TIMBER

Properties- Uses- Classification- Seasoning-

Defects- Preservation- Tests: Hard Board &

Particle Board – Manufacture & Use

TIMBER

• Timber denotes wood which is suitable for building/ carpentry & for

various engineering purposes

• The word timber denotes from an old English word ‘timbrian’ which means to build

TREES

Exogenous Trees Endogenous Trees

Conifers Deciduous

Soft wood Hard wood

Endogenous Trees • In the endogenous trees, a plant grows by the addition of new cells

only at the tips or end

• The trees grows inwards & fibrous mass is seen in their longitudinal section

• Such trees shows very little branches

• Limited engineering applications

• Eg: Bamboo, Cane, Palm etc

Exogenous trees • These trees increase their bulk by growing outwards

• Distinct consecutive rings are formed in the horizontal section of such trees

• These rings are known as annular rings because one ring is added every year

• Such trees grows bigger in diameter

• Mostly used for engineering purposes. Eg: Teak, Sal, Pine

Conifers • Also known as evergreen trees

• These trees bear cone-shaped fruits, thus they

are known as conifers

• Theses trees yield soft wood which are generally

light colored, resinous, light in weight & weak

• Hence they are not used for building constructions

• Eg: pine

Deciduous • Also known as broad leaf trees

• Leaves of these trees fall in autumn & new ones appear in spring

• These trees yield hard wood which are usually close-grained, strong,

heavy, dark colour, durable & non-resinous

• used for building constructions

• Eg: Teak, Sal

Comparison of ‘soft wood’ & ‘hard wood’

ITEM SOFTWOODS HARD WOODS

Annular rings Distinct Indistinct

Colour Light Dark

Fire resistance Poor More

Medullary rays indistinct Distinct

Strength Strong for pull & weak

for resisting shear

Equally strong for

resisting tensile,

compressive and shear

Structure Resinous & split easily Non-resinous & close

grained

Weight Light Heavy

STRUCTURE OF A TREE

Macro Structure Microstructure

I. Macro Structure

• Structure of a wood visible to the eye at a small magnification

The following are different components

1. Pith or medulla

• Innermost part

• It varies in size & shape for different types of trees

• Wood of this zone is black, brown or grey appearance

• It is normally found in the first year of growth of the tree

• It consists entirely of cellular tissues and nourishes the

plant in its young age.

• When the plant becomes old, the pith dies up and decays and the sap is then transmitted by the woody fibres deposited round the pith

Cross Section of an Exogenous Tree

2. Heartwood

• The inner annular rings surrounding the pith constitute the heartwood

• It is usually dark in colour.

• It indicates dead portion of tree & it does not take active part in the growth of tree

• But it imparts rigidity to the tree & hence it provides strong & durable timber for various engineering purposes

3. Sap Wood

• The outer annular rings between heart wood & cambium layer

• It is usually light in colour & weight

• It indicates recent growth & it contains sap

• The annular rings of sap wood are less sharply defined than those of heart wood

• It takes active part in growth of tree & sap moves in upward direction through it

• also known as alburnum

4. Cambium layer

• the thin layer of sap between sap wood & inner bark

• It indicates sap which has yet not been converted into sap wood

• If the bark is removed for any reason, the cambium layer gets

exposed & the cells cease to be active resulting in the death of tree

5. Bark

• It is the outermost zone & makes the skin of the tree

• The function of the bark is to protect the inner tissue from heat,

rain & injury

• Sometimes a second thin membrane is also present inside the bark

& it is called the inner bark

6. Medullary rays

• The thin radial fibres extending from pith to the cambium

• The function is to hold together the annular rings of hard wood &

sap wood

II. Microstructure

• The structure of wood apparent only at great magnifications is known as microstructure

• When studied under a microscope, it becomes evident that wood consists of living & dead cells of various size & shapes

• A living cell consists of four parts, namely membrane, protoplasm, sap & core

• The cells according to their function are classified as conductive cells, mechanical & storage cells

Conductive Cells

• Serve mainly to transmit nutrients from roots to the branches & leaves

Mechanical Cells

• Imparts strength to wood

Storage Cells

• These cells serve to store & transmit nutrients to the living cells & they usually located in medullary rays

Seasoning of Timber

• A newly cut tree contains about 50 % of its own weight as

water in the forms of sap and moisture.

