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| About woods |
About woods |
Myanmar hardwoods |
Care of teak |
Storage of woods
About Wood, Science of Wood
Hardwood and Softwood
Trees are divided into two classes: hardwoods and softwoods. The hardwoods such as Teak, birch, maple have
broad leaves. The terms "hardwoods" and "softwoods" are not directly associated with the hardness or softness
of the wood although in most cases hardwoods are actually harder and tougher than softwoods. In general softwoods
originate from cone-bearing trees and hardwoods from trees that have their seeds contained in a seed-case.
Sapwood
Sapwood plays and important part in a tree's living process. In general only the last few outside layers of sapwood
are alive. The rest of the sapwood carries moisture from the roots to the leaves and store food for the tree. It usually
have a higher moisture content than the heartwood.
Heartwood
During the life of the tree, sapwood gradually changes into heartwood. As it does so, it becomes less permeable.
Since moisture movement is retarded considerably, heartwood dries more slowly than sapwood. Heartwood usually
surface checks and honeycombs more readily than sapwood, hence requires milder drying conditions. It is usually
darker in color and also more resistant to decay than sapwood.
A : Cambium layer inside of inner bark
B : Inner bark
C : Outer bark
D : Sapwood
E : Heartwood
F : Pitch
G : Wood rays
Wood and Moisture
All wood in growing trees contains considerable quantities of water. It is present in two forms, i.e. (a) Free-water in the
cell cavities and (b) Bound-water bounded to the cell walls. Most of this water should be removed in order to obtain
the satisfactory service from the wood in use. The removal of free-water is much easer than that of bound-water from
the wood and there is no change in dimension or in strength properties by removing free-water.
The wood is said to be in a state of Fiber Saturation Point (FSP) when no free-water is present and the cell wall is
saturated with bound-water. In this state, moisture content in wood is between 25% and 30% and removal of water
below fiber saturation point (FSP) results dimensional change (shrinkage) and increase in many of the strength
properties.
Because of its hygroscopic nature wood is always seeking to equilibrate its vapour pressure with that of the
environment. That is wood always try to maintain its Equilibrium Moisture Content (EMC) which correspond to the
environment. It is the unique characteristic of wood that expends when it absorb moisture if its EMC is higher than
its existing moisture content, whereas contracts or shrinks when it loses moisture if its EMC is lower than its existing
moisture content.
Anisotropy of Wood
As wood possesses a complex fiber-composite structure, it varies in its most properties with the directions, called
anisotropy. Moreover its difference in structure occurs not only between different species but also between trees
of the same species grown in different environments or different parts of a single tree. This results undesirable
variation in its properties as a material.
The figure below shows three principal axes of wood with respect to grain direction and growth rings.
The three principal axes of wood with respect to grain direction and growth rings.
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