See Evaluating Moisture Content, below), a small electronic tool with two sharp pins that are inserted into a freshly cut surface of the wood (old cuts dry quickly and give a false reading, so a fresh cut is essential). Most fine hardwood vendors will loan you a meter to examine their stock before you buy, or you can ask them to take a reading in your presence.
EVALUATING MOISTURE CONTENT
A moisture meter will tell you immediately the moisture content of a board. The red glowing light on this meter indicates the moisture content in this board to be 10%, an acceptable level for cutting and for project use. Calibrated moisture meters aren’t cheap, but you may want to invest in one if plan to do fine furniture work or if the humidity in your shop fluctuates widely. It’s also a good idea to test the moisture content of air-dried lumber if you purchase it directly from the mill. Be sure to test the wood on a fresh-cut edge or end—old edges dry quickly and will not provide an accurate reading.
Wood expands and contracts in response to changes in moisture and temperature. Tangential movement (A) occurs parallel to the growth rings, while radial movement (B) happens across the rings. Wood moves very little along its length (C). Generally a board’s tangential movement is about double its radial movement.
As wood absorbs moisture from the air, it expands, and as the moisture evaporates, it will contract. You may be surprised to learn that wood basically moves parallel to the growth rings (tangentially) and across the rings (radially), but almost never along it (longitudinally). Therefore, in a standard plain-sawn board (see Lumber Cuts, previous page), expansion or contraction essentially occur in just two directions: width and thickness. Movement across the width is normally about twice that in thickness. The “greener” the board, the more it will move. It is critical, when designing and building woodworking projects, to consider how these forces of expansion and contraction will affect your project; they cannot be entirely eliminated.
Lumber Distortion & Defects
Often lumber will not expand and contract uniformly, causing it to distort. Four types of distortion, caused largely by improper kiln drying, are cupping, crooking, bowing and winding. Cupping is where the two long edges of the board begin moving toward each other, while the middle remains flat. A cupped cross section resembles the letter C. About the only way to fix this is to rip the board into several small strips after they have attained equilibrium, joint their edges, and then reglue them, alternating the growth rings (See here).
Crooking is evident when a board’s faces are flat but it warps from side to side. This is an easy fix: after the board reaches equilibrium, simply joint one edge of the board, then rip the second edge parallel (See here). Fixing both cupped and crooked boards will incur some degree of waste.
Bowing is a more difficult problem to deal with. In this case, a board cups along its length and resembles a very wide rocking chair runner. About the only solution is to support the ends and place weights on the center with the board’s convex side facing up. In some instances, the board will flatten when it reaches moisture equilibrium.
Sometimes the ends of a board will twist in opposite directions. Twisting is difficult to remedy, but you may be able to flatten the faces of thicker boards by running them repeatedly over the jointer (See here).
LET YOUR LUMBER ACCLIMATE
Once you have purchased lumber for a project, allow it to acclimate to its new environment for a few weeks before building with it. If your shop is particularly damp, insert sticking between each board so that the air can surround it evenly on all sides. Or wrap it completely in 6 mil plastic until you need to use it, then machine and finish the lumber immediately.
COMMON LUMBER DISTORTIONS
Boards distort in four primary ways, due to how internal stresses are released when it is machined as well as how the board absorbs and releases moisture. Moisture distortion is largely a measure of how the wood was dried at the mill. Wood that bows is flat across its width but the faces curve lengthwise. A crooked board is flat across the face but curves along the edges in one direction or the other, like the rocker on a rocking chair. Cupping occurrs when a board is flat along its edges but curls across its width. Twist is the condition where one or both ends of a board twist so the board faces are no longer flat.
Distortions can be spotted easily at the lumberyard by simply sighting down the edges and faces of each board before you buy.
Defects like pitch pockets, spalling or loose knots are easy to spot if you look carefully. Boards with these defects are salvageable by simply cutting away the bad areas. One defect that can’t always be seen until after the lumber is rip-cut is a condition called case-hardening. Case-hardening occurs when the outside faces of the board dry quickly while the center remains wet, causing tremendous internal stresses. Telltale signs of case-hardening are checks (small cracks), shakes (large cracks, most often radiating out from the center across the grain), and a problem referred to as honeycombing: when the board is ripped, the inside looks just like the inside of a beehive, full of tiny honeycombs. To safeguard against case-hardening, check boards along their edges and ends, paying close attention to honey-combing, the worst kind of case-hardening. If one board in a pile is affected, chances are several more from the same batch will have the same defect.
COMMON LUMBER DEFECTS
Spalling is a gray to green permanent discoloration of the wood