• This moisture may lead to fermentation and consequent decay

of timber.

• Hence this water is to be removed before timber can be used

for any engineering purpose.

• This process of removing moisture or drying of timber is

known as seasoning of timber.

Objectives as seasoning of timber:

1. To decrease the weight of timber and lower the cost transport

and handling.

2. To improve hardness, stiffness ,strength & durability.

3. To make the timber safe from the attack of fungi and inserts.

4. To reduce the tendency of timber to crack, shrink and warp

etc.

5. To make timber fit for receiving treatment of paints,

varnishes, preservatives

6. To allow timber to burn easily if used as a fuel.

7. To make the timber easily workable

Natural Seasoning

• In this method, seasoning of timber is carried out by natural

air, hence known as air seasoning

Following procedure is followed in natural seasoning

• Timber is cut & sawn into suitable sections of planks or

scantling

• The timber pieces can either be stacked horizontally or

vertically, the former arrangement being very common

• The platform of stock is made 30cm higher than the round

level

• The timber pieces are stored out according to the length &

thickness

• They are then arranged in layers, one above the other. All

members in a particular layer are of the same thickness

• Each layer is separated by good quality dry wood

• This arrangement is protected from strong blow of wind, rain

& extreme heat or sun

• Air gets circulated between the layers & in course of time,

timber gets seasoned

• This process takes a long time, even upto 2 years

Artificial Boling 1. Boiling

2. Chemical Seasoning or Salt Seasoning

3. Electrical seasoning

4. Kiln seasoning

5. Water Seasoning

1. Boiling

• In this method, timber is immersed in water & water is then boiled

• Boiling of water extended for 3 to 4 hours

• Boiled timber is then dried very slowly under a shed

• The period of seasoning and shrinkage are reduced by this method,

but it affects the elasticity & strength of wood

• This method of seasoning is quick but costly

2. Chemical Seasoning or Salt Seasoning

• In this method, timber is immersed in a solution of suitable salt

• It is then seasoned in ordinary way

• The interior surface of timber dries in advance of exterior one &

chances of formation of exterior cracks are reduced

3. Electrical seasoning

• In this method, high frequency alternating current is used to dry

up the wood

• This is most rapid method of seasoning, but very costly

4. Kiln seasoning

• In this method, timber is arranged in stacks inside kilns & heated air

is circulated.

• This heated air slowly circulates, takes up the moisture from timber

& season it

• The relative humidity is gradually reduced

5. Water Seasoning

• In this method, timber pieces are immersed in running water of

stream

• The larger ends of the timber is kept facing on the upstream side

• The sap, gum etc contained in timber is washed away by running

water

• This process takes 2 to 4 weeks

• It, however, weaken the timber & makes it brittle

Comparison Between Artificial &

Natural Seasoning

ITEM NATURAL SEASONING ARTIFICIAL

SEASONING

Moisture

content

It is difficult to reduce the

moisture content below 15 to 18%

The moisture content can be

reduced to any desired level

Speed Slow process Quick process

Strength Stronger timber Little weaker timber

Nature Simple & economical Expensive & quite technical

Quality of

timber

More liable to the attack of

insects & fungi

Less liable to the attack of

insects & fungi

Space Requires more space for stacking Requires less space for

stacking

DEFECTS IN TIMBER 1. Defects due to conversion

– Chip Mark

– Diagonal Grain

– Torn Grain

– Wane

2. Defects due to fungi

– Blue Stain

– Sap strain

– Brown Rots

– Dry Rots

– Wet rot

– Heart Rots

3. Defects due to insects

– Beetles

– marine borers

– termites

4. Defects due to natural forces

– Burls

– Knots

– Shakes

– Twisted fibers

– Water stain

– Wind cracks

– Chemical stains

5. Defects due to seasoning

– Bow

– Cup

– Check and spilt

– Twist

– Warp

1. Defects due to conversion

The defects mainly occur during the process of converting timber

to commercial forms such as planks, beams, rafters etc.

The common defects are

• Chip Mark

– This defect is indicated by the marks or signs places by the chips on

the finished surface of timber

• Diagonal Grain

– This defect is formed due to improper sawing of timber

Chip Mark

• Torn Grain

– This defect is caused when a small depression is formed on the

finished surface of timber by falling of tools

• Wane

– This defect is denoted by the presence of original rounded surface

on the manufactured piece of timber

Wane

2. Defects due to fungi

• The fungi 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 20%

(ii) There is presence of air and warmth for the growth of

fungi.

