Defects of Timber

IntroductionTimber is a natural product and is usually not free from imperfections. However what is an imperfection to one person may be a feature or an embellishment to another. For instance, twisted grain is sought after by craftspeople interested in turning bowls or in cutting exotic veneers, whereas for a joiner making a door, twisted grain would be totally unacceptable.

A 'defect' is generally an imperfection, which either weakens the wood greatly or makes the wood unsuitable to work with for the purpose it is to be used. Defects in timber can be grouped into the following five categories:

Defects due to conversion Defects due to fungi Defects due to insects Defects due to natural forces Defects due to seasoning

Defects due to conversionDuring the process of converting timber to commercial form, the following defects may occur:

Chip mark Diagonal grain Torn grain Wane

Chip MarkThis defect is indicated by the marks or signs placed by chips on the finished surface of timber. They may also be formed by the parts of a planing machine.

Diagonal GrainThis defect is formed due to improper sawing of timber. It is indicated by diagonal mark on straight grained surface of timber.

Torn GrainThis defect is caused when a small depression is formed on the finished surface of timber by falling of a tool or so.

WaneThis defect is denoted by the presence of original rounded surface on the manufactured piece of timber.

Defects due to fungiFungi are minute microscopic plant organisms. They attack timber only when the following two conditions are satisfied simultaneously.

i. The moisture content of timber is above 12-15%.ii. There is presence of air and warmth for the growth of fungi.

If any of the above condition is absent, decay of wood due to fungi would not occur. Hence, dry wood having moisture content less than 12-14 % will remain sound for centuries. Similarly, wood submerged in water will not be attacked by fungi because of absence of air. Following defects are caused in timber by fungi.

Blue stain Brown rot Dry rot Heart rot Sap stain Wet rot

Blue StainSap of wood is stained to bluish color by the action of certain type of fungi.

Brown RotThe term rot is used to indicate decay or disease of timber. Certain types of fungi remove cellulose compounds from wood and hence, wood assumes the brown color. This is known as brown rot.

Dry RotCertain types of fungi feed on wood and during feeding, they attack on wood and convert it into dry powder form. This is known as dry rot.

Heart RotThis is formed when a branch has come out of a tree. In such a case, heartwood is exposed to the attack of atmospheric agents. Ultimately, the tree becomes weak and it gives out hollow sound when struck with a hammer.

Sap StainCertain types of fungi do not bring about the complete decay of timber. But they feed on cell contents of sap wood. In doing so, sap wood loses its color. This is known as sap stain and it generally occurs where moisture content goes beyond 20% or so.

Wet RotSome varieties of fungi cause chemical decomposition of wood of the timber and in doing so they convert timber into a greyish brown powder. This is known as wet rot.

Defects due to insectsFollowing are the insects which are usually responsible for the decay of timber:

Beetles

Marine borers

Termites

BeetlesThese are small insects and they cause rapid decay of timber. They form pin-holes of size about 2 mm diameter in wood. They attack the sap wood of all species of hard wood. Tunnels are formed in all directions in sapwood by the larvae of these beetles. The timber is converted into fine flour-like powder. They usually do not disturb the outer shell or cover. Hence, timber piece attacked by beetles may look sound till it completely fails.

Marine BorersThese are generally found in salty water. Most of the varieties of marine borers do not feed on wood. But they make holes or bore tunnels in wood for taking shelter. The diameter and length of these holes may go as high as25 mm and 60 mm respectively. The wood attacked by marine borers loses color and strength. It may be noted that no timber is completely immune from the attack of marine borers.

TermitesThese are popularly known as ‘white ants’ and they are found in abundance in tropical and sub-tropical countries. These insects live in a colony and they are very fast in eating away the wood from core of the cross section. They make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover. Hence, timber piece attacked by termites may look sound till it completely fails. Very few good timbers such as teak, sal, etc., can resist the attack of white ants. Such timbers have certain chemicals in their composition and the smell of these chemicals is not favorable for termites.

Defects due to natural forcesThe main natural forces responsible for causing defects in timber are two, namely:

Abnormal growth Rupture of tissues

Following defects are caused by these forces:

Burls Chemical stain Coarse grain Dead wood Knots Shakes Twisted fibres Upsets Water stain Wind cracks

BurlsThese are also known as ‘excrescences’ and they are particularly formed when a tree has received shock or injury in its young age. Due to such injury, the growth of tree is completely upset and irregular projections appear on the body of the timber.

Chemical StainWood is sometimes discolored by the chemical action caused to it by some external agency. This is known as chemical stain.

Coarse GrainIf a tree grows rapidly, annual rings are widened. It is known as coarse grained timber and such timber possesses less strength.

