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Cupping is caused when one face of the lumber shrinks more than the other face. Because the bark side of lumber shrinks more than the heart wise, this defect is found in flatsawn lumber. [...]once the wood temperature is returned to 70 to 90 F, fresh fungal spores are plentiful in the air and can quickly reinfect the wood if growth conditions are satisfactory); moisture (Moisture levels need to be at least 22 percent MC and higher MCs are more favorable); and food (For the blue stain fungus, the food supply is the sugars in the sapwood.) Cures. [...]occurring starches and sugars in the wood will begin to oxidize as soon as the tree is cut down The oxidation colors are different than the natural wood color. [...]long drying times are expensive and may encourage other defects to develop. [...]some wet pockets can be expected even when drying is properly done.
A look at symptoms, causes and cures for a variety of problems.
Side-Bend Warp
Symptom. When a piece of lumber is initially flat and straight, but after drying the lumber is no longer straight but bends or crooks to the left or right, this defect is called side-bend, sweep or crook.
Causes. Wood does not usually shrink in the lengthwise direction, but there are three instances where we see substantial lengthwise shrinkage:
a) Juvenile Wood, which is the wood within about 15 growth rings around the pith (center) of a log.
b) Tension Wood, found only in hardwoods, is caused when the tree is stressed while growing. Tension wood is very weak and characterized by fuzzy or stringy fibers when sawing, but can be hard to identify.
c) Compression Wood, found only in softwoods, is caused when the tree is under stress and is mostly in the juvenile portion of the log. It is denser wood that is very weak and makes the stem oval shaped.
Cures. There is no cure or changes in drying practices that control this warp.
However, if the sawyer can position the growth rings in a piece of flatsawn lumber so that the rings are centered, with the right edge (looking at the end grain of the lumber) a mirror image of the left edge, side bend can be minimized. Quartersawn often will have juvenile wood on one edge and not the other, so quartersawn and rift sawn lumber are prone to side bend warp. Tension wood is spread throughout a log, so it cannot be eliminated or controlled when sawing. Compression wood can only be minimized by eliminating piece with this wood.
Cup Warp
Symptom. Cup is warp in drying that occurs from edge to edge.
Causes. Cupping is caused when one face of the lumber shrinks more than the other face. Because the bark side of lumber shrinks more than the heart wise, this defect is found in flatsawn lumber. The natural difference between two faces is larger when the lumber is closer to the pith, which usually is low grade lumber due to the many knots near the pith; lumber far away from the pith, upper grade usually, has little difference so it is not likely to cup. Cup is accentuated by allowing partly dried lumber to regain moisture (rain, snow, too humid in the kiln). Cup amount is also increased as the lumber gets drier.
Cures. Cupping is a natural occurrence, so we cannot eliminate it. To minimize cupping, avoid rewetting partly dry lumber and avoid over-drying.
Bow Warp
Symptom. Bow is warp in drying along the length that makes the lumber look like a ski; that is, the opposite direction of side-bend warp.
Causes. If the lumber is not stacked flat, then it will be bent and assume this bent shape. Also, bow can occur when one face shrinks more than the other face. Juvenile wood, compression wood or tension wood that is on or closer to one face will cause that face to shrink lengthwise.
Cures. Always stack lumber with enough support and level supports so that it dries flat. The sawyer must be aware of the presence ofjuvenile wood and compression wood and their effect on bow when close to one face.
Twist Warp
Symptom. A piece of lumber that is no longer flat, but is twisted lengthwise has twist warp.
Cause. The basic cause is that the grain of the wood is not parallel to the length of the lumber. This grain angle can be because of slope of grain in the tree (common in the first 20 years of growth of softwoods and common in some hardwood species like mahogany and live oak throughout the tree's life). It can be caused by the sawyer putting the log on the carriage at an angle or by sawing parallel to the pith and not parallel to the bark.
Cure. This is mostly a tree growth issue, so there is no cure by changing drying practices. The sawyer should saw parallel to the bark as much as possible.
Fungal Blue Stain
Symptom. As the name indicates, this is a blue, sometimes dark blue or grey, stain in the sapwood
Cause. This stain is caused by a blue colored fungus. Once the stain has occurred, it can be partially bleached out, but such bleach treatment also causes the grain to fade. There is no strength loss from this fungus. When damage is heavy, the wood does become more permeable, which will affect finishing.
