Understanding & working with wood defects
original Source https://www.woodworkingnetwork.com/best-practices-guide/solid-wood-machining/understanding-working-wood-defects
No tree is perfect. It’s subject to defects from the time it emerges as a seedling to the last stages of seasoning. A defect is simply an abnormality or irregularity found in wood. There are many different types of defects arising from many different causes. For instance, there are natural and acquired defects caused by a broken limb or other injury, insect and fungal attack, or rapid tree growth.
There are innate defects caused by the natural characteristic of wood to shrink or expand in response to water vapor in the air. And, there are artificial and mechanical defects caused by incorrect sawing or machining (conversion), improper drying (seasoning), or improper handling and storage.
Defects may be responsible for reducing wood’s economic value, lowering its strength, durability and usefulness, marring its appearance, and in some cases, causing its decay.
Natural Wood Defects
During its lifetime, a tree is subjected to many natural forces that cause defects in the wood. Woodworkers are quite familiar with these defects – knots, splits, ugly dark streaks or stains, worm holes, even decay. Some of the more common wood defects all woodworkers face include:
Bark pockets – Formed when a small piece of the bark protrudes into the lumber. This area is generally considered unsound.
Bird pecks – Caused by birds, especially woodpeckers, which peck on trees mainly to cause panic to the insects living in or under the bark and in the wood of the tree. This causes the insects to come out enabling the birds to eat them. Bird pecking can cause small injuries to the tree, resulting in grain changes that later show up as various forms of figure in the wood (figure is the “look” or appearance of a piece of wood).
Burls – Burls are a deformed growth formed when a tree receives a shock or injury in its young age. Due to its injury, the tree’s growth is completely upset and irregular projections appear on the body of the timber.
Continued tree growth follows the contour of the original burl deformity, producing all manner of twists, swirls and knots in the wood fiber. Usually, this results in spectacular patterns in the wood that can be used to great effect in woodworking. Burl wood is normally darker than the rest of the tree and, in some cases, may be a significantly different color altogether.
Coarse grain – If the tree grows rapidly, the annual rings are widened. It is known as coarse grain timber and possesses less strength.
Fungal damage – Fungi generally damages timber or wood by discoloration and/or decay. The resulting wood is generally weaker or of a different color than is typical for that species. The more common effects of fungal damage include:
● Blue stain – Common in pine, maple, and many other woods, blue stain (also called “sapstain”) is caused by a fungus that feeds on the sap. It does not live in live trees due to lack of oxygen. The bluish color (sometimes gray or dark gray) is the fungus itself, not the color of the sapwood. The color does not degrade the cellular structure and does not count against wood in the grading process.
● Brown rot – A form of wood decay found only in softwoods that destroys the wood’s cellulose, eventually causing cracks across the grain. Advanced brown rot tends to leave the wood more brown than normal. It is a precursor to dry rot.
● Dry rot – After the wood infected with brown rot dries out, the cell walls of the remaining wood turns into dry powder when crushed. This is called dry rot.
● Heart rot – This is formed when a branch has come out of the tree. The heart wood is exposed to an attack of atmospheric agents. Ultimately, the tree becomes weak and it gives a hollow sound when struck with a hammer.
● Wet rot – Some kinds of fungi cause chemical decomposition of a wood’s timber and in doing so converts timber into a grayish brown powder known as wet rot. Alternative wet and dry conditions favor the development of wet rot. If unseasoned or improperly seasoned timber is exposed to rain and wind, it easily becomes vulnerable to wet rot attack.
● White rot – This is just the opposite of brown rot. In this type of fungi attack, the wood’s lignin and the wood itself assumes the appearance of a white mass consisting of cellulose compounds. Some of the white rots during their early stages of development form what is commercially termed “spalted wood.” This wood has a unique color and figure, and some woodworkers highly prize it.
Insect defects – There are a number of insects that eat wood. Many other insects use wood as a nesting place for their larvae which results in holes and tunnels in the wood. The damage they cause ranges from minor to catastrophic. Some of the more common insects include:
● Wood boring beetles – Wood boring beetles, such as buprestid, powder post, ambrosia, furniture, and longhorn, tunnel through wood to deposit their larvae. Some larvae eat the starchy part of the wood grain. Many species attack live but usually stressed trees, while others prefer recently dead hosts.
