Various defects which are likely to occur in timber may be grouped into the following three: (i) Due to natural forces (ii) Due to defective seasoning and conversions. (iii) Due to attack by fungi and insects. (i) Defects due to Natural Forces: The following defects are caused by natural forces: (a) Knots (b) Shakes (c) Wind cracks (d) Upsets

1. Defects of Timber
Introduction
Timberisa natural productand isusuallynotfree fromimperfections.Howeverwhatisanimperfection
to one personmaybe a feature oran embellishmenttoanother.Forinstance,twistedgrainissought
afterby craftspeople interestedinturningbowlsorincuttingexoticveneers,whereasforajoiner
makinga door,twistedgrainwouldbe totallyunacceptable.
A 'defect'isgenerallyanimperfection,whicheitherweakensthe woodgreatlyormakesthe wood
unsuitable toworkwithforthe purpose itisto be used.Defectsintimbercanbe groupedintothe
followingfive categories:
 Defectsdue toconversion
 Defectsdue tofungi
 Defectsdue toinsects
 Defectsdue tonatural forces
 Defectsdue toseasoning

2. Defects due to conversion
Duringthe processof convertingtimbertocommercial form, the followingdefectsmayoccur:
 Chipmark
 Diagonal grain
 Torn grain
 Wane
Chip Mark
Thisdefectisindicatedbythe marksor signsplacedbychipson the finishedsurface of timber.Theymay
alsobe formedbythe parts of a planingmachine.
DiagonalGrain
Thisdefectisformeddue toimpropersawingof timber.Itisindicatedbydiagonal markonstraight
grainedsurface of timber.
TornGrain
Thisdefectiscausedwhena small depressionisformedonthe finishedsurface of timberbyfallingof a
tool or so.
Wane
Thisdefectisdenotedbythe presence of original roundedsurface onthe manufacturedpiece of timber.

3. Defects due to fungi
Fungi are minute microscopicplantorganisms.Theyattacktimberonlywhenthe followingtwo
conditionsare satisfiedsimultaneously.
i. The moisture contentof timberisabove 12-15%.
ii. There ispresence of airand warmthfor the growth of fungi.
If any of the above conditionisabsent,decayof wooddue tofungi wouldnotoccur. Hence, drywood
havingmoisture contentlessthan12-14 % will remainsoundforcenturies.Similarly,woodsubmerged
inwater will notbe attackedbyfungi because of absence of air.Followingdefectsare causedintimber
by fungi.
 Blue stain
 Brownrot
 Dry rot
 Heart rot
 Sap stain
 Wet rot
Blue Stain
Sap of woodis stainedtobluishcolorbythe actionof certaintype of fungi.
BrownRot
The term rot isusedto indicate decayordisease of timber.Certaintypesof fungi remove cellulose
compoundsfromwoodandhence,woodassumesthe browncolor.Thisisknownas brownrot.
Dry Rot
Certaintypesof fungi feedonwoodandduringfeeding,theyattackonwoodand convertitinto dry
powderform.Thisisknownas dry rot.
Heart Rot
Thisis formedwhenabranch has come out of a tree.In sucha case,heartwoodisexposedtothe attack
of atmosphericagents.Ultimately,the tree becomesweakanditgivesouthollow soundwhenstruck
witha hammer.
Sap Stain
Certaintypesof fungi donot bringaboutthe complete decayof timber.Buttheyfeedoncell contentsof
sap wood.Indoingso,sap woodlosesitscolor.Thisis knownassap stainand it generallyoccurswhere
moisture contentgoesbeyond20%or so.
Wet Rot
Some varietiesof fungi cause chemicaldecompositionof wood of the timberandindoingsothey
converttimberintoa greyishbrownpowder.Thisisknownaswetrot.

