INTRODUCTION TO WOOD

 

Sources of wood: Wood is a naturally occurring material and can be found all over the world with different species in different areas. Approximately 80% of UK wood needs are supplied by other countries.

Structure of wood: All woods are fibrous with the fibres (or grain (the visual effect of the flow of tracheid's)) growing along the length of the trunk or branch. These fibres consist of cells (tracheid's (the cells of woods)) of, mainly, cellulose supported by lignin (the natural resins that hold the cells together in timbers) resin.

• There are two main types of this naturally occurring material, the difference between the two types is botanical:

                          - Hardwood = timbers that are deciduous and slow growing
                          - Softwood = timbers that are evergreen and quick growing

KEY TERMS:
1. Deciduous - a general term used to describe trees that lose their leaves in autumn.
2. Evergreen - a general term used to describe trees that do not lose their leaves in autumn.

 



Examples of hardwoods grown in the UK include oak, ash, beech, sycamore and willow; while examples of imported tropical hardwoods include teak, cedar and mahogany.

There are a smaller number of useable softwoods than there are useable hardwoods. Softwoods such as larch, spruce, Scots pine and Douglas fir are grown in forests and plantations in the UK, but approximately 90% of the UK softwood needs are supplied by countries such as Norway and Sweden.

Woods Strengths: Wood has its greatest strength in the direction of the fibres. If a length of timber were to be put under tension along the grain it would be able to support a far greater load than if it were put under tension across the grain, which would result in the timber splitting at a much lower loading.
Woods Defects: Wood is a natural material and therefore is not as consistent in structure.

KEY TERMS:
1. Knots - natural defects found in timber - the start of branches from the trunk.
2. Conversion - sawing up logs to provide useable wood forms.
     --> Slab Sawn: a form of conversion where the trunk of the tree is cut into slabs - more      prone to warping.
     --> Quarter Sawn: a form of conversion that can prevent warping and can be used to enhance grain.

3. Splits - separation of timber fibres along the grain due to rate of drying.

Wood defects due to drying:

Shrinkage: all timbers shrink due to moisture lost in seasoning.
Moisture content: the amount of 'water' in the timber. Usually shown as a percentage of volume.
Twisting: a form of warping that is due to a combination of a method conversion (sawing the trunk of the tree) and uneven seasoning.
Rot: the breaking down of the lignin resins in the woods that hold the tracheid's (cells) together.
Deathwatch beetle: insect responsible for the destruction of mainly hardwoods, e.g. oak-frame buildings; churches and barns can be affected by the deathwatch beetle.
Kiln drying: a form of seasoning that uses steam in a controlled way to reduce the content of moisture in timber.
Decay: the deterioration of wood.

Seasoning Woods: Is a controlled drying of timber. This can be achieved through natural seasoning of by kiln drying. The ideal is that the moisture content of the timber is the same as that of its surroundings. This is known as the Equilibrium Moisture Content (EMC).
The benefits of seasoning...

• It increases the strength and stability of the timber.
• The reduced moisture content reduces the risk of the timber causing corrosion to the surrounding metalwork.
• It makes the timber less prone to rot and decay.

Not all timbers are seasoned thoroughly. The term 'green timber' refers to woods that have been cut down, converted and stored for up to 12 months.