● Pin-hole borers – They damage fresh-cut logs and unseasoned lumber, but also attack weakened, stressed, dying trees, and healthy trees with bark injuries.
● Termites – Termites not only tunnel through wood in various directions, but eat away the wood from the cross-section core. They usually do not disturb the outer shell or cover. In fact, the timber piece attacked by termites may look sound until it completely fails.
Knots – A knot is the base of a branch or limb that was broken or cut off from the tree. The portion of the remaining branch receives nourishment from the stem for some time and it ultimately results in the formation of dark hard rings known as knots. As the continuity of wood fibers are broken by knots, they form a source of weakness. There are several types of knots:
● Sound (or tight knots) are solid and cannot be knocked loose because they are fixed by growth or position in the wood structure. They are partially or completely intergrown with the growth rings.
● Unsound knots (or loose knots) are knots which fall out of the lumber when pushed or have already fallen out. They are caused by a dead branch that was not fully integrated into the tree before it was cut down.
● Encased knots are those which are not intergrown with the surrounding wood.
● Knothole is a hole left where the knot has been knocked out.
● Spike knots are limbs which have been cut across or cut lengthwise, showing the endwise or lengthwise section of the limb or knot. These knots generally have splits and severe grain deviations near them.
Raised grain – Anything that gives the wood a corrugated feel. Typically, this is caused by the harder summerwood rising above the softer springwood in the growth ring. The growth rings do not separate.
Shake – A lengthwise crack or separation of the wood between the growth rings, often extending along the board’s face and sometimes below its surface. Shakes may either partly or completely separate the wood fibers. The separations make the wood undesirable when appearance is important. Although this is a naturally occurring defect possibly caused by frost or wind stress, shakes can also occur on impact at the time of felling and because of shrinkage in the log before conversion.
There are two types of shakes:
● Star Shake: A group of splits radiating from the pith or center of the tree in the form of a star. It is wider on the outside ends and narrower on the inside ends. Star shakes are usually formed due to extreme heat or severe frost during the tree’s growth. Also referred to as heart shake.
● Ring Shake: Also known as “cup shake” or “wind shake,” this rupture runs parallel to the growth rings. A ring shake is not easily detected in green logs and lumber, but only becomes apparent after drying. It’s caused by any one of numerous factors, including bacteria, tree wounds, tree age, and environmental conditions such as excessive frost action on the sap when the tree is young.
Split – A split is a rupture or separation in the wood grain which reduces a board’s appearance, strength, or utility. One of the more typical ruptures of this type is called ring shake. In a ring shake (also known as cup shake or wind shake), the rupture runs parallel to the growth rings. It’s not easily detected in green logs and lumber, but only becomes apparent after drying. It’s caused by any one of numerous factors, including bacteria, tree wounds, tree age, and environmental conditions.
Stains – Stains are a discoloration that penetrate the wood fiber. They’re caused by a variety of conditions and can be any color other than the natural color of the wood. A number of non-wood destroying fungi can cause stains or discoloration. Some stains may indicate decay or bacteria are present.
Spalting – Any form of wood discoloration caused by fungi. It’s typically found in dead trees, so if the wood isn’t stabilized at the right time it will eventually become rotten wood.
There are three types of spalting that are typically incorporated into woodworking as design elements: pigmentation (“sapstain”), white rot, and zone lines.
Twisted fibers – These are known as wandering hearts and caused by twisting of young trees by fast blowing wind. The timbers with twisted fibers is unsuitable for sawing.
Wood defects due to conversion
Conversion is the process of converting raw timber to forms suitable for woodworking or construction projects. During this process, the following defects may occur:
Chip mark – Shallow depressions or indentations in the surface of a board caused by shavings or chips getting imbedded in the surface during the process of dressing. They may be formed by a planer or jointer.
Diagonal grain – Wood in which the annual growth rings are at an angle with the axis of a piece as a result of sawing at an angle. In other words, rather than running parallel to the long edge of a board, for example, the grain runs at an angle to it. Such wood is not permitted for structural applications in the American Forestry Association guidelines because it lacks the same structural strength as an equal-sized piece that has the grain running parallel to the edge.
Torn grain – An irregularity in the surface of a board where wood fibers below the level of the dressed surface have been torn or broken out by a planer.
