Wood Defects

WOOD DEFECTS:

Definition: “Those abnormalities/ irregularities/ blemishes which tend to lower the economic value or decorative value wood and also reduces the strength of wood, are known as wood defects”.

  • Natural Defects:

Those abnormalities which develop in the standing tree naturally, difficult to control but can be minimized up to some extent eg the development of knot (a natural phenomenon) is difficult to control but can be minimized by the Silvicultural operations such as pruning.

  • Biological Defects:

The abnormalities caused in a tree due to the biological agencies such as fungi, insects and can be controlled by the insecticides, pesticides, or fungicides and biological control is also done to reduce the biological defects in a wood.

  • Seasoning Defects:

These are the defects which are created during the seasoning of the wood. The defects created during the process are known as seasoning defects.

Following are some defects and their reasons:

  1. Cupping: When sawn log makes width wise curve is called cupping.
  2. Bowing: When any converted log makes lengthwise curve is called bowing.
  3. Twisting: When the four corners of a plank are in the same plane.
  4. Checks: It is the separation of fiber that does not extend through the timber from one face to the other face.
  5. Split: It is the separation of fiber extending from face to face.



NATURAL DEFECTS:

Definition: The defects caused in a standing tree naturally are known as natural defects.

Following are the Natural Defects found in wood. i) Knots  ii) Reaction wood  iii) Brashness  iv) Defects due to grains v) Shakes  vi) Frost Injuries  vii) Pitch defects  viii) Mineral streak ix) Bark Pocket x) Chemical Stains xi) Insect Attack

KNOTS:

The portion of the branch wood embedded in main trunk or bole of the tree is called a knot.

Knot - Wood Defects

Knots are of two types:

  • Live Knot: If the branch is alive then it is fully in connection with the main trunk then it is known as a live knot. OR; when the tissues of the main branch are in continuation of the main trunk then it is called live knot. The live knot is also known as “Light knot”.
  • Dead Knot: When the tissue of branch wood is not in continuation with the main trunk. It is also known as loose knot because it may create a hole in the trunk. Due to its color, it is also known as black knot.

Knots - Wood Defects

Rotten Knots: Those knots whose tissues are decayed due to fungal attack or insect attack.

  • These knots are studied in the timber for grading purposes.
  • Greater the size of the knot, lesser will be the strength of wood.
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REACTION WOOD:

“It is an abnormal wood which is developed in leaning trunk or branches.”

Reaction woods are of two types:

  1. Compression wood
  2. Tension wood

 

  1. COMPRESSION WOOD:
    • Definition: Those reaction woods which are developed in leaning soft-woods trees are known as compression woods.

In softwood leaning trees compression wood also develop below in the branches to support them. See fig.

Reaction Wood - Wood Defects

  • Anatomical approach: Normally tracheids, in softwood, are rectangular or square in shape, when they become circular there will be intercellular spaces created which facilitate the development of compression wood.
  • Chemistry of wood: In compression wood, the properties of lignin becomes more than cellulose, as compared to the normal wood, due to which it becomes harder but not stronger.
  • Anatomical differences: Compression wood is developed generally in the latewood portion of the growth ring or in simple words compression wood is restricted to the late-wood.
  • Compression wood is more dark brown in color than normal wood.
  • Compression wood always develops inside the center of the tree and on the lower sides of the branches.
  1. TENSION WOOD:
    • Definition: A type of reaction wood which is developed in the hard-wood leaning trees is called tension wood.
    • Tension wood developed above the branches to support them.

Tension Wood - Wood Defects

  • In tension wood, the proportion of cellulose is more than lignin as compared to that or normal wood.
  • Tension wood is paler in color than normal wood.
  • In tension wood, there are more gelatinous fibers in which there is an extra cellulosic layer.
  • The surface of tension wood remains wooly after planning due to the break out of fiber, which may be due to a lesser quality of lignin

Causes Of Formation Of Reaction Wood/ How Reaction Wood Is Formed:

About the formation of Reaction wood, different views are found, some of them are:

  1. The formation of reaction wood is initiated by the gravitational pull.
  2. Due to the decrease in growth hormone level.
  3. Due to unbalanced growth stresses.
  4. Develop due to wind actions.
  5. Develop due to increase in cambial activity.

According to Sinnott:

“Any factors which force a tree out of its normal growth cause a formation of reaction wood.”

BRASHNESS:

Definition:

Breaking of wood at a force at which normal wood does not break is called brashness.

Explanation:

Brashness a natural defect in the wood of standing trees. The defects develop when all the fibers within the wood are arranged in a single plane. In such an arrangement a less force is required to break it into pieces. In contrast, in normal wood the fibers are arranged in different plans and more force is required to break them.

Hence we can say that brashness is a natural defect in wood which reduces the strength property of wood.

Brashness - Wood Defects

However, this defect is objectionable because it cannot be identified or defined before breaking.



DEFECTS DUE TO GRAINS:

There are two types of grains in the wood.