Dead Wood Timber which is obtained from dead standing trees contains dead wood. It is indicated by light weight and reddish color.

KnotsThese are bases of branches or limbs which are broken or cut off from the tree. The portion from which the branch is removed receives nourishment from the stem for a pretty long time and it ultimately results in the formation of dark, hard rings which are known as knots. As continuity of wood fibers is broken by knots, they form a source of weakness.

Knots are classified on the basis of their size.

Sl. No Type of Knot Size1 Pin knot Diameter upto 6.5 mm2 Small knot Diameter between 6.5 mm and 20 mm3 Medium knot Diameter between 20 mm and 40 mm4 Large knot Diameter greater than 40 mmKnots can also be classified on basis of their form and quality.

Sl. No Type of Knot Description1 Dead knot The fibers of knot are not properly interconnected with those of

surrounding wood. Hence, it can be easily separated out from the body of wood. It is not safe to use wood with such a knot for construction purposes.

2 Decayed knot It is also known as an unsound knot and it is formed by the action of fungi on wood.

3 Live knot It is also known as a sound knot. It is free from decay and cracks. It is thoroughly fixed in wood and hence, it cannot be separated out from the body of wood. Presence of such knots makes wood difficult to plane. However, wood containing such knots can be used for construction purposes.

4 Loose knot It indicates preliminary stage of dead knot. The fibers of this knot are not firmly held in the surrounding wood.

5 Round knot Cross-section of this type of knot is either round or oval .It is obtained by cutting the knot at right angles to its long axis.

6 Tight knot It indicates preliminary stage of live knot. The fibers of knots are firmly held in the surrounding wood

Rind GallsRind means bark and gall indicates abnormal growth. Hence, peculiar curved swellings found on the body of a tree are known as Rind Galls. They develop at points from where branches are improperly cut off or removed.

ShakesThese are cracks which partly or completely separate the fibers of wood. Following are the different varieties of shakes :

Cup Shakes: These are caused by the rupture of tissue in a circular direction. It is a curved crack and it separates partly one annual ring from the other. It develops due to non-uniform growth. It may not prove to be harmful, if it covers only a portion of ring.

Heart Shakes: These cracks occur in the centre of cross-section of tree and they extend from pith to sap wood in the direction of medullary rays. These cracks occur due to shrinkage of interior part of tree which is approaching maturity. Heart shakes divide the tree cross-section into two to four parts.

Ring Shakes: When cup shakes cover the entire ring, they are known as ring shakes. Star Shakes: These are cracks which extend from bark towards the sap wood. They are usually

confined up to the plane of sapwood. They are wider on the outside ends and narrower on the inside ends.They are usually formed due to extreme heat or frost.

Radial Shakes: These are similar to star shakes. But they are fine, irregular and numerous. They usually occur when the tree is exposed to sun for seasoning after being felled down. They run for a short distance from bark towards the centre, then follow direction of annual ring, and ultimately run towards pith.

Twisted FibersThese are also known as wandering hearts and they are caused by twisting of young trees by fast blowing wind. The fibers of wood are twisted in one direction. Timber with twisted fibers is unsuitable for sawing. It can however, be used for posts and poles in an unsawn condition.

UpsetsThese are also known as ruptures and they indicate wood fibers which are injured by crushing or compression. Upsets are mainly due to improper felling of tree and exposure of tree in its young age to fast blowing wind.

Water StainWood is sometimes discolored when it comes into contact with water. This is known as water stain and this defect is usually found in converted timber.

Wind CracksIf wood is exposed to atmospheric agencies, its exterior surface shrinks. Such a shrinkage results in cracks .These are known as wind cracks.

Defects Due to SeasoningFollowing defects occur in seasoning process of wood.

Bow Case-hardening Check Collapse Cup Honey-combing Split Twist Wrap

BowThe defect is indicated by the curvature formed in the direction of length of timber.

Case-hardeningThe exposed surface of timber dries very rapidly. It, therefore, shrinks and is under compression. The interior surface which has not completely dried is under tension. This defect is known as case-hardening and it usually occurs in timbers which are placed at the bottom during seasoning.

CheckA check is a crack which separates fibers of wood. It does not extend from one end to the other.

CollapseDue to uneven shrinkage, wood sometimes flattens during drying. This is known as collapse.

CupThis defect is indicated by the curvature formed in the transverse direction of timber.

Honey-combingDue to stress developed during drying, various radial and circular cracks develop in the interior portion of timber. Timber thus assumes honey-comb texture and the defect so developed is known as honey-combing.

SplitWhen a crack extends from one end to the other, it is known as a split.

TwistWhen a piece of timber has spirally distorted along its length, it is known as twist.

WarpWhen a piece of timber has twisted out of shape, it is said to have warped.