April - June 2013
Wood Science 101 (10) - Where Does Lumber Come From?
by Chuck Ray
In the early 1970's, the US Forest Service developed a computer program that mathematically calculated the highest volume of lumber that could be sawn from a log of specified dimensions based on what it called the "best opening face". Soon, computerized sawing equipment incorporated this computer algorithm into their equipment along with scanning technology that allowed the log to be spun and scanned prior to sawing, thereby allowing the computer to determine just exactly where that first critical cut should be made. The resulting "face" of the log then, would produce the widest pieces of lumber, and subsequent narrower lumber would be produced as the log is turned. The sawyer, or the computer he operated, determined where the best first cut would be. The cut was made just at the edges of the top piece of bark, producing a "slab" from which the top two narrow boards were re-sawn. Then, once the slab was sent on its way the third and fourth boards from top were sawn and sent on to an "edger" where the square edges of the boards were formed as the rounded corners were sawn away. The log was then rotated and sawing continued on the next face, with most of the pieces in this case being sent on to a "re-saw" or a "gang-saw" to produce the narrower strips.
Not long after the computerized saws were capable of producing the highest amount of lumber, or "yield" from a log, technologists figured out how to allow the mill operators to assign market values to the different sizes of lumber in "value tables" built into the software. This allowed the mill operator to then produce not the highest "yield" of lumber in board feet (one board foot is equal to a square piece of wood 12 inches long, by 12 inches wide, by 1 inch thick), but the highest value of lumber in dollars based on ever-changing current lumber market values.
This system works well for softwood lumber, for which most of the value is determined by the dimension of each piece. But in hardwood lumber production, the real value of the lumber is determined by the internal characteristics of the log...the number and size of knots and other defects, the coloring and figuring of the wood, and the surface area of "clear units" in each piece of lumber. These characteristics are determined again by the sawing technique used for each log. The three most common methods of sawing hardwood logs are called "plain or flat sawn" (the most common and highest yielding method), quarter-sawn (the most popular for certain applications where highly figured wood is desired and for sawing staves used for barrels), and rift sawn (used when straight-grained lumber is highly desired).