TIMBER
1.0 What is Timber? Definition & Types of Timber & Lumber
1.1 What is Timber?
Timber is a type of wood which has been processed into beams and planks. It is also
known as “lumber” in US and Canada. Basically, timber or Lumber is a wood or firewood
of growing trees. Any wood capable of yielding a minimum dimensional size can be
termed as a timber or lumber. It is a stage in the process of wood production. Timbers
are used for the structural purpose. Those woods which are adapted for building purposes
are timbers. Finished timber is supplied in standard sizes for the industry. Timber is used
for building houses and making furniture.
There is high demand for timber as a building material. From building construction to
furniture making, timbers have numerous uses. These uses have made timber an
important building material.
1.2 Types of Timber and Lumber
Timber can be divided into two categories – hardwoods and softwoods. There are many
types of timber under these two categories. They are * Bamboo
* Birch
* Cedar
* Cherry
* Cross-laminated
* Glulam
* Green timber
* Lime
* Mahogany
* Oak
* Pine
* Plywood
* Sapele wood
* Tulipwood
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* Walnut
* Wood ash
* Spruce
* Fir
2.0 Properties of Timber - Qualities of Good Timber Wood
2.1 Properties of Timber
The quality of timber must be ensured before using it for a purpose. The quality can b
ensured by investigating the properties of timber. Here we have discussed both physical
and mechanical properties of timber which affects timber quality.
Followings are the physical and mechanical properties of timber:
* Colour
* Appearance
* Hardness
* Specific Gravity
* Moisture Content
* Grain
* Shrinkage and Swelling
* Strength
* Density
* Toughness
* Elasticity
* Warping
* Durability
* Defectless
* Workability
* Soundness
* Free of abrasion
While selecting timbers for use, one must check these properties to ensure the quality. At
the same time, it is also essential to ensure that the timber is defectless..
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These properties are briefly discussed below:
2.1.1 Colour
Color is a uniform property by which most trees are characterized as they show variation
from tree to tree. Light color indicates weak timber. For example, freshly cut teak, Deodar,
and Walnut have a golden yellow, whitish and dark brown shades respectively.
2.1.2 Appearance
Smell is a good property as timbers for few plants as they can be identified by their
characteristic aroma. Fresh cut timbers have a good smell. For example resinous smell
from pine.
2.1.3 Hardness
For the resistance of any kind of damage, hardness is an obvious property.
2.1.4 Specific Gravity
Variation of umber in specific gravity (0.3 - 0.9) is found. It depends on pores present
inside timber. The specific gravity of this light material is less than that of water (<1). But
in case of compact wood where pores are almost absent and become heavier, their
specific gravity increases up to 1.5.
2.1.5 Moisture Content
Timbers are hygroscopic and gain water from nature (atmosphere). The absorption of
water or dehydration depends on atmospheric humidity. If timbers moisture content is
high that means the timber quality is low. Water content is the risk of fungal attack.
2.1.6 Grain
Several types of grain arrangement found. On the grain structure quality of timber varies.
Grains remain closely related.
1.
Straight grain: Arrangement of vascular tissue (xylem and phloem)
is important which grow parallel to the length of the timber that is termed as straight
grain.
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2.
Coarse grain: vascular tissue and fibre arranged broadly and widely.
3.
Interlocked grain: Instead of parallel arrangement twisted, a spiral arrangement
may be found.
2.1.7 Shrinkage and Swelling
The percentage of shrinkage and swelling varies from plant to plant. Some give higher
percentage after drying. Shrinkage starts when cell walls of timber start to release water.
In moisture atmosphere timber swells when cell walls absorb water. Good quality timbers
swell less. Timbers having thicker wall swell more than a thinner one.
2.1.8 Strength
Best quality timbers have the highest strength. Strength means capable to bear loads.
Anisotropic material like timber has different structure at the different portion. So, the
strength of timber is different at different points. Grain structure determines the strength
of the timber. Some types of strength are
1.