In order for this and all wood fungi to grow, the fungi need four elements: oxygen; temperature (The fungi are most active between 70 to 90F; slower activity occurs between 50 to 70F and 90 to 120F. Freezing does not kill the fungi; however, heating to 130F will result in death. However, once the wood temperature is returned to 70 to 90 F, fresh fungal spores are plentiful in the air and can quickly reinfect the wood if growth conditions are satisfactory); moisture (Moisture levels need to be at least 22 percent MC and higher MCs are more favorable); and food (For the blue stain fungus, the food supply is the sugars in the sapwood.)
Cures. Eliminate one of the four elements, and growth will stop. Oxygen -In the log yard, aggressive watering of logs is suggested; that is, keep the logs soaking wet so no oxygen can get into the wood. Temperature - Not much we can do about temperature. Moisture -The quicker that we can dry the wood, especially the surface, the less opportunity for the fungi to grow. In other words, at high moisture contents, avoid slow drying (high humidity and low air flow). In air drying, piles spaced well apart; air piles with roofs on top avoid rain wetting; piles elevated above the ground. Food - Not much we can do about the food supply. In extreme cases, we can poison the food in the outer 1/100 of an inch with commercially designed and approved fungicides that is effective for several weeks.
Chemical Brown Stain, Sticker Stain, Gray Stain
Symptoms. The wood color changes to a variety of different colors including brown, blue, grey, pink and dingy brown. Often the color change is not well seen when drying rough lumber but becomes obvious when planing. This discoloration bleeds through some water-based finishes, especially noticeable on light colored paint or lacquer finishes.
Causes. Naturally occurring starches and sugars in the wood will begin to oxidize as soon as the tree is cut down The oxidation colors are different than the natural wood color. This oxidation is fairly slow, but is accelerated by heat (roughly, each 20F warmer means twice as fast). In order for the oxidation to occur, an enzyme in the wood (enzymes are protein molecules and are not alive) facilitates the chemical reaction. The oxidation reaction is a multi-step reaction that occurs mainly at high moisture contents, with the final step being the development of a darker color.
The apparent requirements for oxidation are warm temperatures within the wood (70F or warmer, but not above 130F, as the hot temperature deactivates the enzyme. (High temperatures cause another darkening reaction, however, so that is why we do not use high temperatures in drying.); moisture over 45 percent MC; which means the humidity in the air is likely over 80 percent RH; time (at least one day) at high moisture and warm temperatures.
Special note: Rain water on drying lumber also causes an appearance similar to oxidation stain. This effect is likely the result of migration and concentration of water soluble chemicals at or near the surface of the lumber when drying is slow.
Cures. The initial oxidation reactions can occur within the log, especially if the log is stored in warm conditions. Sprinkling water should be cold. Rapid drying of lumber can control stain by eliminating the necessary high moisture. However, fast drying does not reverse reactions that occur previously; that is, the further the reaction has proceeded before fast drying starts, the less successful drying will be in controlling the stain. In other words, the only effective control today for brown stain is to avoid slow drying by avoiding long log storage (especially on valuable logs), use low RHs in the kiln or in air drying, and using brisk air flow.
Wet Pockets
Symptom. Wet pockets are small zones, perhaps several inches along the grain and several inches wide in the interior of lumber, that have an extraordinary high moisture, compared to the adjacent moisture within the same piece of lumber. They are rare in 4/4 lumber, but a somewhat common in 8/4. They are more common in wide lumber pieces. Wet pockets seem common in cottonwood, aspen, elm, white pine, redwood and hemlock.
Cause. The cause of such pockets is likely related to a bacterial infection in the living tree, and they cannot be identified when the wood is green. In my experience, wet pockets are more common when drying is accelerated.
The problem with a wet pocket is that eventually the water will leave this wet pocket and, when the water leaves, the pocket can shrink and then can develop collapse or checking.
Cure. The only reliable cure for wet pockets is patience in drying. But, long drying times are expensive and may encourage other defects to develop. Therefore, some wet pockets can be expected even when drying is properly done.
Technology now exists to use an in-line moisture meter to find wet pockets in dry lumber. Such wet pocket lumber pieces can be withdrawn from dry lumber production and allowed to remain in storage for 4 weeks (give or take), thereby allowing the wet pocket to dissipate. +
Copyright CCI Media Nov 2018