Q.
a) Explain the classification of wood based on botanical aspects. (10 marks)
b) Describe the methods of seasoning and preservation of timber. (15 marks)
(25 marks, 2014 Q6)
A.
a) Wood is generally classified under 'Hard Wood' and 'Soft Wood'.
A good write up about wood classification is here.
b) Methods of seasoning and preservation of timber.
TIMBER SEASONING
Seasoning is the reduction of moisture content of timber to that of the-surrounding air - Sydney area approx. 12.3%.Timber is seasoned to dry out the timber to bring it to a usable and workable condition. More than half the weight of freshly cut timber consists of moisture or sap. Before the timber can be used a large part of the moisture must be removed.
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Reasons for Seasoning
Air - natural circulation of air around & through timber stacks. --- Time - 3-9 months, sometimes years. - Kiln - forced drying of timber stacks in large containers called kilns - Time 1-3 weeks. - Combination of Air & Kiln - combines advantages of both air and kiln. - Solar kilns – rely on solar energy to aid the drying process - Time 1-3 weeks For more information on seasoning of timber go to Drying timber in Australia.Seasoned timber has many advantages over unseasoned timber, such as: improved strength and hardness
- To prevent shrinkage, splitting, checking and warping.
- To achieve greater stiffness and strength.
- To allow penetration of preservatives
- To obtain a surface that will accept paint, polish or glue.
- To protect against decay.
Methods of Seasoning
Air Seasoning
The timber is stacked on foundations to keep it off the ground. Strips of wood (about 38 x 19 mm) are placed between the layers of boards to allow air to circulate. Ends of boards may be painted to retard drying and prevent splitting. A waterproof covering is placed over the stack to protect the timber from the sun and the weather. Air seasoning is slow, taking up to a year for a piece 25 mm thick and considerably longer for thicker pieces. But the timber produced is well suited to outdoor work.
Kiln Seasoning
Kiln seasoning is done in a large oven-like structure in which air circulation, humidity and temperatures are controlled. The timber is stacked on the trucks in the same way as for air drying. The trucks are on rails so that they can be moved into the kiln. Care must be taken to see that the drying process while removing the free water is not too rapid, as at this stage the timber is susceptible to degrade (damage) by checking and collapse. Many types of softwood, such as Radiata Pine, can be kiln dried easily and fairly cheaply from the green condition. But many of the Australian hardwoods are so slow in drying that their kiln drying is uneconomical. It is, however, satisfactory and economic to season them by the combined process of air and kiln drying mentioned below.
Ref:
http://timber.lk/Seasoning/index.html