Compressive strength: 500 kg/cm2 to 700 kg/cm2 load is enough to test timbers
strength.
2.
Tensile strength: When timber is enough strong to the tensile force. If
perpendicular force is made then timber is weaker. 500-2000 kg/cm2 is the range
of tensile strength
3.
Transverse strength: Enough bending strength indicates good quality timber.
2.1.9 Density
Timber having higher density have a thicker wall. An important property that quality of
timber. Moisture content: Presence of defects: There may be some of the natural and
artificial defects in timber such as cross-grain, knots, and shakes, etc. All of them
decrease in the strength of the timber.
2.1.10 Toughness
Timber has to have the capability to bear shocks, jerk. Anti-bending and ant splitting
characteristic is needed. Old timbers have annual rings which indicate their age is a good
indicator.
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2.1.11 Elasticity
Another property elasticity means timber should attain its own shape after use. Because
of this quality, it is used in sports bat.
2.1.12 Warping
Environmental change with season can’t effect good quality timber.
2.1.13 Durability
A good quality timber has the property to resist the attack the infection of fungus or other
insects. This resistance quality makes timber better.
2.1.14 Defectless
This property is gained if the timber is from a sound tree. A defectless tree is free from
sap, shakes, and dead knots. To know more about timber defects read: Defects in Timber
2.1.15 Workability
A good timber is always easy to work on it. Easy to drag using saw on good timber. The
finishing can be done well.
2.1.16 Soundness
A good quality timber gives good sound.
2.1.17 Texture
The texture of good timber is fine and even.
2.1.18 Free of Abrasion
Timber should not be damaged by the external environment. It has to gain the ability to
protect skin
3.0 Defects in Timber/Defect Types & Causes
Timber is a natural product and every natural product has some imperfections. Timbers
are not excluded from that. Most of the defects in timber cause weakness or others sorts
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of difficulties. However, some defects can be beneficial for a specific type of work, for
example, twisted wood is good for making a bowl out of timber.
3.1 Defects in Timber
The followings are the five main types of defects in timber:
1.
Defects due to Natural Forces
2.
Defects due to Attack by Insects
3.
Defects due to Fungi
4.
Defects due to Defective Seasoning
5.
Defects due to Defective Conversion
These timber defects are briefly discussed below.
3.1.1 Defects in Timber due to Natural Forces
a.
Knots: Knots are the most common defects caused due to natural forces. During
the growth of a tree, branches close to the ground or lower branches die. Bases of
those branches remain in the tree as the trees grow. These bases may create
imperfection known as knots.
Types of Knots: Knots are of two types.
i.
Dead Knots: The remains of damaged branches after drying
out
they become loose and fall out.
ii.
Live Knots: They are sound and firm. If small, are not great of a defect.
Live knots are usually not a problem as they remain firmly attached to the timber.
But in dead knots, they are loosely attached and reduce strength. Knots decrease
the strength of the wood and thus lower its value for structural uses, knots cause
serious defects when the load is perpendicular to the grains.
Image of Dead Knot and Live Knot
Dead Knot and Live Knot.
Source: woodexportchile.com
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b.
Twist: Twist in timber rotates the ends of the timber in opposite directions. The
main reason behind this defect is twisting of the trees by the strong wing.
twisting-of-the-trees-by the-strong-wind
Twisting of the trees by the strong wind.
Source:commons.wikimedia.org
c.
Shakes: Shakes are timber defects that occur around the annual ring or growth
ring of a timber. In other words, cracks or splits in the woods are called shakes.
It may or may not be a structural problem depending upon depth and use. The
main problem is aesthetic. Where the appearance is important, shakes are
undesirable.
Types of Shakes: Shakes can be classified into three main categories:
i.
