I am constantly encountering manufacturing problems with wood that are related to issues with the wood itself. In fact, I am surprised how many manufacturers just buy wood on the open market and do nothing to assure that they have received the right grade, the right footage and the right quality needed for their processing. So, here are some items (maybe discussed too briefly, so drop me an email for more info) about checking the quality of incoming lumber.
I divide the quality issues into two groups: Those related to tree quality issues; those related to lumber manufacturing issues including moisture content.
Part 1. Tree Quality
Anyone that has worked with wood knows that no two pieces of wood are the same; wood is variable, which is part of wood’s beauty indeed. However, part of this variability does include special types of wood that cause quality issues (warp, checks, splits, stain).
Reaction wood (Tension wood and compression wood). When a growing tree is subject to outside stress (from another tree leaning against it, from the wind blowing mostly in one direction, from a heavy branch, from having to bend a little to get more sunlight, etc.), the tree will react to this outside force. If the tree is a softwood (needle tree), it will create compression wood; if it is a hardwood, tension wood. This special reaction wood is quite weak (checks likely) and also shrinks lengthwise while “normal” wood does not (bow, twist and sideband are quite likely). Tension wood will develop fine fuzz when sanding and will absorb stains differently (blotchy appearance) than the rest of the wood.
There is nothing we can do to offset the quality loss of reaction wood. Therefore, we need to learn to identify it and eliminate it from our incoming raw material.
|
KNOWLEDGE CENTER Find more Wood Doctor at FDMCdigital.com Gene Wengert, aka The Wood Doctor, troubleshoots wood related problems, and explores lumber and veneer qualities and performance, species by species, in Wood Explorer, inside FDMC's Knowledge Center. |
|
Juenile wood. When a tree stem is very small, it has to be really flexible so that it does not break off when there is high wind, heavy snow, ice storms, heavy birds, etc. So, the wood within about 15 years of the center of the log is not as strong as the rest of the wood. This wood also likes to shrink lengthwise.
Virtually all sideband in dried lumber, which dramatically affects yield, is due to juvenile wood and the sawmill’s sawing pattern used rather than due to drying or manufacturing procedures.
Lumber containing wood from close to the center of the log (identified by highly curved growth rings when viewing the lumber from the end) should be eliminated.
Spiral grain. We consider it normal for the wood fibers to run vertically in the tree. But sometimes a tree has fibers that are at an angle rather than being perfectly vertical. That angle can be seen in a tree or log if the bark is removed and the wood develops some lengthwise cracks. In lumber, this slope of grain in a small area is called birds eye and fiddleback in sugar maple. In mahogany and several other species, the spiral reverses annually from clockwise to counterclockwise giving a beautiful ribbon stripe appearance to the wood. Many softwood trees develop a spiral in the juvenile wood that then becomes straight and actually reverses the spiral direction in the later growth.
With spiral grain, the normal shrinkage of the wood during drying now occurs a bit lengthwise, creating twist. This twist can become evident any time the wood’s MC changes.
Note that a crooked stem will also have grain that does not run vertically. Again, lumber from crooked trees will likely warp (especially twist) during MC changes.
Although slope of grain is hard to detect, when seen (cracks or splits at an angle), it should be eliminated from production.
Growth stress.
A living tree, when small weighs several hundred pounds. But a mature tree can easily weigh several tons. All that weight on the stem creates stress in the tree while it is growing. Such stress, called growth stress, will remain in the tree and the logs as well as in the sawn lumber. As might be expected, such stress will cause warp—both when the MC changes and when pieces are machined. So, warped lumber after drying, indicating this growth stress, should be eliminated or else cut into short pieces.
Bacterial infection. There can be bacterial infections in the living tree. The bacteria weaken the wood, essentially causing shake or wind shake (a crack parallel to the growth rings rather than across them), weaken the wood making checking and honeycomb likely, give the wood a foul odor, increase the moisture content creating wet pockets in green and dry wood, and discolor the wood. Identification of this wood will help to explain many drying defects.
Any piece with shake or foul odor needs to be eliminated from further production.
Growth rate. Oak trees, and other species with large pores within each annual growth ring, that grow much slower than normally expected (such as in an old growth forest with lots of competition from other trees) create wood that is very weak. Such wood is called brash wood. This weakness will be seen in the final product. Brash wood is not caused by the drying process. Fast grown oaks are a bit stronger than slow grown. On the other hand, slow grown pines produce wood that it much stronger than fast grown. Basically, be cautious about any species that has an unusual growth rate.
Species. There is quite a variation between wood species, especially with the grouping of species, such as hickory (pecan hickory vs. true hickory), soft maple (silver vs. red), and oak.
Example: Consider the oaks: There are four types of oak lumber. Each type has different manufacturing behavior, so it is critical to initially determine the correct type of oak you need The four types are
Upland red oak
Upland white oak
Lowland red oak
Lowland white oak
The difference between upland and lowland is the site where the tree was growing. In a lowland site, there will be a longer growing season with adequate water to result in annual growth rings that are at least ¼ inch apart. As a generalization, lowland oak (which is often imprecisely called Southern oak), compared to upland oak (often imprecisely called Appalachian or Northern oa-has more sapwood
-is more check-prone
-shrinks more
-has more risk of staining
-has more bacterial problems
-has growth rings ¼ inch or more apart
As another generalization, white oak, compared to red oak,
-shrinks more
-has lower initial green MC (65 percent MC vs 75 percent MC)
-has a higher risk of checking
-dries more slowly (25 percent longer or more)
Even within these four groupings there is a lot of variation. For example, with the upland red oak grouping, northern red oak shrinks 6.9 percent from green to 6 percent MC; black oak 8.9 percent, southern red oak 9.0 percent. Within the white oak upland group, white oak shrinks 8.4 percent; bur oak 7.0 percent; overcup oak 10.2 percent; swamp chestnut 8.6 percent. These values apply to the movement of wood after drying whenever the MC changes.
