Teak timber has qualities as: durability, workability, beautiful
figure and non-splitting. Some desirable features are: straight grain,
knotless and defect free. Teak is exceptionally stable under changes
of temperature and moisture: free from warping in drying. Four major
teak color types are recognized in Java:
1. Uniform Golden color, straight grained and even texture and with
few markings. Such teak is found in BAGO YOMA forests and the forest
of Upper Myanmar. It is the best teak of the country and is much
preferred by foreign customers.
2. Darker in color and harder, mostly found in the dry forests of
the country. It is a good teak apart from a higher density and
comparatively harder in working properties. The color is uniform
grey-brown and mostly cross-grained and found in the northern part
of the country.
3. Sometimes bee holes are observed. This variety could be said
to be the worst quality of the quality of the country, but it may
yet be better than the standard quality teak of other origins in the
teak-bearing region.
4. Black stripe teak in Thayet and Taunndwungyi areas with a
limited availability. It is a good ornamental timber and previously
a high price was obtained. It is said that black stripe teak also
occurs in Java, seeking black
walnut.
Quality of Teak
Durability, stability, workability and ease in pre-treatment are the
major determining factors of good quality teak. Beautiful figure,
color, density and rate of growth are other wanted qualities.
Straight-grained, knotless and defect-free timbers are also desirable
qualities.
Durability of Teak
Teak is one of the most durable timbers in the world. On some dry
sites, it has lasted for more than seven hundred years. In contact
with ground, it can remain serviceable for more than twenty-five
years. Extractives present in the heartwood are believed to be
responsible for its durability. Several studies of teak heartwood
extractives and decay resistance showed that most teak heartwood
extractives directly contribute to the natural durability of teak.
Decay resistance increases from the pith to the sapwood and is also
related to the age, rate of growth and extractive content. It is also
found out that a wide range of variation exists in decay resistance
both between tress and within a tree. These variations are largely
dependent on genetic rather than environmental influences. The basal
portion is said to be not very durable, while the most durable portion
is about 12.5 feet from the base. The middle to outer heartwood is the
durable part and also contains the highest concentration of
extractives. A teak tree consists of more heartwood than sapwood.
Normally, the sapwood has about 0.5 Inch thickness in old-aged large
trees whereas it consists of about 0.6-1 Inch sapwood in young tress
of age 20 years and below. Natural durability is one of the
prerequisites that make teak a high quality timber.
Stability and Movement Teak is an exceptionally stable
timber under changes of temperature and moisture. It is free from
warping in drying. Movement due to its low shrinkage is unrivalled
among the timbers of the world.
Shrinkage
The shrinkage and the specific gravity of teak from different
localities are given in Table 1. It consists of both natural and
plantation-grown teak, as well as girdled and green-felled teak. The
specific gravities are on the basis of oven dry weight and green
volume. Shrinkage data are from green condition to the oven dry state.
In Table 1 are presented data from other teak bearing countries and
planted area.
Table 1: Specific gravity and shrinkage of teak
Locality
Condition
Specific
Gravity
Shrinkage %:Green to Ovendry
Radial
Tangential
Myanmar
Green
0.586
2.1
3.0
Myanmar
Girdled
0.594
2.2
3.0
Malabar, India
Natural
0.614
2.5
6.0
C. Provinces, India
Natural
0.526
2.2
4.0
Bihar/Orissa, India
Natural
0.536
1.8
4.0
Honduras(a)
Plantation
0.560
2.1
5.0
Philippines(a)
Plantation
0.490
2.2
4.0
Thailand
Plantation
0.640
2.5
5.0
Indonesia
Plantation
0.646
0 - 2.0
0 - 3.5
Nigera
PLantation
0.509
0 - 3.0
0 - 5.0
Papua New Guinea
Plantation
0.509
2.1 - 3.0
3.6 - 5.0
South Africa
Plantation
0.646
2.1 - 3.0
3.6 - 5.0
It is observed that wherever teak is grown, the specific gravity is
quite comparable except that of the Philippines. Similarly the
shrinkage of teak from natural habitat is rather consistent and
shrinkage from teak outside the natural habitat are rather high. Teak
is so classified as "low" in shrinkage.
Measurement
Seasoned timber in services is liable to fluctuate dimensionally
according to the Equilibrium Moisture Content of the locality. Some
movement tests were carried out in the Forest Product Laboratory in
Prince Risborough. Movement percvents of some selected species,
including teak, are reproduced in Table 2.