Star Shakes: This type of shake starts propagating from the bark towards
the sapwood and sometimes even towards the heartwood along the lines
of medullary rays. Cracks are wider on the outer edge or bark and narrower
on the inside (usually sapwood, sometimes heartwood). The main reasons
behind star shakes are extreme heat or frosting during the growth of the
trees and rapid or uneven seasoning after cutting off the timber. Extreme
heat or frost causes temperature difference, which causes shrinkage
leading to the crack.
ii.
Cup and /or Ring Shakes: Cup shakes follow the annual growth ring. It is
capable to separate the growth ring partially or completely. When the crack
separately, it is called ring shakes. So all ring shakes are cup shakes, but
all cup shakes are not a ring shape. Excessive frost action is the main
reason for this type of crack.
iii.
Heart Shakes: Unlike star shakes, heart shakes starts propagating from
the pith to the sapwood along the lines of medullary rays. Shrinkage of the
interior part of the timber causes this crack.
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d.
Rind Galls: The meaning of rind is bark and gall is abnormal growth. So abnormal
growth of the bark of the trees is called rind galls. Improper cutting of branches
causes this abnormal growth. Wood from this portion of the timber lacks strength
and desirable in structure.
e.
Upsets: Upsets in various wood indicate that the tree was subjected to crushing
or compression. Improper felling of trees, heavy wind blowing during the young
age of the tree these are the main reasons behind this type of defect.
3.1.2 Defects of Timber due to Attack by Insects
Insects like beetles, termites or marine boars eat wood, make holes and weaken the
strength of the wood.
Beetles are small insects that make holes in almost all the sapwoods. The larvae make
tunnels through the sapwood in all directs and turn wood into powder.
Termites live in a colony. They are very fast in eating woods and making tunnels through
it. Only a few good kinds of wood can withstand are the action of termites.
Marine boars are found in salt water. Usually, they make tunnels in wood to take refuge
or shelter. All kinds of wood or timber are vulnerable to this kind of insect.
3.1.3 Defects in Timber due to Attack by Fungi
a.
Stain: When fungi feed only on sapwood, where the food materials are stored, it
causes a stain. Heartwood doesn’t contain these kinds of food materials and is not
affected by it. Stain action causes color but does not affect the strength of the
wood.
b.
Decay: Wood eating or wood destroying fungus is responsible for this type of
defect in wood. This type of fungi breaks down the cell structure. Both sapwood
and heartwood are affected by them. Considerable strength reduction occurs.
3.1.4 Defect in Timber due to Defective Seasoning
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Faulty method of seasoning causes serious defects in woods. During seasoning of timber,
exterior or surface layer of the timber dries before the interior surface. So, stress is
developed due to the difference in shrinkage. In a perfect seasoning process, stress is
kept minimum by controlling the shrinkage. Some of the defects resulting from defective
seasoning are as follows:
a.
Bow: Curvature formed in direction of the length of the timber is called bow.
b.
Cup: Curvature formed in the transverse direction of the timber is called a cup.
c.
Check: Check is a kind of crack that separates fibers, but it doesn’t extend from
one end to another.
d.
Split: Split is a special type of check that extends from one end to another.
e.
Honey Combing: Stress is developed in the heartwood during the drying process
or seasoning. For these stresses, cracks are created in the form of honeycomb
texture.
3.1.5 Defects of Timber due to Defective Conversion
a.
Boxed Heart: This term is applied to the timber which is sawn in a way that the
pith or the center heart falls entirely within the surface throughout its length.
b.
Machine Burnt: Overheating is the main reason for this defect.
c.
Machine Notches: defective holding and pulling causes the defect.
d.
Miscut: erroneous cutting or sawing of wood causes this defect. Lack of
experience in sawing and carelessness is the main reason for erroneous cutting.
e.
Imperfect Grain: Mismatch in grain alignment.
4.0 Uses of Wood
A porous and fibrous hard structured plant stem which is the secondary xylem of the
vascular tissue consisting of cellulose, hemicellulose and lignin can be defined as wood.