  1. Straight Grains: These are those grains which run longitudinally parallel to each other.
  2. Cross Grains: The grains which deviate from straightness are called cross grains.
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Cross grains of several types such as spiral grains, wavy grains, interlock grains, etc.

In above grains, the spiral grains are considered as defective because it causes a reduction in the strength of wood.

Defects due to Grains - Wood Defects

SHAKES:

Definition: “Separation of fiber along the grain, this may be caused due to improper seasoning or in natural standing trees.”

  • In natural standing trees, these may be caused due to the variation in day and night temperature in cold climate area due to rapid contraction and expansion. During night time due to fall in temperature the fibers contract and during daytime the rise in temperature cause expansion in the fibers contract and during daytime the rise in temperature cause expansion in the fivers. This rapid expansion and contraction cause them to break and an abnormality or defect develop in a standing tree, called Shakes.
  • When separation of the fibers along the growth rings tangential direction is called Ring Shake, Cup Shake or Wind Shake.
  • When the separation of fibers perpendicular to the growth ring then this condition is known as Heat Shake.
  • When more than one separation appeared perpendicular to the growth ring, then such an arrangement of fiber separation is called Star Shake. These shakes develop along the radiating rays.

Shakes - Wood Defects

FROST INJURIES:

It is also a natural defect in standing trees, occur in the cold climate areas. The abrupt variation in day and night temperature in the cold climatic areas results in the Frost Injuries.

Frost injuries are of following types:

  • Frost Rings: It is a natural defect in which dark brownish lines appear within the growth rings due to the freezing temperature.

Frost Injuries - Wood Defects

  • Frost Cracks: A natural defect occurring in the trees of cold climatic areas in which longitudinal splitting appears at the basal portion of the trees.

Mostly it occurs in the old trees due to the low temperature at the basal portion of the tree.

 

  • Frost Ribs: At the basal portion of trees, in the cold climatic areas, the gradual opening and healing of frost crack due to the variation in day and night temperature, results in the formation of frost ribs. OR; it is the radial longitudinal splitting in the bark near the basal portion of the trees. See fig.

Frost Cracks - Wood Defects

PITCH DEFECTS:

It is a natural defect which normally develops in those trees which produce resin (in softwood trees) like Pinus roxbrughii.

The pitch defect develops due to excessive accumulation of the resin.

The pitch defects may be of the following types:

  • Pitch Streak:

A natural defect in the standing trees in which excessive accumulation of resin occurs in the form of brownish lines.

  • Pitch Pocket:

A natural defect in standing trees in which lens-shaped opening is filled with resin.

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Pitch Defects - Wood Defects



MINERAL STREAK:

  • A natural defect caused due to the excessive accumulation of minerals in the form of lines is called Mineral Streak.
  • The color of mineral streaks varies from spp to spp.
  • It is defective because it makes the wood harder and causes dulling effects.
  • When a wood is seasoned cracks develop at the area of mineral streaks.

BARK POCKETS:

  • Small patches of bark that are embedded in the wood apparently developed from some injury to the tree resulting in the death of small areas of cambium.
  • The surrounding tissue continues to function and new cambium forms over the gap in the inner bark thus embedding a portion of the bark in the wood.
  • Bark pocket may arise as injuries inflicted on the tree by insects (bark beetles) or as a result of bird pecks (work of sapsuckers).
  • The shallow tunnels of certain insects may become partly overgrown with cambium and subsequently embedded in the wood by the activity of the cambium.
  • When the resin is filled in these bark pockets, then this defect is also known as a black check or black streaks eg western hemlock

CHEMICAL STAINS:

  • Stains of these sorts are caused by chemical changes in Lumina of cells.
  • These defects are caused by wood staining fungi which are of two types:
    • Molds: Grow on the surface of wood causing superficial discoloration. Can be brushed or planned off.
    • True Sap Stain: Fungi which penetrate the sapwood and cause staining to deep. Cannot be removed.
  • As fungi feed on the cell contents causing stain and are confined to the sapwood so-called sap stain.
  • Chemicals stains are beloved to be due to the oxidation of certain constituents of the protoplast and therefore the term oxidation stains are used.
  • Chemical stains in color ranging from the shade of yellow and orange to brown.
  • If the stain is developed during air seasoning of lumber called as Yard brown stain.
  • If the stain is developed in the kiln seasoning of lumber called as Kiln brown stain.
  • Chemicals stain develop in both hard and softwoods.
  • Strength is not affected by stain production.
  • The important chemical stains are brown discolorations that are especially prevalent in sugar and ponderosa pine.

INSECT ATTACK:

  • Insects cause pith flecks, pinholes and grub holes in wood before it is utilized.

Pitch Flecks:

  • Pitch flecks or medullary spots are confined to hardwoods. They appear as small areas of wound tissue within the limit of growth in cross section. Along the grain, pith flecks appear to be dark streaks.
  • Defects of this type are produced in wood through injury in the cambium resulting from infestation by the larvae of flies Agromyze spp.

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