• If any of the above condition is absent, the decay of wood

due to fungi would not occur.

• Hence the dry wood having moisture content less than 20%

will remain sound for centuries.

• Similarly the wood submerged in water will not be attacked

by fungi because of absence of air.

• Blue Stain

– certain type of fungi attack the sap wood and it changes the colour. This is known as stain.

– If fungi changes the sap wood to blue colour. It is known as blue stain.

• Sap strain

– Fungi of certain type do not bring about the complete decay of timber but they feed on cell contents of sap wood

– In doing so sapwood loses its colour

– This is known as sap strain & it generally goes beyond 25%

• Brown Rots

– certain type of remove the cellulose compound from the wood and it becomes brown colour .This is known as brown rots.

Blue Stain

Brown Rots

Heart Rots

Wet Rots

• Dry Rots

– certain types of fungi feed on wood& during feeding they

attack on wood and convert it into dry powder form . This

is known as dry rot.

• Wet rot

– Some varieties of fungi cause chemical decomposition of

wood & they convert timber into a greenish brown powder.

This is known as wet rot

• Heart Rots

– When a branch comes out of a tree, fungi attack the heart

wood & this makes the timber weak

– In this case, the tree become weak and it gives out hollow

sound when struck with a hammer.

3. Defects due to inserts • Following inserts are usually causing defects in timber : beetles,

marine borers and termites.

Beetiles

• These are small insects , form pin –holes like tunnels in all

directions thus affecting the sap wood.

• In this process beetles convert the timber into fine flour like

powder

• Since these insects do not disturb the outer cover of the timber,

the timber attacked by beetles may look sound till it completely

fails.

Marine borers

• These insects generally found in salty water

• These marine borers do no feed on wood, but they make holes or

tunnels in wood under water and taking shelter .

• The timber attacked by marine borers loses colour and strength

Termites

• Termites are white ants and they are found in abundant in

tropical and sub –tropical countries .

• These live in a colony and they are very fast in eating away the

wood from core of the cross-section thus making the timber not

fit for any engineering purpose .

• Very few timbers like teak , Sal etc. can resist the attack of

termites.

• This is because of their smell which is not favorable for

termites.

4. Defects due to natural forces

Burls

• A tree has received shock or injury in its young age, the

growth of tree is completely upset and irregular projections

appear on the body of the timber .

• These projections are called burls.

Knots

• Knots are the bases of branches which are broken or cut off

from the tree.

• Since the knots break the continuity of wooden fibres, they

from a source of weakness.

BURLS

Shakes

• These are cracks which partly or

completely separate the fibres of

wood.

• If the cracks are curved one,

they are called cup shakes.

• If the cup shakes cover the entire

ring they are known as called

ring shakes.

• If the cracks are radial then they

are called heart shakes.

• If the cracks extend from bark

towards the sap wood, then they

are called star shakes .

• Radial shakes are similar to star

shakes but they are fine,

irregular and numerous.

Heart Shakes

Star Shakes

Twisted Fibres

Twisted Fibers

• These are caused by twisting young trees by fast blowing wind.

• The fibres are twisted in one direction making it unsuitable for

sawing.

Water Stain

• When the wood comes into contact with water, sometimes it is

discolored this is known as water stains

Wind Cracks

• If the wood is exposed to atmosphere agencies its exterior

surface shrinks which results into cracks.

Chemical Stains

• If the wood is discoloured by the chemical action caused by

some external agency. It is known as chemical stain

5. Defects due to seasoning

Defects due to seasoning process are Bow, cup, check and

split, twist and warp etc…

Bow

• This is indicated by the curvature formed in the direction of

length of timber.

Cup

• This defect is indicated by the curvature formed in the

transverse direction of timber.

Check and spilt

• A crack which separates the fibres of wood, and it does not

extend from one end to other is known as a check

• If the same crack which separates the fibres of wood and

extending one end to other end is known as split.

Twist

• When a piece of timber has spirally distorted along its length ,

it is known as twist.

Warp

• If a piece of timber has twisted out of shape , it is called warp.