The Technique of FURNITURE MAKING

Portable chain mills

These are, in essence, large chainsaws that operate horizontally on a felled log to produce sawn planks to any thickness required. Their great advantage is that they can be taken into the woodlands and the log can be converted on the site, which enables the individual boards to be easily transported by normal van or trailer.

This machine does, therefore, enable the furniture-maker to recover logs that might otherwise simply rot away due to their not being economically viable in terms of normal commercial considerations. Furthermore, in some parts of the world it is the only way to obtain complete logs of sawn timber as opposed to the squared up and graded boards.

SEASONING AND MOISTURE CONTENT

The growing tree which is full of sap can contain anything from 50 per cent free water up to 200 per cent in certain swamp-grown softwoods. A single cubic foot (0.02832 m3) of green oak, for instance, can contain up to 28 pt (16 1) of water which must be reduced to at least 4 pt (2.27 1) before the timber is fit for use. The object of seasoning is, therefore, to accelerate the evaporation of this surplus moisture, rendering the wood lighter, harder, stronger, less susceptible to discoloration and mould attack, and much less liable to shrinkage and distortion.

The water in the tree is contained in the actual fibres which are saturated (bound moisture), and in the open pores or vessels which are filled with a mixture of water and water vapour. Immediately the tree is felled the vessels will start to dry. This process is very slow in whole logs, but can be speeded up by converting the log into plank form, when the water-level will fall steadily until a fibre saturation point is reached (usually 30 per cent moisture content) in which the vessels are empty but the fibres remain saturated. From then on the water loss will be at a slower rate until the board has reached an equilibrium with the surrounding atmosphere. The amount of moisture thus left in the wood is termed the 'equilibrium moisture content' according to the 'relative humidity' of the prevailing atmosphere, and before describ­ing methods of seasoning it is necessary to define these terms.