Seasoning : Definition/Types/Behavior of wood on seasoning/Stacking timber /Seasoning sheds and kilns/Compartment and progressive kilns

1. Course Title : Wood Science and
Technology
Ankush

2. 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.

3.  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.

4. 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.

5. 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.

6.  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).

7. 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).

8. 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

9.  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.

10.  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.

11.  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.

12. 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.

13.  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.

14.  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.

15. Air seasoning Shed
Horizontal tacking
Vertical Stacking

17. Thank you
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