During its growth, a tree depends on the rapid transfer of water through its internal system to take in nutrients from its roots and spread them to the tips of its leaves. Therefore timber is by nature hygroscopic – that is, it is affected by the moisture content of the atmosphere surrounding it. If its environment is too wet the timber will swell; if it is too dry it will shrink. Both of these can cause dimensional change in the timber and this can result in warping.

Although modern preparation techniques reduce this hygroscopic characteristic it is never totally eliminated, even after processing.

The changes a piece of timber undergoes when it is exposed to damp or dry air are not directionally uniform as wood swells more tangentially than radially. Pine for instance has a ratio of about 2:1 tangentially to radial. The dimensional changes and the more severe warping of the sapwood as compared with the heartwood cause deformation and often cracking when the water content is allowed to change.

Dimensional variation

Obviously, the susceptibility of wood to deformation depends on the type of wood, the volumetric weight, the growth ring position and the grain. The percentage dimensional variation is considerable even if the changes in water content are only small.

In the case of veneered boards and furniture the effect of humidity is further complicated by variation in thickness. Every room in which dried veneers are stored or worked should receive constant artificial humidification at least in winter to counteract any fluctuations.

The effects on wood of a dry atmosphere include the joints at the corners of frames becoming loose, delicate parts of furniture and internal woodwork beginning to warp, middle layers and edging (veneers) showing through and ground surfaces becoming uneven and developing surface cracks.

If wood is not exposed to direct sources of moisture, such as rain, its moisture content will come into equilibrium with the humidity of the ambient air. For the purposes of manufacture, the ideal water content of wood is 9-12%. These values result automatically at a level of between 50-60% relative humidity, producing humidity equilibrium between the air and the wood so no further exchange of moisture takes place.

This balance is known as the equilibrium moisture content (EMC). Ambient temperature has an effect on the EMC of wood. At 45% relative humidity a piece of timber with an EMC of 8.6% at 15 degrees C would have an EMC of 8.1% at 32 degrees C.

Maintaining equilibrium

The problem is how to maintain equilibrium. During the summer months the relative air humidity in Europe is between 50-60%. It is, however, a different matter in winter when artificial heating is used. The relative humidity can then drop to between 20-30%. Such a low value, particularly following a rapid drop in relative humidity, represents an acute danger to wood. Humidification systems should be installed in storage areas to ensure such a drop does not occur.

Production areas with a low humidity do not present such a danger to wood as the timber is rarely in this environment for long. However, by installing a humidification system around cutting and sawing areas, sawdust is suppressed, producing a more pleasant working environment.

Dust suppression

The need for dust suppression is a major reason why the most popular form of humidifier in the timber industry is a cold water spray system. The other main advantage of using this type of humidifier is that it is low energy, requiring a fraction of the running costs of other types of humidifiers that generate and release steam into an atmosphere.

An atomising nozzle system will combine compressed air and water to achieve a fine spray that evaporates the moment it is released, raising the humidity to the desired level.

Any mistakes in humidity control can be very expensive in an industry where the quality of the product is so heavily influenced by dry air. Humidification system design is therefore very important and advice should be sort from a reputable company which can give reliable back-up and after-sales support.