Moisture in Timber and its Seasoning!

Moisture occurs in timber in as free water in the cell cavities or as bound water in the cell walls.

When all of the free water in the cell cavities has been removed, the fibre saturation point is searched and at this point the timber normally has a moisture content of between 25-30%. And any reduction of moisture in the timber below the fibre saturation point will lead to shrinkage. The timber should be dried out to a moisture content which is approximately equal to the surrounding atmosphere in which it will be used. This moisture content is known as equilibrium moisture content and provided the moisture content and temperature of the air remains constant, the timber will remain stable and not shrink or expand.

Determining the moisture content (%)

This refers to the water in the timber compared to the dry weight of the timber. In order to determine the average moisture content of a stack of timber, select a board from the center of the stack, cut the end 300mm off and discard it as this will normally be dryer than sections nearer the center. Cut off a further 25mm sample and immediately weigh it. This is the wet weight of the sample. Place this sample in a drying oven and remove it periodically to check its weight. When no further loss of weight is recorded, assume this to be the dry weight of the sample.

moisture content = wet weight – dry weight x 100

dry weight

The alternative way of finding moisture content of timber is to use an Electric meter. Although not as accurate, it has advantage of giving an on-the-spot readings and it even be used for determining the moisture content of timber already fixed in position. The moisture meter measures the electrical resistance between the 2points of a twin electrode which is pushed into the surface of the timber. Its moisture content can then easily be read off a calibrated dial.

Seasoning of Timber

Seasoning is controlled drying by natural or artificial means of converted timber and there are a number of reasons as to why seasoning is done and these include;

• To ensure that the moisture content of timber is below the dry rot safety line of 20%.

• To ensure that any shrinkage takes place before the timber is used.

• Using seasoned timber, the finished article will be more reliable and less likely to split or distort.

• In general, dry timber is stronger and stiffer than wet timber.

• Wet timber will not readily accept glue, paint or polish.

Timber can be seasoned either by natural means (air seasoning) or artificial means (kiln seasoning);

Air seasoning; timber is stacked in open sided, covered sheds which protect it from rain whilst still allowing a free circulation of air. In most cases, moisture content of between 18% and 20% can be achieved in a period of 2 or 12 months, depending on the size and type of timber.

Key points to note when setting up an ideal timber stack for the air seasoning of softwoods:

1. Brick piers and timber joists keep the bottom of the stack well clear of the ground and ensure good air circulation underneath.

2. The boards are laid horizontally, largest at the bottom, smallest at the top; this reduces risk of timber distorting as it dries out.

3. The boards on each layer are spaced approximately 25mm apart.

4. Piling sticks of stickers as introduced between each layer of timber at approximately 600mm distances to support the boards and allow a free air circulation around them. (it should be noted that these sticks should be the same type of timber as that being seasoned to avoid stains)

5. The ends of the boards should be painted or covered with strips of timber to prevent them from dying out too quickly and splitting.

Hardwood can be seasoned in the same air-seasoned sheds but the boards should be stacked in the same order as they were cut from the log.

Kiln seasoning; most timber for intended use is kiln seasoned; as this method, if carried out correctly, is able to safely reduce moisture content of the timber to any required level, without any danger or degrading (causing defects).

Although timber can be completely kiln seasoned, sometimes when a saw mill has a low kiln capacity, the timber is air seasoned before being placed in the kiln for final seasoning. The length of time the timber needs to stay in the kiln normally varies between 2-6 weeks according to the type and size of timber being seasoned.

The 2 main types of kiln in general use:

Compartment Kiln; this is normally a brick/concrete building in which the timber is stacked. The timber will remain stationary during the drying process, while the conditions of the air are adjusted to the correct levels as the drying progresses.

Note: the drying of the timber depends on 3 factors namely;

• Air circulation, which is supplied by fans.

• Heat, which is normally supplied by heating coils through which steam flows.

• Humidity (moisture content of the air). Steam sprays are used for raising the humidity. They are installed along the whole length of the compartment.

Progressive Kiln; this is a tunnel full of open trucks containing timber that are progressively moved forward from the loading end to the discharge end. The drying conditions in the kiln become progressively more severe so that the loads at the different distances from the loading end are at different stages of drying. This type of kiln is mainly used in situations where there’s need for continuous supply of timber.

Storage of seasoned timber

Since seasoning of timber is a reversible process, great care must be taken in the storage of seasoned timber. Carcassing and external joinery timber that is delivered on the site at an early stage should be stacked clear of the ground using piling sticks between each layer and then covered with waterproof tarpaulins. Integral joinery or low moisture timber shouldn’t be delivered or installed before the building is fully glazed and it’s heating system in operation so as to maintain a low humidity.

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