4. Defects due to insects
Followingare the insectswhichare usuallyresponsible forthe decayof timber:
Beetles
Marine borers
Termites
Beetles
These are small insectsandtheycause rapiddecay of timber.They formpin-holesof size about2 mm
diameterinwood.Theyattackthe sap woodof all speciesof hardwood.Tunnelsare formedinall
directionsinsapwoodbythe larvae of these beetles.The timberisconvertedintofine flour-like powder.
Theyusuallydonotdisturbthe outershell orcover.Hence,timberpiece attackedbybeetlesmaylook
soundtill itcompletelyfails.
MarineBorers
These are generallyfoundinsaltywater.Mostof the varietiesof marine borersdonotfeedonwood.
But theymake holesorbore tunnelsinwoodfortakingshelter.The diameterandlengthof these holes
may go as highas25 mmand 60 mm respectively.The woodattackedbymarine borerslosescolorand
strength.Itmay be notedthat no timberiscompletelyimmune fromthe attackof marine borers.
Termites
These are popularlyknownas‘white ants’andtheyare foundinabundance intropical and sub-tropical
countries.These insectslive inacolonyandtheyare veryfast ineatingawaythe woodfromcore of the
cross section.Theymake tunnelsinsidethe timberindifferentdirectionsandusuallydonotdisturbthe
outershell orcover.Hence,timberpiece attackedbytermitesmaylooksoundtill itcompletelyfails.
Veryfewgoodtimberssuchas teak,sal,etc.,can resistthe attack of white ants.Such timbershave
certainchemicalsintheircompositionandthe smell of these chemicalsisnotfavorable fortermites.

5. Defects due to natural forces
The main natural forcesresponsible forcausingdefectsintimberare two,namely:
 Abnormal growth
 Rupture of tissues
Followingdefectsare causedbythese forces:
 Burls
 Chemical stain
 Coarse grain
 Deadwood
 Knots
 Shakes
 Twistedfibres
 Upsets
 Water stain
 Windcracks
Burls
These are alsoknownas ‘excrescences’andtheyare particularlyformedwhenatree has receivedshock
or injuryinitsyoungage. Due to such injury,the growthof tree iscompletelyupsetandirregular
projectionsappearonthe bodyof the timber.
ChemicalStain
Wood issometimesdiscoloredbythe chemical actioncausedtoitby some external agency.Thisis
knownas chemical stain.
CoarseGrain
If a tree growsrapidly,annual ringsare widened.Itisknownascoarse grainedtimberandsuchtimber
possesseslessstrength.
Dead Wood
Timberwhichisobtainedfromdeadstandingtreescontainsdeadwood.Itisindicatedbylightweight
and reddishcolor.
Knots
These are basesof branchesor limbswhichare brokenorcut off from the tree.The portionfromwhich
the branch isremovedreceivesnourishmentfromthe stemfora prettylongtime and itultimately
resultsinthe formationof dark,hard ringswhichare knownasknots.As continuityof woodfibersis
brokenbyknots,theyforma source of weakness.

6. Knotsare classifiedonthe basisof theirsize.
Sl. No Type of Knot Size
1 Pinknot Diameterupto6.5 mm
2 Small knot Diameterbetween6.5mm and 20 mm
3 Mediumknot Diameterbetween20mm and 40 mm
4 Large knot Diametergreaterthan40 mm
Knotscan alsobe classifiedonbasisof theirformand quality.
Sl. No Type of Knot Description
1 Deadknot The fibersof knotare not properlyinterconnectedwiththose of
surroundingwood.Hence,itcanbe easilyseparatedoutfrom
the bodyof wood.It isnot safe touse woodwithsucha knot
for constructionpurposes.
2 Decayedknot It isalso knownasan unsoundknotand itis formedbythe
actionof fungi onwood.
3 Live knot It isalso knownasa soundknot.It isfree fromdecayand
cracks. It isthoroughlyfixedinwoodandhence,itcannotbe
separatedoutfromthe body of wood.Presence of suchknots
makeswooddifficulttoplane.However,woodcontainingsuch
knotscan be usedforconstructionpurposes.
4 Loose knot It indicatespreliminarystage of deadknot.The fibersof this
knotare not firmlyheldinthe surroundingwood.
5 Roundknot Cross-sectionof thistype of knotiseitherroundoroval .It is
obtainedbycuttingthe knotat right anglestoitslongaxis.
6 Tightknot It indicatespreliminarystage of live knot.The fibersof knotsare
firmlyheldinthe surroundingwood
Rind Galls
Rindmeansbark andgall indicatesabnormal growth.Hence,peculiarcurvedswellingsfoundonthe
bodyof a tree are known asRind Galls.Theydevelopatpointsfromwhere branchesare improperlycut
off or removed.

