COURSE CODE : AGR 308Course Title : Wood Science and

Technology

Ankush

Wood Seasoning

Definition : Seasoning is defined as the removal of moisture that is present in the timber in its

green state.

A piece of green wood, on exposure to atmosphere; loses moisture, and the rate of drying

depends on the nature of timber, thickness of the piece, temperature, relative humidity and the rate

of circulation of air around the piece.

If the piece is left for a sufficiently long period in the air, it finally attains a moisture content

which is in equilibrium with the humidity of the atmosphere , or in other words , the piece would

not loose any more moisture if kept longer under the same atmospheric conditions. The wood is

then said to be air seasoned or air dried.

Air seasoning , air drying, open air seasoning or natural seasoning, as the process is known is

therefore the drying of wood, under ordinary atmospheric conditions to a certain moisture

content, which is in equilibrium with the temperature and humidity of the atmosphere in

which it is seasoned.

This process may take from a few months to several years, depending on the nature and

thickness of the wood, and the atmospheric conditions under which the seasoning is carried

on.

Wood Seasoning

Objectives of seasoning

To reduce risk of fungal and insect attack.

To reduce weight.

To avoid seasoning defects like shakes, splits and cracks.

To make wood more secure for proper penetration of preservatives.

To make timber fit for painting and polishing.

Seasoning of logs

The process of seasoning starts from the time the tree is felled or, sometimes in the standing

trees itself, as in the case of girdled trees.

The initial seasoning of timber in the log is just as important as the ultimate seasoning of

sawn timber.

The actual drying of wood in log form is extremely slow, and the amount of drying that takes

place in a log is negligible as compared with the rapid loss of moisture in converted materials.

The outside portions of logs dry out to a certain extent, but owing to the tension set up in the

outside layers, as the result of the inside of the log remaining wet while the outside portion is

dried, surface cracking may take place.

According to the ease or difficulty with which the woods season, these have been divided

into three classes.

a) Highly Refractory Woods : Woods , which are liable to crack, split and warp are called

refractory woods.

Species classified as highly refractory :

Aegle marmelos (Bael), Madhuca latifolia (Mahua), Casia fistula (Amaltas), Shorea

robusta (Sal), Syzygium cumini (Jamun), Diospyros melanoxylon (Tendu).

b) Moderately Refractory Woods : Woods, which are moderately liable to splitting and

cracking during seasoning are called medium refractory.

Species classified as medium refractory :

Acacia nilotica (Babul), Adina cordifolia (Haldu), Albizia lebbeck (Siris), Toona ciliata

(Toon), Dalbergia sissoo (Sheesham), Dalbergia latifolia (Rosewood), Juglans regia

(Walnut), Melia azadirachta (Neem), Tectona grandis (Teak), Terminalia arjuna (Arjun).

Seasoning Behaviour of Woods

c) Non-Refractory Woods : Woods which are capable of withstanding rapid seasoning.

Species classified under this category :

Abies pindrow (Silver fir), Bombax ceiba (Semul), Cedrus deodara (Deodar), Mangifera

indica (Mango), Morus alba ( Mulberry), Picea smithiana (Spruce), Pinus roxburghii

(Chir), Pinus wallichiana ( Kail), Boswellia serrata (Salai).

Seasoning Behaviour of Woods

Methods of Seasoning

1) Air Seasoning : In air seasoning, air is the only medium that carries heat to the timber and

removes the moisture evaporated from the timber by the heat supplied to it.

To make the drying of timber uniform, in ordinary air seasoning practice two important

points should be kept in mind.

i. Piling or Stacking of timber

ii. Protection of the stack

Piling of Timber : Piling of timber is very important, as poor, uneven piling results in

crooked and twisted pieces. The timber is stacked either in :

Horizontal Stacking

Vertical Stacking

Close Crib Method

Open Crib Method

One in Nine Method

Horizontal Stacking : This is the most common method of stacking timber for air seasoning,

and is suitable for all forms of sawn material from thin veneers to thick scantlings, with slight

modifications to suit different conditions as regards species of wood, thickness of material and

the climatic conditions of the locality.

