Common wood defects can occur from the time it spawns from a seedling to the final steps of the seasoning process. Wood defects are usually an abnormality or irregularity in the wood that arise from many different causes. For example, natural and acquired wood defects can be caused from insect and fungal attack, or rapid tree growth. There are also natural wood defects that we cannot control. Such common wood defects can be caused by the woods natural characteristic to absorb or expel moisture in the air. And, there are man-made wood defects caused by incorrect milling, sawing or machining. Other common mechanical wood defects are known as (conversion); improper drying, (seasoning); or improper handling and storage.
Common wood defects is what is responsible for reducing wood’s monetary and utilized value. Wood defects is the number one culprit in lowering woods strength, durability and usefulness. Defects causes marring to the wood’s appearance, and in some cases, causes it to rapidly decay.
Always remember, no tree is perfect or exactly like another.
Wood Defects Glossary
Natural Wood Defects
During its lifetime, a tree is exposed to natural elements that cause defects in the wood. Woodworkers are quite familiar with these defects. Knots, splits, dark streaks or stains, worm holes, even decay are some commonly known natural wood defects. Other common wood defects all woodworkers encounter are:
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 when trying to feed on the insects living in or under the bark and in the wood of the tree. Bird pecks cause small injuries to the tree, resulting in grain changes that later show up as various forms of figure or imperfections 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. As the tree continues to grow it follows the contour of the original burl deformity, producing all manner of twists, swirls and knots in the wood fiber. This type of growth in the wood usually turns out to have 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 can often 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 and durability.
Fungal damage – Fungi generally damages timber or wood by discoloration in the grain and/or decay. Wood affected by fungi is generally weaker or of an undesirable color than what 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 the beginning stages of dry rot.
- Dry rot – After the wood that was once infected with brown rot dries out, the cell walls of the remaining wood begins to break down and can be crushed to a dry powder.
- Heart rot – This is formed when a branch has come out of the tree. The heart wood is exposed to an attack from insects and the elements . Ultimately, the tree becomes weak and it gives a hollow sound when struck with a hammer.
- Wet rot – Kinds of fungi cause chemical decomposition of a wood’s timber. In doing so it 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. It is a type of fungi attack where the wood’s lining and the wood itself acquires the appearance of a white mass consisting of cellulose compounds. Trees that develops white rots during their early stages of growth form what is commercially termed “spalted wood.” Spalted wood has a unique color and figure, and some woodworkers highly prize it.
Insect Defects – There are a number of insects that eat wood. There are many insects that do not eat wood, but choose 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 of insects prefer to attack live tress that are usually stressed out. Other insects 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 core. They usually do not disturb the outer shell or cover. In fact, the timber piece attacked by termites may look sound until it is ready to be milled.
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 results in the formation of dark hard rings known as knots. As the wood fibers continue to break down in the area of the knot it forms a source of weakness. Several types of knots include:
- 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 within 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 end grain or length 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. This is a naturally occurring defect usually caused by frost or wind stress in colder climates. Shake also occurs at the time of impact when felling. Shrinkage of the log before conversion is also a cause of shake. 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 from 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 from 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.