It’s a supportive organ of woody plants forming the branches and stems give them a firm
structure.
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4.1 Uses of Wood in Different Sectors
Wood is a plant part having multipurpose uses those are impossible to deny and difficult
to note all in our daily life. From the ancient time wood is used by human and this
continuation still remains in the modern civilization. A few of many uses of wood are
mentioned below:
4.1.1 Construction and Fencing
a. Home Construction:
During the early periods, use of wood in domestic construction was a common
scene and this is still followed in this twenty-first century. In different parts of the
world in the making of houses, wood is used commonly like the flooring, frames of
doors and windows for its strength and internment quality. e.g. Deodar (Cedrus
deodara),walnut wood (Juglans sp) is used in Pakistan widely, teak (Tectona
grandis) in South Asia and all over the world, Chir pine (Pinus roxburghii) etc. In
Bangladesh during construction of buildings woods from mango (Mangifera
indica), burflower three (Neolamarckia cadama) are used for casting and piling.
b. Fencing and Decorating Gardens:
In modern decoration system woods are also used for building the fencing and
simple decoration for artificial gardening inside a home of on roofs. e.g. Cedar
(Cedrus libani), redwood (Sequoia Semipervirens), Shorea sp, Acasia sp.
4.1.2 Household Uses
a. Utensils:
Utensils made up of wood instead of plastic and steel are a symbol of elegance
which increases the charm and loveliness of the home corners. e.g. Black walnut
(Juglans nigra) wood is used in the west for home utensils.
b. Hand Tools:
The handles of most common hand tools made of wood help as heat resistant
when they are kitchenware used in an oven and closes the chance to shock while
used on electricity.
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4.1.3 Art Industry
a. Artworks:
For artworks such as statues, sculptures, carvings and making decorative objects
woods are widely used. The frames of art board, color plate are also made from
wood in many cases. e.g. Pine (Pinus sp), maple (Acer sp), cherry (Prunus sp)
wood for framing work.
b. Musical Instrument:
The musical instruments such as Piano, violin, cello, drums, flute, guitar, double
bass and a number of other music instruments material requires wood for making
a perfect tune. e.g. Mahogany (Swietenia macrrophyila), maple, ash wood
(Fraxinus sp) for guitars.
4.1.4 Sports Equipment
Wooden Toys:
These are preferred to plastic towards the health conscious people which were
supposed as a fashion before. Plastic is nothing but the combination of chemicals
which is hazardous to children’s health. Cricket, hockey, billiard, table tennis etc.
Toys and sports equipment have long made use of wood for handles and main
parts. e.g. Willow (Salix sp) wood for cricket, tennis bat; Mulberry (Morus sp) wood
for hockey sticks.
4.1.5 Commercial Uses
a. Furniture:
At present, the marker for wooden furniture is very profitable. No one can deny the
demand for wooden furniture as it is a sign of aristocracy since ancient time. e.g.
Teak (Tectona grandis) wood is the best for making furniture. Some other woods
from Mahogany (Swietenia macrophyiia), Shimul (Bombax ceiba), Sundari,
Jackfruit (Artocarpus heterophyllus), Mango trees are used in South Asia for
making different types of furniture.
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b. Shipbuilding:
Ships and rural fishing boats were made from wood. For constructing boats and
ships wood is one of most important construction material. Hardwood and
softwood were used in the past for ship industry. e.g. Teak, sgal (Shorea robusta),
mango, Arjun (Terminalia arjuna) were frequently used in the past. Now Cypress
(Cupressaceae sp), redwood (Sequoioideae sp), white oak (Quercus aIba) are
water resistant and used for shipbuilding and boat building. Woods like Kauri
(Agathis australis) is used for making the frames of ships.
c. Fuel:
Wood is an age-old source of energy all over the world. Before the exploration of
gas, fuel was the main source we can also define as only one source of energy
that people used by burning as woods were available in the forest easily.
