More about Teak wood

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 Indonesia:

1. Uniform Golden color, straight grained and even texture and with few markings. 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.
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. It is a good ornamental timber and previously a high price was obtained.

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 percents 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 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 splitting 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 countries.

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.

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.

Outdoor living is becoming popular and garden furniture is in large-scale production, where durable and light teak timber is much admired.

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.

 
 
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