Seasoning of Wood, Joseph Bernard Wagner [best beach reads txt] 📗

greater for the upper than the lower cuts, greater for the limbs than the stems, and greatest of all in the roots.

Different trees, even of the same kind and from the same place, differ as to the amount of water they contain. A thrifty tree contains more water than a stunted one, and a young tree more than on old one, while the wood of all trees varies in its moisture relations with the season of the year.

Seasonal Distribution of Water in Wood

It is generally supposed that trees contain less water in winter than in summer. This is evidenced by the popular saying that "the sap is down in the winter." This is probably not always the case; some trees contain as much water in winter as in summer, if not more. Trees normally contain the greatest amount of water during that period when the roots are active and the leaves are not yet out. This activity commonly begins in January, February, and March, the exact time varying with the kind of timber and the local atmospheric conditions. And it has been found that green wood becomes lighter or contains less water in late spring or early summer, when transpiration through the foliage is most rapid. The amount of water at any one season, however, is doubtless much influenced by the amount of moisture in the soil. The fact that the bark peels easily in the spring depends on the presence of incomplete, soft tissue found between wood and bark during this season, and has little to do with the total amount of water contained in the wood of the stem.

Even in the living tree a flow of sap from a cut occurs only in certain kinds of trees and under special circumstances. From boards, felled timber, etc., the water does not flow out, as is sometimes believed, but must be evaporated. The seeming exceptions to this rule are mostly referable to two causes; clefts or "shakes" will allow water contained in them to flow out, and water is forced out of sound wood, if very sappy, whenever the wood is warmed, just as water flows from green wood when put in a stove.

Composition of Sap

The term "sap" is an ambiguous expression. The sap in the tree descends through the bark, and except in early spring is not present in the wood of the tree except in the medullary rays and living tissues in the "sapwood."

What flows through the "sapwood" is chiefly water brought from the soil. It is not pure water, but contains many substances in solution, such as mineral salts, and in certain species—maple, birch, etc., it also contains at certain times a small percentage of sugar and other organic matter.

The water rises from the roots through the sapwood to the leaves, where it is converted into true "sap" which descends through the bark and feeds the living tissues between the bark and the wood, which tissues make the annual growth of the trunk. The wood itself contains very little true sap and the heartwood none.

The wood contains, however, mineral substances, organic acids, volatile oils and gums, as resin, cedar oil, etc.

All the conifers—pines, cedars, junipers, cypresses, sequoias, yews, and spruces—contain resin. The sap of deciduous trees—those which shed their leaves at stated seasons—is lacking in this element, and its constituents vary greatly in the different species. But there is one element common to all trees, and for that matter to almost all plant growth, and that is albumen.

Both resin and albumen, as they exist in the sap of woods, are soluble in water; and both harden with heat, much the same as the white of an egg, which is almost pure albumen.

These organic substances are the dissolved reserve food, stored during the winter in the pith rays, etc., of the wood and bark; generally but a mere trace of them is to be found. From this it appears that the solids contained in the sap, such as albumen, gum, sugar, etc., cannot exercise the influence on the strength of the wood which is so commonly claimed for them.

Effects of Moisture on Wood

The question of the effect of moisture upon the strength and stiffness of wood offers a wide scope for study, and authorities consulted differ in conclusions. Two authorities give the tensile strength in pounds per square inch for white oak as 10,000 and 19,500, respectively; for spruce, 8,000 to 19,500, and other species in similiar startling contrasts.

Wood, we are told, is composed of organic products. The chief material is cellulose, and this in its natural state in the living plant or green wood contains from 25 to 35 per cent of its weight in moisture. The moisture renders the cellulose substance pliable. What the physical action of the water is upon the molecular structure of organic material, to render it softer and more pliable, is largely a matter of conjecture.

The strength of a timber depends not only upon its relative freedom from imperfections, such as knots, crookedness of grain, decay, wormholes or ring-shakes, but also upon its density; upon the rate at which it grew, and upon the arrangement of the various elements which compose it.

The factors effecting the strength of wood are therefore of two classes: (1) Those inherent in the wood itself and which may cause differences to exist between two pieces from the same species of wood or even between the two ends of a piece, and (2) those which are foreign to the wood itself, such as moisture, oils, and heat.

Though the effect of moisture is generally temporary, it is far more important than is generally realized. So great, indeed, is the effect of moisture that under some conditions it outweighs all the other causes which effect strength, with the exception, perhaps of decided imperfections in the wood itself.