Generally, sticks, pellets, sawdust, and charcoal are used as an energy source
from wood. Usually, woods from cheap plants are used in this sector.
d. Stationary:
Some stationeries like paper pencil are made of wood. Wood pulp is used for
making paper. Wood is used for making pencils too. e.g. in the past Cyper papyrus
trees were used to make paper. In Bangladesh woods from Keora (Sonneratia
apetala), Bine (Avicennia alba), Sundari (Heritera fomes), Kakra (Bruguiera
gymnorhyza), Geoa (Excoecaria agallocha) are used for making pulp of paper.
Dhundal wood (Xylocarpus granatum) is used for pencil production.
5.0 Seasoning of Timber – Reason – Methods - Benefits
Reduction of moisture content along with improving some qualities before the use of
woods is called seasoning of timer. By seasoning, generally, the moisture is reduced to
about 15% where new cut woods bear about 50%.
5.1 Reasons for Seasoning
Seasoning of timber is done to fulfill some specific requirement. Followings are the
reasons to perform timber seasoning,
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1.
To change and improve the properties of wood
2.
To make a correct percentage of shrinking of woods
3.
To make a confident use of woods
4.
To reduce the adverse behavior of woods
5.2 Methods of Seasoning of Timber
There are mainly two methods of seasoning of timber. These are:
1.
Natural Seasoning
2.
Artificial Seasoning
Following tree diagram can be used to illustrate all the methods of timber seasoning.
Seasoning of Timber
Natural Seasoning
Water Seasoning
Air Seasoning
Artificial Seasoning
Seasoning by Boiling
Chemical Seasoning
Kiln Seasoning
Progressive Kiln Seasoning
Electrical Seasoning
Compartmental Seasoning
Detail explanation along with advantages and disadvantages of these methods of
seasoning is discussed below.
5.2.1 Natural Seasoning
Seasoning of woods or timbers using natural elements is called Natural Seasoning. e.g.
water and air seasoning.
(a) Water Seasoning:
Removal of wood sap immersing logs into water flow is called water seasoning. It is
carried out on the banks of the river while thicker ends are kept towards upstream. After
that, the logs are allowed to dry. Disadvantage: It is time consuming such as 2 to 4 weeks
generally.
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(b) Air Seasoning:
Exposing the wood to air for seasoning. At first, a platform is required that is built on the
ground at 300mm height above the ground.
Secondly, the arrangement of woods in layers. Air circulation is maintained between logs
because it helps to reduce the moisture which is important for seasoning. The
environment for this need to maintain some conditions. A clean, shady, dry, cool place is
preferred. Sometimes logs are coated by the impermeable substance to reduce extreme
moisture. To improve the quality oil coating, thick paint coating is maintained. To prevent
fungal infection logs are treated with petrol or gasoline.
Advantage of natural seasoning
*
Good quality of seasoned wood.
*
A large amount is convenient in this process.
*
Well-seasoned timber is formed.
Disadvantage of natural seasoning
*
It’s a slow process.
5.2.2 Artificial Seasoning
a. Seasoning by Boiling
Seasoning by boiling wood logs in hot water is called seasoning by boiling. Dry is done
after proper boiling. For a large amount of wood, it is done in an enclosed place where
hot steam is passed.
Advantages:
*
It takes a short amount of time.
General, 3-4 hours is good enough.
*
Develops the strength and elasticity.
Disadvantages:
*
It is serviceable basically for a small quantity of wood, not convenient for a large
amount.
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*
The cost is high.
b. Chemical Seasoning:
Reduction of moisture using salt solution is called chemical seasoning. After the
absorption of water by the solution logs are let to dry.
Advantages:
*
It increases the strength of the timber.
*
It is less time-consuming.
Disadvantages:
*
Chemical reagent can sometimes reduce strength
*
It can cause a problem in gluing or finishing or corrosion while using.
c. Kiln Seasoning:
Seasoning of wood by using a large chamber or oven where there is a good process for
the circulation of hot air.