Vertical Stacking : This method of stacking timber is used only for the rapid surface drying

of certain species of non-refractory woods. These are liable to develop mould and

discoloration, soon after conversion, unless surface moisture is removed rapidly to prevent

germination and growth of moulds and fungi, which cause the staining of wood.

Close Crib Method : Sleepers are stacked in alternate layers placed in crisscross manner

without leaving gaps. Rate of seasoning is slow as the circulation of air is reduced. This

method is recommended for staking heavy structured timbers like Sal in hot and dry localities.

Open Crib Method : This method is the modification of close crib method and in this

method gaps are allowed between adjacent sleepers in layer this allows more air circulation.

The effect of drying in this method resembles the one in nine method.

One in Nine Method : Nine sleepers are laid and each one having gap between adjacent

sleepers The successive layers are separated from one another by placing one sleeper across

alternatively at either end of stack. Rate of seasoning is faster in contrast to Close Crib. This

method of stacking timbers is best suited for moderately heavy coniferous timbers in hot

climate and for heavy timbers in moist climates.

Protection of the stack : Some kind of protection is required for protection of the stack from

hot, dry prevailing winds, the blazing sun and rain.

The shelter provided to the green timber affords the control over the elements of nature. It is

the mean for controlling the rate of drying within certain limits.

Hot winds and sun are the chief sources of too rapid drying of wood and consequent splitting

and cracking.

The amount of shelter required for air seasoning, varies with the characteristic of the wood to

be seasoned and the climatic conditions of the region.

Woods which have a tendency to crack and split excessively are those which suffer most from

rapid seasoning and therefore require means of reducing the rate of drying.

In case of non-refractory woods the surface should dry as quickly as possible, the only

protection required is protection against rain.

Medium refractory woods require a midway treatment.

Air seasoning sheds: It is necessary that sizeable quantities of timber have to be seasoned

regularly. It gives protection against direct sun and rain The sheds are long building provided

with raised foundation pillars of stacking. For Indian condition the air seasoning shed should

be in east –west direction.

2) Kiln Seasoning : In case of kiln seasoning, steam is used as the source of heat, and the

heated air is directed through a chamber containing the pile of timber. The air is made

artificially humid; according to the stage of drying by means of steam or water sprays. Thus

a proper control can be obtained over the three factors which mainly influence the rate of

drying, viz., :

Temperature of circulating air.

Humidity of circulating air.

The rate of circulation of air through the kiln.

Temperature of circulating air : Heat is used in the kiln to produce rapid evaporation and

to hasten transfusion of moisture from interior to the surface of wood. Steam is the source of

heat, in all types of kilns. Steam is made to pass through a radiator, placed either inside or

outside the kiln. The air in contact with the radiator after getting heated passes through the

pile of timber and thus supplies the heat required for evaporation of moisture from the

timber. The temperature of the heated air can be easily regulated by shutting or opening the

valve.

Methods of Seasoning

Humidity of circulating air : The ability of air to dry any substance varies with the amount of

additional moisture it can hold before becoming saturated. The amount of water vapor in the air

expressed as a percentage of the amount of water it can hold at saturation is called ‘relative

humidity’. It is this factor which determines the rate of drying. The relation between the

humidity of the air and moisture in wood is an important one, since it forms the basis for

drying. The humidity in the kiln can be controlled either by increasing the humidity by steam

sprays, hot water sprays , or by reducing the humidity of air by cold water sprays and cold

water condensers.

Circulation of air : It is necessary to have a certain amount of circulation, as it not only carries

heat from the source to the pile of timber but also removes the evaporated moisture from the

timber which, if left there, would retard further evaporation.

Process of seasoning is under control; any desired conditions of temperature, humidity and

circulation of air are obtainable according to the requirements of the timber.

Maximum temperature at which timber is seasoned can be raised much above the temperature

of air prevailing under ordinary atmospheric conditions.

Process of drying can be carried on continuously without any interruption.

Total length of time is reduced considerably.

Advantages of Kiln Seasoning

Air seasoning Shed

Horizontal tacking

Vertical Stacking

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