Table 2: Movement of Teak and other species
Species
Movement %
Trade Name
Scientific Name
Tangential
Radial
Teak
Tectona grandis
1.2
0.7
Gurjan
Dipterocarpus spp
3.3
2.0
Kuruing
Dipterocarpus spp
2.5
1.5
Mahogany
Swietenia macrophylla
1.3
1.0
Oak
Quercus spp
2.5
1.5
Pyinkado
Xylia dolabriformis
2.1
1.7
Rhodesian Teak
Baikiaea plurijuga
1.0
0.7
It is noticeable that teak has a small movement. Rosewood has
similarly small movement, whereas the so-called Rhodesian teak is a
little higher in movement. Movement is an important index for fine
works, cabinet making and parquet flooring.
Workability
Apart from the teak from drier areas of the country, the rest are
quite easy to work in all aspects. The resulting surfaces of planing,
boring and turning process are smooth. Resistance to spliting when
nailed is rather good. Tools tend to become dull in machining process
because of the presence of silica.
Silica is said to be present in the vessel elements. Silica content in
teak varies up to 1.4%. Teak form the northern part of Myanmar has
slight fiber pricking in planing, and due to some cross-grained
nature, it is not a good quality teak compared to those from other
parts of the country.
Pre-Treatment
Preservative treatment and the drying of timber are the two main
pre-treatments to be considered.
Preservative treatment
As teak is a naturally durable timber, no preservative treatment is
necessary. Even if treatment is carried out, it is rather hard to
apply.
Timber drying
Teak is said to be moderately refractory and is not liable to check,
split and warp.
(a) Air Drying
A slight spliting is said to occur but practically there is little or
no Surface checking and warping. There are no signs of insect attack,
Discoloration or decay. End coating to prevent spliting is Advisable.
(b) Kiln Drying
Different kiln schedules were tested in drying teak. Table 3 Show the
most suitable schedules for minimising drying defects:
Table 3: Kiln Schedule for Teak
Moisture Content %
Temperature (F)
Relative Humidity
Initial
120
80
35
125
75
30
130
70
25
135
65
20
140
60
15
145
50
10
150
40
(c) Solar Drying
Drying of teak by solar means has been carried out by one local
Researcher, who used a semi-green house type. It takes 26 days to
reduce 1-Inch thick board of 39%.1 % initial moisture content to 12.2%
moisture content. No drying defects were observed. It is thus observed
that parquet blocks that are solar dried are quite satisfactory.
(d) Drying by Girdling
is a common practice in Myanmar, India and Thailand, the sapwood being
removed and leaving the tree standing before felling, normally for
three years. This is to allow the wood to float for water
transportation and at the same time reduce the drying time with less
drying defects. Girdled teak that stands for 27 months has a moisture
content of 33.6% still left in the wood. It is still above the normal
fiber saturation point and therefore it is only partially dry.
Rate of growth
Growth rings of 2-13 per inch are usual and yet up to 40 rings per
inch have been found occasionally; plantation teak has wide growth
rings. Teak Plantation in Thayawaddy showed an average growth ring of
4.3 per inch. In Thingan-Neenaung and Koloon area, the averages are
5.5 and 9.6 rings per inch. In the natural forests of Bago Yoma, the
average growth ring was found to be 12 per inch. An optimum number of
growth rings per inch is said to be six, which increase the strength
properties. Beyond that the strength decrease in such a way that it is
about the same as that of 15-20 rings per inch, which is extremely
poor strength value or comparable to fast grown pieces of 2 rings per
inch. On the other hand growth rate may not be of prime importance, if
the timber is to be used for ornamental or other purposes apart from
structural use. In the early years of the tree life, the strength
properties of wood are comparatively low, but under normal growth
conditions, rise steadily for twenty years to a level at which thy
remain more or less constant. It is rather obvious that slow grown and
fast grown teak have different properties and one may be useful for
one purpose but it may not be good for another purpose. From the
research, it appears that for the same stem diameter, there is more
sapwood if the growth rate is faster as is the case in the better
quality areas. It also appeared that the sapwood increases with the
height of the cross-section above the ground. Teakwood of very fast
growth is weaker and spongy.