Advantage:
*
Most effective and economic seasoning.
Kiln seasoning can be done by 2 processes such as:
i.
Progressive Kiln Seasoning: Wood log is entered through the kiln and the
temperature and humidity diferentials are maintained through the length of the kiln
to maintain proper drying.
ii.
Compartmental Seasoning: Its maintained by enclosed container or buildings.
Advantage: It accelerates the process because external energy is used.
d.
Electrical Seasoning:
Dry wood is non-conductor of electricity while green timber is a conductor, so, can pass
alternating current. Thus in this method alternating current is used for drying the cells of
wood by creating heat. As electricity is used, it’s called electrical seasoning.
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Advantage:
*
Using this method quick drying is obtained. A French electrical seasoning method
is used to season overnight.
Disadvantages:
*
The equipment required is very costly
*
It is an uneconomic process as a high rate of electricity is consumed.
*
During heating the cells of wood or timber they lose their strength and become
weak.
5.3 Qualities Improved by Seasoning:
By seasoning, some common known qualities are improved which are mentioned below:
1.
Strength
2.
Hardness
3.
Durability
4.
Weight
5.
Painting and finishing
6.
Gluing
7.
Resistance to insect attack
8.
Electrical resistance
9.
Heat content
5.4 Precautions:
Following precautions must be taken during seasoning of timber.
*
During seasoning, the moisture should be removed under an environment
maintaining conditions.
*
Moisture should be extracted almost at an equal rate from all logs because
differentiated dryness causes irregular shape.
*
Seasoned timber should be protected from exposure to the rain and excessively
high humidity during air seasoning.
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*
During seasoning, a proper gap between logs should be maintained for easy and
uniform air or water or hot air passing.
6.0 Difference Between Softwood and Hardwood.
Softwood and hardwood are two different types of wood. To choose between these two
types of wood one must know the differences between them. The main differences
between Softwood and Hardwood are given below.
Softwood VS Hardwood:
Characteristics
Source
Softwood
Hardwood
Softwood is collected from
Hardwood is obtained from
conifer trees which are
deciduous trees (loses
evergreen having needle-
leaves in autumn). They are
shaped leaves. These are
basically angiosperms.
generally gymnosperms.
Fibre
Less dense, strait fiber is
In case of hardwood, the
found.
fibers are quite close and
dense.
Resin Quality
It is resinous wood contains
Non-resinous woods have
good and regular fragrant
enough acid.
smell.
Weight
Colour
Lightweight and softer than
Heavyweight and harder than
hardwood.
softwood.
Softwood is light in colour.
Normally these are dark
coloured woods.
Resistance to Fire
Poorer than hardwood, burn at
Better than softwood. Burn at
a high rate.
a very slow rate.
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Weather
If they are modified or treated
Naturally resistant to whether
resistance
then these may become
having a less environmental
resistant to whether having an
impact.
environmental impact.
Duration
Less durable wood
Hardwoods are highly
durable and last for several
decades.
Ring Structure
Distinct annual rings are found.
The annual rings are not
distinct.
Medullary rays
Indistinct medullary rays.
Distinct medullary rays.
Growth rate
Softwood trees grow faster
The growth rate of this type
than hardwood trees.
of trees is slower.
Wood branching
Creates more branch or shoots. Have fewer shoots.
Tensile and shear
Well tensile and comparatively
Good tensile and shear
strength
weaker shear strength.
strength.
Cost
Less expensive
More expensive
Uses
Paper pulp, paper, solid wood
Generally flooring and
products, Woodwares like
furniture. Also used for
homes and cabins and also for
papermaking.
furniture.
Workability
Easier to carve.
Difficult to curve.
Example
Black willow Salix nigra),
Sugar maple (Acer
Redwood (Sequoiodeae sp).
saccharum), Eucalyptus
(Eucalyptus globulus).
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