Timber Details of structure and Different methods of seasoning

 TIMBER

A tree basically consists of three parts namely,trunk,crown and roots.the function of the trunk

is to support the crown and to supply water and nutrients from the roots to the leaves through

branches and from the leaves back to the roots.the roots are meant to implant the trees in the

soil ,to absorb moisture and the mineral substancesit contains and to supply them to the

trunk.

Details of structure:

From the visibility aspect,the structure of tree can be divided into two categories :-

(1). Macrostructure

(2). Microstructure

Macrostructure:-

The structure of wood visible to the naked eye or at a small magnification is called the

macrostructure.Following are its different components:

(1) Pith:

the innermost central portion of the core of the tree is called the pith or medulla. It varies in

size and shape and for different types of trees.It consist entirely of cellular tissues and it

nourishes its plant at 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.the pith of the

branches is nothing but merely a prolongation of the stem.

(2) Heartwood:

The inner annular rings surrounding the pith constitute the heart wood.It is usually dark in

colour .as a matter of fact ,it indicates the dead portion of tree and as such,it does not take

active part in the growth of the tree.But it imparts rigidity to the tree hence it provides strong

and durable timber for various engineering purposes.

(3) Sapwood:

The outer annular rings between heartwood and cambium layer is known as sapwood.It is

usually light in colour in light and weight.It indicates recent growth and it contains sap.The

annual rings of sap wood are less sharply defined than those of heartwood.It takes active part

in the growth of the tree and the sap moves in upward direction through it. The sapwood is

also known as the alburnum.

(4) Cambium layer

The thin layer of sap between sapwood and inner bark is known as the cambium layer.It

indicates sap which has not yet been converted into sap wood. If the bark is removed for any

reason,the cambium layer gets exposed and the cells cease to be active resulting in the death

of the fibre.

(5) Inner bark:

The inner skin or layer covering the cambium layer is known as the inner bark.it gives

protection to the cambium layer from any injury.

(6) Outer bark

The outer skin or cover of the tree is known as the outer bark.it is the outermost protective

layer and it sometimes contain cracks and fissures.it consists of cells and wood fibre and is

also known as the cortex.

(7) Medullary rays

The thin radial fibres extending from pith to cambium layer are known as the medullary

rays.the function of these rays is to hold together the annular rings of heartwood and

sapwood.these rays are sometimes broken and in some varieties of trees,they are not very

prominent.

Microstructure:

The structure of wood apparent only at great magnifications is called the microstructure.

A living cell consists of four parts namely membrane ,protoplasm,sap and core.The cell

membrane consists mainly of cellular tissues and cellulose.The protoplasm is a

granular,transparent ,viscous vegetable protein composed of

carbon,hydrogen,oxygen,nitrogen and sulphur.The core of cell differs from protoplasm

merely by the presence of phosphorus and it is generally oval.

The cells ,according to the function they perform,are classified into the following three

categories:

- Conductive cells

- Mechanical cells

- Storage cells

(1).conductive cells:

These cells serve mainly to transmit nutrient from root to the branches and leaves.

(2).mechanical cells

These cells are elongated ,thickwalledand have tightly interconnected narrow interior

cavities.these cells impart strength to the wood.

(3).storage cells:

These cells serve to store and transmit nutrients to the living cells in the horizontal direction

and they are usually located in the medullary rays.

Defects in timber:

Various defects occurring in timber are grouped into following five categories:

(1)defects due to conversion

(2)defects due to fungi

(3)defect due to insects

(4)defects due to natural forces

(5)defects due to seasoning

Defects due to conversion:

During converting timber to commercial form the following defects may occur:

(i) chip mark: this defect is indicated by marks placed by chips on the finished surface of

timber .they may also be formed by parts of a planning machine.

(ii)Diagonal grain: this defect is formed due to improper sawing of timber.it is indicated by

diagonal mark on straight grained surface of timber.

(iii)Torn grain:this defect is caused when a small depression is formed on the finished

surface of timber by falling of a tool or so.

(iv)Wane:this defects is denoted by the presence of original rounded surface on the

manufactured piece of timber.