Wane – The presence of bark or the absence of wood on the corners or along the length of a piece of lumber. Wane, in the form of bark, is more commonly associated with rough milled lumber. In the case of construction lumber (e.g., 2x4s), it can be bark or missing wood.
Machine burn – A darkening of the wood due to overheating by machine knives or rolls when pieces are stopped in the machine.
Machine bite – A depressed cut of the machine knives at the end of the piece.
Machine gouge – A groove cut by the machine below the desired line.
Wood defects due to seasoning
Seasoning is the process of drying lumber (either in a kiln or air drying) to an appropriate level of moisture for woodworking and other commercial uses. During this process, a board may become warped.
The term “warped” is a nonspecific term that refers to a distorted or misshapen board. More specific terms for warping include cupping, twisting, bowing, crook, and spring. Common seasoning defects, including types of warping, include:
Bowing – A curvature formed in the direction of the length of timber. A bowed board is flat, but bent, like a road going over a hill.
Check – A check is a crack which separates the fibers of wood. It does not extend from one end to the other. It occurs across the growth rings and is usually caused by poor or improper drying processes.
Crook – Where the board remains flat, but the ends move away from the center. Another type of warp.
Twisting – Where the board curves in length and width like a propeller.
Cupping – Where the face of a board warps up across its width such that if one looks at the end of the board, it will look like a shallow letter “U.” Is common with plain-sawn lumber.
Spring – Occurs when the board remains flat in width, but curves in length like a river going around a bend.
Case hardening – When lumber or timber is dried too quickly, wood shrinks much at the surface, compressing its damp interior. This results in unrelieved stress. Case-hardened wood may warp considerably and dangerously when the stress is released by sawing. Extreme cases of case hardening leads to honeycombing.
Honeycombing – During drying, internal stresses cause various radial and circular cracks to develop in the interior portion of the wood resembling a honeycomb texture. Honeycombing is among the worst of drying defects because it’s irreversible and usually cannot be detected by looking at the face of the lumber.
Dealing with defects
Many woodworkers prefer to avoid wood with defects because they detract from the beauty or value of the finished product. For others, though, defects in their works are often highly prized.
Take, for example, wood microphone maker Greg Heumann of Geyersville, Calif., and tree sculptor Cecil Ross of Bainbridge Island, Wash. They often seek wood with certain defects because they believe they add character to the wood and ultimately their final product.
“I love wood with color variations, knots, and insect holes because they make each microphone unique,” says Heumann. “As long as there is no threat to the mic’s structural strength, I normally leave them in as is.”
Ross also seeks wood with blemishes, knots, and holes. “They form the basis or focal point of my sculpture’s design,” he notes.
On the other hand, mandolin maker Austin Clark of Boise, Idaho purposely avoids all defects in the spruce and maple he uses to create his mandolins.
“My mandolin customers are pretty traditional, so they want the wood in their instruments to be straightforward. Meaning they need to be free of any defects,” he says.
Many woodworkers rely on suppliers to ship them the wood they want. Most suppliers are reputable and they are careful to send their customers wood that is generally free of deformed or irregular wood.
On the other hand, woodworkers who buy their wood from a lumberyard or big box store should carefully check the wood and select only those pieces that are usable. They may have to get permission to sort through the lumber and also promise to restack everything when done.
Not all woodworkers, however, want to sort through stacks of lumber. Instead, they may opt for the most expensive-grade available, whether it’s needed or not.
Of course, there are times when the best grade is the best choice, especially for premium furniture makers, but more often, woodworkers can save money and get good wood for their projects by using lower grades. Often, lower-grade boards display more beautiful figure and character than better boards. The catch to finding these pieces, though, is to know the basics about buying lumber.
Some woodworkers may even obtain wood from fence rows and fields, and perhaps old abandoned barns or other buildings. But they need to be wary of getting wood that might contain bullets, nails, or barbed wire which could damage a saw blade.
For most woodworkers, the easiest way to deal with natural defects is to simply avoid using the wood. Another option, though, is to hide the defect. Furniture maker John Landis of Warminster, Pa., says, “At times I’ll use a knot in a tabletop as long as I can keep it underneath and it doesn’t pose a structural problem. I won’t sacrifice a beautiful side of a board I can use for making a tabletop if the knot is underneath, not visible, and isn’t a structural problem.”
