Читать книгу Seasoning of Wood - J. B. Wagner - Страница 9

Certain general properties of wood may be discussed briefly. We know that wood substance has the property of taking in moisture from the air until some balance is reached between the humidity of the air and the moisture in the wood. This moisture which goes into the cell walls hygroscopic moisture, and the property which the wood substance has of taking on hygroscopic moisture is termed hygroscopicity. Usually wood contains not only hygroscopic moisture but also more or less free water in the cell cavities. Especially is this true of sapwood. The free water usually dries out quite rapidly with little or no shrinkage or other physical change.

In certain woods—for example, Eucalyptus globulus and possibly some oaks—shrinkage begins almost at once, thus introducing a factor at the very start of the seasoning process which makes these woods very refractory.

The cell walls of some species, including the two already mentioned, such as Western red cedar and redwood, become soft and plastic when hot and moist. If the fibres are hot enough and very wet, they are not strong enough to withstand the resulting force of the atmospheric pressure and the tensile force exerted by the departing free water, and the result is that the cells actually collapse.

In general, however, the hygroscopic moisture necessary to saturate the cell walls is termed the "fibre saturation point." This amount has been found to be from 25 to 30 per cent of the dry wood weight. Unlike Eucalyptus globulus and certain oaks, the gums do not begin to shrink until the moisture content has been reduced to about 30 per cent of the dry wood weight. These woods are not subject to collapse, although their fibres become very plastic while hot and moist.

Upon the peculiar properties of each wood depends the difficulty or ease of the seasoning process.