Defects due to fungi:

Fungi attack timber only when

(1)The moisture content of timber is above 20%.

(2)If there is a presence of air and warmth for the growth of fungi.

Due to attack of fungi following defects occur:

(i)Bluestain:the sap of the wood is stained to bluish colour by the action of certain type of

fungi.

(ii) Brown rot:the fungi of certain types remove cellulose compound from wood and hence

the wood assumes the brown colour.this is known as the brown rot.

(iii) Dry rot:the fungi of certain types feed on wood and during feeding ,they attack on wood

and convert it into powder form.This is known as dry rot.

This type of defect occur in place where there is dampness and no free circulation of air.The

dry rot may be prevented by using well seasoned timber free from sap.

(iv) Heart rot: this is formed when a branch has come out of a tree.It occurs when heart

woodis exposed to atmospheric agent.

(v) Sap stain:certain types of fungi feed on cell contents of sap wood.In doing so ,the sap

wood loses its colour.this is known as sap stain.It generally occurs when moisture content

goes beyond 25 % or so.

(vi) Wet rot:some varietiesof fungi cause chemical decomposition of wood of timber in

doing so timber is converted into a greyish brown powder.This is known as wet rot.

(iv) White rot:this defect is opposite of brown rot.In this defect the wood assumes the

appearance of a white mass consisting of cellulose compounds.

Defect due to insects:

Defects in timber occur dueto various types of insects.

Such as:

(1) beetles

(2)marine borers

(3)termites

Decay of timber occurs due to the above insects.

Defect due to natural force:

The main natural forces responsible for causing defects in timber two,namely,abnormal

growth and rupture of tissues.

(i) Burls:these are also known as the excrescences and they are particularly formed when a

tree has received shock or injury in is young age. Due to such injury, the growth of tree is

completely upset and irregular projections appear on the body of timber.

(ii) Callus:it indicates soft tissues or skin which covers the wound of a tree.

(iii) Chemical stain: the wood is sometimes discoloured by the chemical action caused with

it by some external agency.

(iv) Coarse grain: if a tree grows rapidly, the annual rings are widened. Such timber

possesses less strength.

(v) Dead wood: the timber which is obtained from dead standing trees contains dead wood.

(vi)Druxiness: this defect is indicated by white decayed spots which are concealed by

healthy wood.

(vii) Foxiness: this defect is indicated by red or yellow tinge in wood or reddish brown stains

or spots round the pith of tree discolouring the timber. It is caused due too poor ventilation.

(viii) Knots: these are the 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 the knots.

(ix) Rind galls: the rind means bark and gall indicates abnormal growth. Hence peculiar

curved swelling found on the body of a tree known as the rind gall.

(x) Shakes: these are cracks which partly or completely separate the fibres of wood.

Following are the different types of shakes: cup shake, heart shake, ring shake, star shake,

radial shake.

(xi) Twisted fibres: these are also known as the wandering hearts and they are caused by

twisting of young trees by fast blowing wind.

(xii) Upset: these are also known as the ruptures and they indicate the wood fibres which are

injured by crushing or compression.

(xiii) water stain: the wood is sometimes discoloured when it comes into contact with water.

Thi defect is usually found in converted timber.

(xiv) Wind cracks:if wood is exposed to atmospheric agencies, its exterior surface shrinks.

These are known as the wind cracks.

Defects due to seasoning:

Following defects occur in the seasoning process of wood.

(i) Bow: the defect is indicated by the curvature formed in the direction of length of timber.

(ii) case-hardening: the exposed surface of timber dries very rapidly. It therefore shrinks and

is under compression. The interior surface which has not completely dried under tension.

This defect is known as the case-hardening.

(iii) Check: a check is a crack which separates fibres of wood. It does not extend from one

end to the other.

(iv)Collapse: due to uneven shrinkage, the wood sometime flattens during drying. This is

known as collapse.

(v) Cup: this defect is indicated by the curvature formed in the transverse direction of timber.

(vi) Honey-combing: due to stresses developed during drying, the various radial and circular

cracks in the interior portion of timber. This defect is known as honey-combing.

(vii) Radial shake: these are radial cracks.

(viii) Split: when a check extends from one end to the other, it is known as a split.