Defects can also play a supporting role – provided they enhance the beauty of a piece. For instance, when Landis comes across wood that has attractive burls, he rarely uses it as a slab in and of itself because the wood is usually unstable. Instead, he slices it into quarter-inch thick pieces and applies it to a stable backing inside door panels or uses it as a veneer on furniture.
Wood sculptor Jose Rivera from San Antonio, Texas, works with various woods, especially mesquite. He often encounters mesquite with knots and longhorn beetle holes, but has learned to incorporate them into his finished pieces.
“Knots are common in mesquite. To me they add to the beauty and characteristic of the wood,” Rivera says. “As for the insect holes, I prefer not having them. But they do appear in some of my pieces. Actually, the holes are acceptable to my clients because most of them know that’s how mesquite wood is.”
Wood is a hygroscopic material. This means it naturally absorbs and releases water (moisture) to balance its internal moisture content with the surrounding environment.
Because wood retains its hygroscopic nature after it is put into use, it is subjected to fluctuating humidity. Shrinkage and swelling may occur in wood when there are changes in humidity and temperature. This may eventually result in cracks, gaps, and weak joints.
One major problem that occurs when drying wood is the tendency of its outer layers to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the fiber saturation point while the interior is still saturated, drying stresses are set up because the shrinkage of the outer layer is restricted by the wet interior. Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses across the grain exceed the strength across the grain.
Since wood shrinks and changes shape as it dries, the bulk of that shrinkage and change of shape should occur before a woodworker starts working with it.
The amount of shrinkage varies from species to species, but generally wood shrinks 8 to 10 percent tangentially, 4 to 5 percent radially, and close to zero percent lengthwise. In other words, the surface of the board where the grain intersects it perpendicularly, or close to perpendicularly, shrinks the most. This means woods of different shapes will shrink differently based on how they’re cut from the tree.
Some kiln-dried wood can change 1/8” to ¼” in width for every foot. They may not seem like much, but when you add it up, a 4-foot wide table can vary in width by as much as an inch from dry season to wet season.
Moisture changes and wood defects
Moisture, therefore, is certainly the most important factor affecting the performance and service life of wood and wood products. Because moisture affects the dimensional movement of wood and wood products, under certain conditions moisture change can lead to major dimensional change.
Once kiln-dried wood arrives, woodworkers should store the wood in a climate-controlled shed or shop in order to avoid wood movement. Seasonal changes in humidity can cause wood to shrink or swell.
Some woodworkers, however, prefer to buy green lumber and dry it themselves – either by air drying or using a solar kiln. This can help offset the high price of kiln-dried lumber.
Depending on where wood is stored and under what conditions, wood can easily absorb or release moisture. For that reason, all wood should be monitored regularly and certainly before use in order to avoid moisture-related problems.
This is one of the reasons why more woodworkers today are using moisture meters to prevent problems such as warping, loose joints, or cracked table tops.
Mandolin maker Clark says, “When I work with wood, moisture is a big issue. Most of my wood comes from the Pacific coast region. It arrives more wet than what it ends up being here.”
To ensure his wood gets down to an ideal 7 percent moisture content, Clark always uses a moisture meter. “I won’t work with any wood until the moisture comes down,” he adds.
Tree sculptor Cecil Ross, who lives in the Northwest where moisture is constant, says he’s used a pinless moisture meter all his career. “In our wet climate, the best you can hope for is working with wood that’s stable and has a constant moisture.”
Wood microphone maker Heumann says he learned the hard way when he first started making wood mics. “I paid no attention to moisture content. But I’ve had woods change size and warp by as much as 0.50 inch over a 2.5-inch diameter – and that is more than enough to cause trouble for me. After a few mishaps, I started paying attention,” he adds.
Huemann has since purchased a wood moisture meter, the Wagner MMC220. “It not only measures wood moisture in all wood species, including the rare tropical species I use, but it tells me exactly what I need to know. It has been worth every penny I spent for it in saved time and increased product quality,” he says.
Ron Smith is a sales manager for Wagner Meters, and has over 30 years of experience in instrumentation and measurement systems in different industries. In previous positions, he has served as a regional sales manager, product and projects manager, and sales manager with manufacturers involved in measurement instrumentation. Call Wagner Meters today at (800) 634-9961 and ask for Ron, or visit WagnerMeters.com.