Girdled and Green-Felled Teak
There is a tendency for girdled teak to be stronger and better than
green-felled teak. Regarding the Bending Strength, the survey showed
as per Table 4.
Table 4: Bending strength of girdled and green felled teak
Locality
Bending
Strength (psi)
Green-Felled
Girdled
Zigon, Bago Division
16,397
17,414
Tharyawaddy, Bago Division
11,580
11,670
The Table shows that there is a tendency of the timber from girdled
trees to be a little bit stronger in both localities, but the
difference is not significant. A similar subject was studied by
Pearson and Brown(1932). The results of the test are given below as
per Table 5.
Table 5: Strength Properties of Girdled and Green Felled Teak
Condition
Girdled
Green Felled
MC (%)
14.0
14.1
Density (Lb/Cft)
43.0
40.0
Static Bending (psi)
FS@PL
MOR
MOEx1000
9145
14965
1913
9425
14465
1830
Compression Parallel to Grains (psi)
FS@PL
Max.Crush
5345
8525
5385
8350
Compression Parallel to Grains (psi)
1445
1280
Sher Parallel to Grains (psi)
1355
1140
Hardness
End (Lb)
Side (Lb)
1225
1240
975
860
Note:
MC = Moisture Content, FS@PL = Fibre Stress at the Proportion Limits
MOR=Modulus of rupture, MOE = Modulus of elasicty
In term of strength as a whole, the difference between girdled teak
and green felled teak is not significant. On the other hand, there is
a trend that the girdled teak has a slightly higher strength value,
except in the Fibre Stree at the Proportion Limits. In hardness, it is
quite obvious that girdled teak is harder. Although the girdled tree
is stronger than the green felled tree, the magnitude of difference is
not that great. However, great care should be taken in extracting
green felled teak. Unless every precaution is taken in different
stages of processing, green felled timber will naturally suffer
various defects and be devalued. At the same time, timber volume loss
could be greater. So, it is advisable to extract and process green
teak efficiently to avoid loss. Otherwise green felling teak may not
have the similar strength value to girdled teak.
According to the research, the shrinkage and swelling properties and
other properties of wood are not altered by the length of the period
during which the trees stand girdled. Sawing and seasoning of timber
did not show difference girdled and green felled teak trees. After one
year, green felled trees reduced their moisture contents to About 66%
and girdled trees to about 64%. After two years, the respective
moisture contents were found to be about 47% and 34%.
End User Requirements selling
teak furniture
Naturally, buyers like to get timber of good quality for the money
they pay. The physical quality of wood normally required is defect
free timber, straight and free from curvature or twists even exposed
to weather. Spiral grain, large knots, sapwood, shakes of different
types, too fast grown, bee holes, and springs, are the defects most
undesirable by the buyers. Girdled Teak, free from visible defects,
less sapwood, small tolerances in sawing, and having specified
moisture content, is much preferred. Discoloration is an unwanted
defect.
Apart from the physical quality, some of the qualities to be
considered in the international trade consist of the following:
In term of wood quality, traders prefer natural grown teak as it
is harder and more stable than the fast grown plantation teak.
Natural grown teak is said to have a more oily nature and is
better uniformity in colour. These characteristics are much
preferred in the use of teak as a long lasting high quality
timber.
Outdoor living is becoming popular and garden furniture is in
large-scale production, where durable and light teak timber is
much admired.
The quality of Myanmar Teak from the government plantation is naturally already good. The stem is
normally cylindrical and straight apart from the teak of the northern
area, which is rather flatted and cross-grained. Teak being a
light-demander competes among its associates and this naturally
favours straight and clean holes. The branches in younger aged trees
are naturally pruned in both the natural and plantation teak. Since
the size of the trees harvested is rather large, the presence of knots
is less pronounced.
Apart from naturally favoured qualities, some work on the collection
of good and sound seeds from better mother trees is being carried out
in forming plantations. Seeds orchards, selection of plus trees for
seed collection, and the development of hybrids are in a state of
research and development.
Normal timely silvicultural operations, namely thinning in plantations
and improvement felling in the natural teak bearing area, are carried
out for better timber quality. Correct selection of sites for
plantations is of great importance.
Pre-Treatment in teak harvesting, accelerated transportation and
processing, and the proper cutting season and drying are processes
that may improve the wood quality in pre- and post harvesting
operations.