(ix) Twist: when a piece of timber has spirally distorted along its length, it is known as a

twist.

(x)Warp:when a piece of timber has twisted out of shape, it is said to have warped.

TIMBER

TIMBER is the oldest material used by humans for construction after stone. Despite its

complex chemical nature, wood has excellent properties which lend themselves to human

use. It is readily and economically available; easily machinable; amenable to fabrication into

an infinite variety of sizes and shapes using simple on-site building techniques;

 Exceptionally strong relative to its weight

 A good heat and electrical insulator

 It is a renewable and biodegradable resource.

However, it also has some drawbacks of which the user must be aware. It is a “natural”

material and is available in limited amount.

Preservation:

Preservation of timber means protecting timber from fungi and insects attack so that its life is

increased. Timber is to be seasoned well before application of preservatives. The following

are the widely used preservatives:

1.Tar

2.Paints

3.Chemicalsalt

4.Creosote

5. ASCO

1.Tar

Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the

attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of

this method of preservation is that appearance is not good after tar is applied it is not possible

to apply other attractive paints. Hence tarring is made only for the unimportant structures like

fence poles.

2.Paints

Two to three coats of oil paints are applied on clean surface of wood. The paint protects the

timber from moisture. The paint is to be applied from time to time. Paint improves the

appearance of the timber. Solignum paint is a special paint which protects the timber from the

attack of termites.

3. Chemical salt

These are the preservatives made by dissolving salts in water. The salts used are copper

sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with

these chemical salt paints and varnishes can be applied to get good appearance.

4. Creosote

Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in an air tight

chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of

0.8 to 1.0 N/mm2 at a temperature of 50°C. After 1 to 2 hours timber is taken out of the

chamber.

5.ASCO

This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1

part by weight of hydrated arsenic pentoxide (As2O5, 2 H2O), 3 parts by weight of copper

sulphate. (CuSO4⋅5 H2O) and 4 parts by weight of potassium dichromate (K2Cr2O7) or

sodium dichromate (Na2Cr2O7⋅2 H2O). This preservative is available in powder form. By

mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution

is then sprayed over the surface of timber. This treatment prevents attack from termites. The

surface may be painted to get desired appearance.

Physical Properties:

Specific Gravity (SG):

Generally, specific gravity (SG) and the major strength properties of wood are directly

related. SG for the major , usually used structural species ranges from roughly 0.30 to 0.90.

Higher allowable design values are assigned to those pieces having narrower growth rings

(more rings per inch) or more dense latewood per growth ring and, hence, higher SG.

Thermal Properties/Temperature Effects:

Although wood is an excellent heat insulator, its strength and other properties are affected

adversely by exposure for extended periods to temperatures above about 100°F. The

combination of high relative humidity or MC and high temperatures, as in unventilated attic

areas, can have serious effects on roof sheathing materials and structural elements over and

above the potential for attack by decay organisms. Simple remedies and caution usually

prevent any problems.

At temperatures above 220°F, wood takes on a thermoplastic behavior. This characteristic,

which is rarely encountered in normal construction, is an advantage in the manufacture of

some reconstituted board products, where high temperatures and pressures are utilized.

Environmentally friendly

Timber is the most environmentally responsible building material. Timber has low production

energy requirements and is a net carbon absorber. Timber is a renewable resource. Wellmanaged

forests produce timber on a sustained continuous basis, with minimal adverse

effects on soil and water values.

In plentiful and growing supply

Timber is readily available. Australia has significant forest resources including a plantation

estate covering more than 1.6 million hectares, and the area is growing rapidly.

Strong and lightweight

Timber is strong, light and reliable making timber construction simpler and safer than steel or

concrete construction. A comparison with steel and concrete shows that radiata pine

structural timber, for example, has a strength for weight ratio 20 percent higher than

structural steel and four to five times better than unreinforced concrete in compression. The

lightweight structures possible in wood confer flow-on advantages in terms of reduced

foundation costs, reduced earthquake loading and easier transport. Building components and

complete constructions are simple and safe to erect, and cheaper to deconstruct or reuse at the

end of a buildingís useful life.

Chemical Properties

Though, would is chemically inert as compared to other materials but is affected by some

acids and bases. Some species have proven very useful for food containers (berry boxes and

crates) because they are nontoxic and impart no taste to the foods contained therein. Wood

structures have also found widespread use as storage facilities for salt and fertilizer chemicals

DRY ROT

The turning of timber tissues to almost dry powder by fungi is called dry rot.The fungus feed

upon the wood and eats the wood tissue, thus penetrating the wood fibres from all direction.

Prevention:

1.well seasoned timber should be used.

2.timber should be used where there is free circulation and access of air.

Remedy:

1. the timber should be painted with a solution of copper sulphate

2.the high temperature of seasoning of kiln helps in killing the Fungi.

WET ROT

The disintegration of tissue of timber due to alternate wetting and drying is called wet rot.

The attacks take place through the wounds in bark by the access of water.

Prevention:

All timber for exterior or underground work should

be first properly seasoned and then coated with tar to keep out the dampness.

Remedy:

The best remedy for treating wet rot is by using a suitable preservative

Seasoning of Timber

From day to day, most people have some contactwith "seasoned" timber. From childhood

days woodencots and toys, to school desks and, eventually, to wooden furniture and flooring

in homes or places of employment - seasoned timber is to be found. Yet how many people

really understand what seasoned timber is?

Only when cracks appear in furniture or floor, orwhen a door shows some degree of warping,

is anythought given to this concept. It is to be regretted thateven some people associated with

the timber tradehave little knowledge of what seasoned timber is andthe best method of

obtaining it.

What is "seasoned" timber?

The process of drying out the water from "wet" or"green" timber is termed

"seasoning", or more simply"drying". Water is just as essential to the life of atree as it is for

all living matter. Together with thevarious minerals, it enters through the roots of the treeand

is carried in the sapwood - the outer woody part tothe leaves. The food, that is the sugars and

starch,are made in the leaves by photosynthesis and aretransported in solution down the inner

bark to thegrowing cells. The whole trunk of the tree is made upof cells, which are like small

tubes, having walls ofcellulose and a more or less hollow' cavity filled withwater and other

materials known as sap. Consequently,when the tree is felled and the resulting logis sawn into

timber, the sawn sections consist ofinnumerable small cells containing water. Drying

themoisture out of wood enhances its properties to suchan extent that the resulting timber is

given the specialname "seasoned" rather than "dried" although theterms are identical.

Why is timber seasoned?

Seasoning timber causes many changes in its properties, and in practically every case

the change is animprovement. There is only one principal disadvantagein drying timber,

namely, the loss in volume dueto shrinkage. However, by a correct understandingof the

shrinkage of timber this effect can be minimized,and timber can then be confidently

usedwithout fear of adverse behaviour subsequently inservice.

Types of Seasoning

(i) Natural Seasoning: It may be air seasoning or water seasoning. Air seasoning is

carried out in a shed with a platform. On about 300 mm high platform timber balks

are stacked as shown in Fig. 1.8.

(ii) Care is taken to see that there is proper air circulation around each timber balk. Over a

period, in a natural process moisture content reduces. A well-seasoned timber

contains only 15% moisture. This is a slow but a good process of seasoning.

Water seasoning is carried out on the banks of rivers. The thicker end of the timber is

kept pointing upstream side. After a period of 2 to 4 weeks the timber is taken out.

During this period sap contained in the timber is washed out to a great extent. Then

timber is stalked in a shed with free air circulation.

(iii) Artificial Seasoning: In this method timber is seasoned in a chamber with regulated

heat,controlled humidity and proper air circulation. Seasoning can be completed in 4

to 5 days only. The

different methods of seasoning are:

(a) Boiling

(b) Kiln seasoning

(c) Chemical seasoning

(d) Electrical seasoning.

(a) Boiling: In this method timber is immersed in water and then water is boiled for 3 to 4

hours.Then it is dried slowly. Instead of boiling water hot steam may be circulated on

timber.The process ofseasoning is fast, but costly.

(b) Kiln Seasoning: Kiln is an airtight chamber. Timber to be seasoned is placed inside it.

