Several natural defects occur in all kinds of timber depending upon the soil upon which they grow and climatic conditions to which they are subjected while growing. These defects should be avoided or removed as far as possible during conversion for use. The following are the most common defects in timber.
(i) Heart shakes:
These are the cracks or splits in the centre, starting from the pith and extending in the direction of the medullary rays in one or opposite directions towards sap wood. Such defects are found in over matured trees but sometimes they may be caused by quick drying of the central part of the tree, if a tree nearing maturity, is felled and left unbaked for a long time. This defect is caused due to shrinkage of heart wood.
(ii) Star shake:
These are cracks or splits which extend from bark towards sap wood. This defect is caused when the tree is subjected to severe heat or frost during its growth. The width of these cracks or shakes is more at the outer ends and reduces when they extend towards centre.
(iii) Cup-shakes and Ring-shakes:
These are formed by the rupture of tissues in a circular direction across the cross- section of a log, usually along annual rings. When the rupture extends only a part round, it is called a cup shake and when the whole way round or almost so, it is called a ring shake. This defect is caused either due to unequal growth or due to sudden contraction of timber under atmospheric changes assisted by twisting action due to wind.
(iv) Radial shakes:
These are similar to star shake but occur due to exposure to sun when felled timber is placed for seasoning. These cracks are fine, irregular and numerous. Many cracks or split run for a short distance from bark towards centre, then follow the course of annual ring and lastly go towards the centre radially.
(v) Rind galls:
These peculiar curved swellings found on living or dead tree, generally caused by the growth of layers over the wounds left after branches have been imperfectly cut off or removed.
(vi) Upsets or Ruptures:
These are ruptures of fibres of wood caused due to some sort of impact, injury or pressure. They are also caused due to unskillful felling and violent wind effect.
(vii) Twisted fibres or Wandering hearts:
This defect is due to the tree being twisted constantly in one direction by the force of prevalent wind. This wood is unsuitable for squaring or sawing and hence can be used for posts or poles in unsawn condition or for fuel purposes.
(viii) Wind cracks:
These are shakes or splits on the sides of a bark of timber due to the shrinkage of the exterior surface exposed to atmospheric agencies like sun, wind, etc.
They are the roots or bases of small branches of the tree. They may be alive or dead but break the continuity of fibres. In the beginning, the base portion gets food from stem but finally results in formation of dark and hard rings known as knots. These knots are not very harmful when small, hard and round, and used for members other than tie bars and beams. Knots are classified on the basis of diameter. If the diameter of knot is less than 6 mm, it is called a ‘nail knot’ , if the diameter ranges between 6.5 and 20 mm, then it is called ‘small knot’ , and if diameter ranges between 20 and 40 mm then it is called ‘medium knot’ and knots with diameter more than 40 mm are called ‘large knots’ . Timbers containing large dead knots or many smaller ones should be rejected as they are poorer in appearance and strength.
(x) End splits:
These are cracks extending from one face to another.
(xi) Dead wood:
Timber obtained from dead standing tree (i.e., after maturity) is known as dead wood. It is light in weight, deficient in strength and reddish in colour.
It is an early decay of healthy wood indicated by whitish spots. It is due to fungi giving access and setting decay.
Find More About,
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber