Part 12 (1/2)

Even in the living tree a flow of sap from a cut occurs only in certain kinds of trees and under special circ.u.mstances. 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 const.i.tuents 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 alb.u.men.

Both resin and alb.u.men, 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 alb.u.men.

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 alb.u.men, 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 cla.s.ses: (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.