7. Shakes
These are cracks whichpartlyor completelyseparate the fibersof wood.Followingare the different
varietiesof shakes:
 Cup Shakes:These are causedby the rupture of tissue ina circulardirection.Itisa curvedcrack
and itseparatespartlyone annual ringfromthe other.Itdevelopsdue tonon-uniformgrowth.
It may notprove to be harmful,if itcoversonlya portionof ring.
 Heart Shakes:These cracks occur in the centre of cross-sectionof tree andtheyextendfrom
pithto sap woodinthe directionof medullaryrays.These cracksoccur due to shrinkage of
interiorpartof tree whichisapproachingmaturity.Heartshakesdividethe tree cross-section
intotwo tofour parts.
 Ring Shakes: Whencup shakescoverthe entire ring,theyare knownasring shakes.
 Star Shakes:These are cracks whichextendfrombarktowardsthe sap wood.Theyare usually
confineduptothe plane of sapwood.Theyare wideronthe outside endsandnarroweronthe
inside ends.Theyare usuallyformeddue toextremeheatorfrost.
 Radial Shakes: These are similartostar shakes.Buttheyare fine,irregularandnumerous.They
usuallyoccurwhenthe tree isexposedtosunfor seasoningafterbeingfelleddown.Theyrun
for a short distance frombarktowardsthe centre,thenfollow directionof annual ring,and
ultimatelyruntowardspith.
Twisted Fibers
These are alsoknownas wanderingheartsandtheyare causedby twisting of youngtreesbyfast
blowingwind.The fibersof woodare twistedinone direction.Timberwithtwistedfibersisunsuitable
for sawing.Itcan however,be usedforpostsandpolesinan unsawncondition.

8. Upsets
These are alsoknownas rupturesand they indicate woodfiberswhichare injuredbycrushingor
compression.Upsetsare mainlydue toimproperfellingof tree andexposure of tree initsyoungage to
fastblowingwind.
Water Stain
Wood issometimesdiscoloredwhenitcomesintocontactwithwater. Thisisknownaswater stainand
thisdefectisusuallyfoundinconvertedtimber.
Wind Cracks
If woodis exposedtoatmosphericagencies,itsexteriorsurface shrinks.Suchashrinkage resultsin
cracks .These are knownas windcracks.

9. Defects Due to Seasoning
Followingdefectsoccurinseasoningprocessof wood.
 Bow
 Case-hardening
 Check
 Collapse
 Cup
 Honey-combing
 Split
 Twist
 Wrap
Bow
The defectisindicatedbythe curvature formedinthe directionof lengthof timber.
Case-hardening
The exposed surface of timberdriesveryrapidly.It,therefore,shrinksandisundercompression.The
interiorsurface whichhasnotcompletelydriedisundertension.Thisdefectisknownascase-hardening
and itusuallyoccursin timberswhichare placedatthe bottomduringseasoning.
Check
A checkis a crack whichseparatesfibersof wood.Itdoesnotextendfromone endtothe other.
Collapse
Due to unevenshrinkage,woodsometimesflattensduringdrying.Thisisknownascollapse.
Cup
Thisdefectisindicatedby the curvature formedinthe transverse directionof timber.
Honey-combing
Due to stressdevelopedduringdrying,variousradial andcircularcracks developinthe interiorportion
of timber.Timberthusassumeshoney-combtextureandthe defectsodevelopedisknownashoney-
combing.
Split
Whena crack extendsfromone endtothe other,itis knownas a split.
Twist
Whena piece of timberhasspirallydistortedalongitslength,itisknownastwist.
Warp
Whena piece of timberhastwistedoutof shape,itis saidto have warped.