Part 2. Lumber Characteristics
Lumber grande and footage
Lumber grade and footage greatly affect the yield (or volume) of parts, the quality of parts and time to process the lumber into parts. You should receive the full range of quality allowed within a grade; that is, another lumber customer should not have received the best quality within the grade and you received the lower quality. There are many lumber grading short courses offered throughout the U.S., so you can learn the basic concepts and check your own incoming lumber.
My experience also shows that footage measurements are often incorrect; usually the actual footage is lower than the value on the invoice.
You will find that some suppliers are perfect, and these would not have to be double checked.
Moisture content
Anyone reading this column knows the importance of correct MC. Moisture content of the lumber must be at the correct level. If much drier than 6.0 percent MC, the wood will behave in a brittle manner when machining, and will absorb glue rapidly potentially leading to a poor glue joint in panels. If the lumber is much above 7.5 percent MC (perhaps above 8.0 percent MC, but I suggest 7.5 percent MC is better), the wood will shrink in processing (poor glue joints, un-smooth surfaces and joints), and potentially in use (delayed shrinkage with numerous potential problems).
Shake
See the bacterial discussion above, as all shake indicates bacterial infection.
End checks
The lack of end coating for logs and lumber means that potentially 2 inches or more from the end or every piece of lumber will have to be trimmed off. (The risk is greatest with the denser species.) Further, as end checks are difficult to see as they move up the piece, it is possible that an end check might accidentally be included in a stave, leading to a small crack in the final product. With properly applied (mainly, put on thick enough) end coating, the trimming required will be ¼ inch in most cases. For 8 foot lumber, this 3-1/2 percent gain is worth about $25 per MBF or so, but the cost of applying the coating at the sawmill is only $4 per MBF.
Large end splits
The presence of large (over ¼ inch wide) end splits indicates that there is stress in the tree that is also in the lumber and that will be in the components manufactured. If the humidity and moisture content of the wood products never changed, then this stress would not be an issue. But with changes in moisture, there is a strong likelihood that the wood will twist and warp lengthwise slightly. Further, when wood with stresses is machined, even though the machines, such as a planer or belt sander, are perfectly flat, the stress will cause immediate warp when machining…mostly bow or side bend.
Side bend and twist
The presence of side bend warp or twist warp in the raw lumber indicates the presence of stress, steep grain angles, and/or tension wood. All these characteristics within the wood will cause a slight amount of warp—bow, twist and side bend—in the components when machining and in the final product if and when the moisture changes. Keeping rings centered (left to right edges are mirror images) prevents side bend warp.
Cupping
The presence of cup warp indicates that the wood is cut from near the center of the tree, which again means that some warp—bow, twist, cup and side bend can be expected if the moisture changes.
Cupping toward the bark is a natural event that is controlled by not over-drying and not rewetting partly dried lumber.
Lumber width
Narrow lumber increases the piece count and also potentially reduces the overall yield. Both of these characteristics result in higher processing costs. The major concern would be for a load or bundle with quite a few narrow pieces. I would think that under 5 inches would be “narrow.” Of course, a few narrow pieces in every load would be normal. (As a test, you might rip two 5-inch pieces and compare the yield to ripping one 10-inch piece.)
Lumber length
The lumber length is important, with longer lumber being less expensive to process.
Checks and honeycomb
The presence of checks (surface or end checks) and honeycomb in oak when looking at dry lumber indicates that the lumber was not dried properly and that yields will be lowered substantially. Checked lumber should be withdrawn from production and returned to the supplier for a full credit.
Oftentimes these checks cannot be easily seen with the “naked eye” because moisture in the atmosphere has swelled the checks closed. Surface check samples and end check samples are easily done and reveal the presence and extent (depth) of preexisting checks.
Discoloration
Any discoloration, including the presence of gray or pink discoloration, blue stain or sap stain, and sticker stain, is preventable through proper handling and drying of the lumber. (So-called mineral stain, usually caused by an insect, occurs in the tree and is not controllable during sawing, or drying.) Oftentimes, such staining is not obvious from the rough lumber. When visible, the grade and price should be 2B Common, at best; or the lumber can be returned. When some lumber pieces appear stained, it is likely that more stain will be seen as processing proceeds.
Drying stress
The presence of drying stresses (also called casehardening) will cause immediate warp when machining and will make flatness of panels and doors difficult to achieve. Such stress can be easily removed in the dry kiln during normal drying procedures. The presence of such stresses indicate poor drying and often highly variable final moisture contents, along with other drying defects.
Grain
The presence of the correct grain, including quartersawn grain, absence of sapwood, and so on, can be important.
Have something to say? Share your thoughts with us in the comments below.