Thenfully saturated air with a temperature 35°C to 38°C is forced in the kiln. The heat

gradually reachesinside timber. Then relative humidity is gradually reduced and temperature

is increased, and maintained till desired degree of moisture content is achieved.

The kiln used may be stationary or progressive. In progressive kiln the carriages

carrying timber travel from one end of kiln to other end gradually. The hot air is supplied

from the discharging end so that temperature increase is gradual from charging end to

discharging end. This method is used for seasoning on a larger scale.

(c) Chemical Seasoning: In this method, the timber is immersed in a solution of

suitablesalt.Then the timber is dried in a kiln. The preliminary treatment by chemical

seasoning ensures uniform seasoning of outer and inner parts of timber.

(d) Electrical Seasoning: In this method high frequency alternate electric current is passed

through timber. Resistance to electric current is low when moisture content in timber is high.

As moisture content reduces the resistance reduces. Measure of resistance can be used to stop

seasoning at appropriate level.

However it is costly process. This technique has been tried in some plywood industries but

not in seasoning of timber on mass scale.

Different methods of seasoning:

Air Seasoning

The traditional method for drying wood, air seasoning is also the longest, taking six to nine

months. To air season wood, stack logs or planks outside on pallets in such a manner that air

can circulate vertically and horizontally through the timbers. The raised pallets also keep

wood away from vegetation and damp ground. Plank and log ends are often wrapped or

sealed to prevent excessive moisture loss through these areas. Protect the drying wood from

the elements with an overhead canopy.

Kiln Seasoning

The most common and effective commercial process for drying wood is kiln seasoning,

which accelerates the process of removing moisture through the use of external energy.

Drying takes two days to one weekend, depending on the type of wood. Two methods,

progressive and compartmental, are used for kiln seasoning. In a progressive kiln, timber

enters at one end and travels on a trolley through chambers with different air conditions to

progressive dry the wood. This method produces a constant flow of seasoned timber. Wood

seasoned via the compartmental process remains in a single building where it is subjected to a

program of varying conditions until the moisture content is removed. This process is used for

hard-to-dry or expensive wood.

Solar Kiln

This method combines the speed of kiln seasoning with the low energy of air drying. Solar

kilns have single-thickness windows on the south side of the structure that work as collectors

to trap the sun’s energy. Heat collectors, made from black metal are attached near the top of

the window sashes. Various methods force the heated air to circulate through the kiln to dry

the wood. Some solar kilns have insulation to retain heat at night. This process takes

approximately twice as long as traditional kiln seasoning. Because of its gentle nature, it is

well suited to producing wood for furniture fabrication.

Microwave Seasoning

Microwave seasoning uses pulsed energy directed into timbers to drive out moisture in

a manner that will not cause seasoning degrade. This method also provides advantages such

as high speed and high quality and is well suited for seasoning lumber, blocks, veneer, chips,

paper and wood-based composite materials. Areas in the wood with the most moisture absorb

the most energy resulting in even temperature during the drying process and a uniform

moisture content. These factors enhance quality and reduce timber checking and warping.

Advantages of seasoning:

Three most important advantages of seasoning have already been made apparent:

1. Seasoned timber lasts much longer than unseasoned. Since the decay of timber is due to

the attacks of wood-destroying fungi, and since the most important condition of the growth of

these fungi is water, anything which lessens the amount of water in wood aids in its

preservation.

2. In the case of treated timber, seasoning before treatment greatly increases the

effectiveness of the ordinary methods of treatment, and seasoning after treatment prevents the

rapid leaching out of the salts introduced to preserve the timber.

3. The saving in freight where timber is shipped from one place to another. Few persons

realize how much water green wood contains, or how much it will lose in a comparatively

short time. Experiments along this line with lodge-pole pine, white oak, and chestnut gave

results which were a surprise to the companies owning the timber.

Freight charges vary considerably in different parts of the country; but a decrease of 35 to

40 per cent in weight is important enough to deserve everywhere serious consideration from

those in charge of timber operations.

When timber is shipped long distances over several roads, as is coming to be more and

more the case, the saving in freight will make a material difference in the cost of lumber

operations, irrespective of any other advantages of seasoning

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