When a tree is felled, the majority of its weight is water. To ‘convert’ a trunk into usable timber, the sawmill cuts it into various board thicknesses. This method involves starting on one side of the trunk and, as each pass of the saw cut is complete, the next cut is made, and so on, until the entire trunk has been sawn.
Different methods of sawing timber yield different types of board with regard to stability, appearance and cost. There are several questions to ask yourself when choosing and buying wood: what method of sawing has been used to cut the log?; and how are the boards stored in the yard while the timber stocks are air-drying? Has the timber also been kiln-dried?
Many years ago, when labour costs were far lower than they are now, timber merchants were prepared to saw a log in half lengthways, and in half again to give a quadrant, or quarter circle, at the end of the each length. The reason for this `quartersawing’ was to reveal the medullary rays that would otherwise be lost. These highly prized rays, or flecks, are present in all trees to a greater or lesser extent, although they are most often seen in oak boards. Quartersawing is very wasteful, because it involves turning the timber frequently before the next saw cut, and it is now rare to find a sawmill converting a tree to boards in this way. For this reason, it is worth buying a board displaying medullary rays should you see one.
Through and through sawing
The most common cutting method, through-andthrough sawing simply involves the trunk being sawn from one edge. As each saw cut is complete, the log is moved the required distance to produce another cut until the trunk is sawn into a number of boards. Logically, the centre boards show the medullary rays and are produced without the cost of quartersawing. This method also produces the widest boards, although the heart is prone to splitting.
In some countries a log is sawn to produce the maximum wood from the trunk, regardless of whether or not the resulting boards have an attractive figure. This system of sawing around the log is known as `tangential’ cutting because each cut forms a tangent to one of the annual rings. It is not unusual for one face to include sapwood. It is advisable to reject any sapwood in your finished cabinetwork: it is prone to beetle attack, it shrinks badly, and its immature fibres do not accept glue or polish as well as the heartwood section.
How figure is produced by different sawing methods
Green timber has a moisture content of about 30 per cent, known as fibre saturation point. Once sawn, individual boards are stacked in the open air, using small strips of wood called ‘stickers’, to separate them from one another. Air flowing over each board draws the water to the surface by capillary action; this then evaporates, reducing the moisture content to about 25 per cent. The time required to ‘season’ the wood in this way depends on the thickness to which the boards have been cut. A rough guide is one year per 25 mm (1 in) thickness of board.
Air-drying worked reasonably well before the advent of central heating and air conditioning. However, the climate in many countries means that even the best air-dried timber retains about 16 per cent moisture content, no matter how long it has been dried, which is too high for most modern homes. Today, any wood used indoors must be able to remain stable, even when the central heating system is full on. In order to achieve this, air-dried timber is placed in a kiln to extract more moisture until it reaches an average of 15 per cent moisture content.
Always ask whether the wood you buy has been kiln-dried: the letters ‘KD, which conform to most trade description acts, should be printed on your receipt. In the event of subsequent extreme movement in your timber, it may be necessary to prove that your timber was sold as kiln-dried. Most timber merchants are conscientious and will replace the odd board that appears to have too high a moisture content despite being kiln-dried.
Kiln- or air-dried timber?
The best moisture content for timber in today’s well-heated homes is around 10 to 12 per cent. This is difficult if the sawmill supplies only timber averaging 15 per cent! Most good cabinet-makers will rough-machine their wood and then expose it to a well-heated workshop prior to finishing a piece to its final dimensions.
Air-dried timber is usually cheaper than kiln-dried, but the moisture content is unlikely to be lower than about 16 per cent. This level makes it suitable for external use such as constructing garden sheds, outdoor structures and timber framing. Timber with a moisture content of more than 20 per cent is susceptible to dry rot, however.
Do not assume that you can reduce the moisture content of this less expensive timber by leaving it in a warm room. You cannot, and if you use the timber for furniture-making, your work will suffer from bad shrinkage and distortion.
Kiln-drying timber takes a few days, as opposed to the months or years required for air-drying. For furniture and internal joinery items, timber should be kiln-dried down to about 11 per cent. Timber in close proximity to heat sources, such as radiators or fireplaces, should ideally be as low as 9 per cent. Sometimes, the kilning process leaves surface splits across the board face, which can cause problems. They indicate that the drying took place too rapidly, leaving the inner part of the board wet with the outer part too dry. This is called ‘case-hardening’. Avoid such boards: once you have machined the surface, the wet inner fibres will distort the board when subjected to the heat in your workshop.
Sawn timber is rarely bone dry: it gains or loses moisture depending on the surrounding atmosphere, and expansion or shrinkage occurs as it adapts to humidity and temperature.
Moisture content is the weight of moisture shown as a percentage of the timber’s dry weight. For example, in a board with 20 per cent moisture content there is actually 2 kg (4 ½ lb) of water for every 10 kg (22 lb) of dry wood. Moisture content is usually measured with an electronic meter, its pin electrodes detecting electrical resistance. Alternatively, a small timber sample can be weighed before and after being dried in an oven, then a simple calculation carried out.
When you are buying timber beware of claims like `average moisture content 15 per cent.’ This could mean some boards retain as much as 19 per cent, while other boards from the same kiln could be as low as 12 per cent.
Word Discription :
Crown-cut Another term for through-andthrough sawing (see left). Crown-cut boards display almost flat or slightly curved growth rings in the end grain. Board faces show the flame effect of the growth rings.
In stick Air-dried boards are stacked as they come off the log with spacer battens, or ‘stickers’, spaced evenly between them. Seasoning The process of removing moisture from the cell walls of wood.