The Fibre Saturation Point in Wood

Water exists in green wood in two forms: (1) As liquid water contained in the cavities of the cells or pores, and (2) as "imbibed" water intimately absorbed in the substance of which the wood is composed. The removal of the free water from the cells or pores will evidently have no effect upon the physical properties or shrinkage of the wood, but as soon as any of the "imbibed" moisture is removed from the cell walls, shrinkage begins to take place and other changes occur. The strength also begins to increase at this time.

The point where the cell walls or wood substance becomes saturated is called the "fibre saturation point," and is a very significant point in the drying of wood.

It is easy to remove the free water from woods which will stand a high temperature, as it is only necessary to heat the wood slightly above the boiling point in a closed vessel, which will allow the escape of the steam as it is formed, but will not allow dry air to come in contact with the wood, so that the surface will not become dried below its saturation point. This can be accomplished with most of the softwoods, but not as a rule with the hardwoods, as they are injured by the temperature necessary.

The chief difficulties are encountered in evaporating the "imbibed" moisture and also where the free water has to be removed through its gradual transfusion instead of boiling. As soon as the imbibed moisture begins to be extracted from any portion, shrinkage takes place and stresses are set up in the wood which tend to cause checking.

The fibre saturation point lies between moisture conditions of 25 and 30 per cent of the dry weight of the wood, depending on the species. Certain species of eucalyptus, and probably other woods, however, appear to be exceptional in this respect, in that shrinkage begins to take place at a moisture condition of 80 to 90 per cent of the dry weight.

SECTION VII WHAT SEASONING IS

Seasoning is ordinarily understood to mean drying. When exposed to the sun and air, the water in green wood rapidly evaporates. The rate of evaporation will depend on: (1) the kind of wood; (2) the shape and thickness of the timber; and (3) the conditions under which the wood is placed or piled.

Pieces of wood completely surrounded by air, exposed to the wind and the sun, and protected by a roof from rain and snow, will dry out very rapidly, while wood piled or packed close together so as to exclude the air, or left in the shade and exposed to rain and snow, will dry out very slowly and will also be subject to mould and decay.

But seasoning implies other changes besides the evaporation of water. Although we have as yet only a vague conception as to the exact nature of the difference between seasoned and unseasoned wood, it is very probable that one of these consists in changes in the albuminous substances in the wood fibres, and possibly also in the tannins, resins, and other incrusting substances. Whether the change in these substances is merely a drying-out, or whether it consists in a partial decomposition is at yet undetermined. That the change during the seasoning process is a profound one there can be no doubt, because experience has shown again and again that seasoned wood fibre is very much more permeable, both for liquids and gases than the living, unseasoned fibre.

One can picture the albuminous substances as forming a coating which dries out and possibly disintegrates when the wood dries. The drying-out may result in considerable shrinkage, which may make the wood fibre more porous. It is also possible that there are oxidizing influences at work within these substances which result in their disintegration. Whatever the exact nature of the change may be, one can say without hesitation that exposure to the wind and air brings about changes in the wood, which are of such a nature that the wood becomes drier and more permeable.

When seasoned by exposure to live steam, similiar changes may take place; the water leaves the wood in the form of steam, while the organic compounds in the walls probably coagulate or disintegrate under the high temperature.

The most effective seasoning is without doubt that obtained by the uniform, slow drying which takes place in properly constructed piles outdoors, under exposure to the winds and the sun and under cover from the rain and snow, and is what has been termed "air-seasoning." By air-seasoning oak and similiar hardwoods, nature performs certain functions that cannot be duplicated by any artificial means. Because of this, woods of this class cannot be successfully kiln-dried green from the saw.

In drying wood, the free water within the cells passes through the cell walls until the cells are empty, while the cell walls remain saturated. When all the free water has been removed, the cell walls begin to yield up their moisture. Heat raises the absorptive power of the fibres and so aids the passage of water from the interior of the cells. A confusion in the word "sap" is to be found in many discussions of kiln-drying; in some instances it means water, in other cases it is applied to the organic substances held in a water solution in the cell cavities. The term is best confined to the organic substances from the living cell. These substances, for the most part of the nature of sugar, have a strong attraction for water and water vapor, and so retard drying and absorb moisture into dried wood. High temperatures, especially those produced by live steam, appear to destroy these organic compounds and therefore both to retard and to limit the reabsorption of moisture when the wood is subsequently exposed to the atmosphere.

Air-dried wood, under ordinary atmospheric temperatures, retains from 10 to 20 per cent of moisture, whereas kiln-dried wood may have no more than 5 per cent as it comes from