Uses - the Structure of Wood

USES - THE STRUCTURE OF WOOD Few of either the makers or the users of forest products—and this includes all of us—have any conception of the real structure of the material with which they deal. The botanist tells one tree from another by differences in foliage, flowers, fruits, and bark; the microscopist, by differences in the structure and arrangement of cells which may be visible only through a high-powered microscope; the woodsman, by general notions of appearance; the carpenter, by characteristics of texture and weight learned in the working of wood; and the ordinary user, by any combination of these methods that may have impressed him in the course of his experience. While the botanist and the microscopist use scientifically exact means of determining species of trees and kinds of wood, the lumberman, the cabinet-maker, and the man in the street use methods which, while unscientific and even impossible to describe, nevertheless often suffice for their own particular needs.

Wood is bought, sold, and used with far less knowledge of its composition, strength, stiffness, density, and other qualities than is any other substance that enters largely into our daily life. A steel rail is made according to a formula prepared by the metallurgist; there are standard mixtures for cement and concrete; the physical properties of metals and stone are accurately known; but even the best grading rules of the manufacturers are only approximations to the actual values of the different classes of lumber. To a large extent this is an unavoidable condition. A tree is not made according to any chemical or mechanical formula. It is the product of soil, moisture, and sunshine in constantly varying combinations. Buffeted by storms and subjected to extremes of heat and cold, drouth, and flood for a century or more, each year's growth is different from that which precedes or follows; and the resulting mass of wood is a highly complex substance of which we know far too little.

But the problems of modern construction and utilization demand that our knowledge of wood be increased in scope and accuracy. Therefore

the timber-testing engineer, with his ponderous machines, determines the strength, stiffness, and elasticity of beams of a specified size and kind; the timber-treating expert, with cylinders and pressure pumps, finds out the best means of impregnating wood with creosote, zinc chloride, and other decay-preventing substances; the pulpmaker cooks and grinds different woods to get the best kind of paper; the chemist puts wood into his retort, and gets alcohol, turpentine, acid, and many other products; but all of these problems go back to the fundamental one of the structure of the wood itself.

Porous and Woods

The unit of woody structure, as of all vegetable and animal growth, is the cell; and the scientific classification of woods is based upon the properties and combinations of these ultimate units. When cross-sections of certain woods are examined, they are seen to contain relatively large, irregularly placed openings called "pores." Other woods, even under the microscope, show no such openings. A natural, fundamental division, therefore, is into "porous" and woods. Of porous woods, the common hardwoods are familiar examples; while the pines, firs, spruces, cedars, etc., are nonporous. Again, the porous woods are divided into two classes according to the arrangement of the pores in the yearly ring of wood. In some woods, very large pores develop early in the season; while only small pores or none at all appear later, so that a cross-section of such a wood shows, even to the naked eye, concentric circles of openings. These woods are called "ringporous" woods. In other woods, pores of small and approximately equal size are scattered throughout, with little if any discernible grouping. Such woods are called "diffuse-porous" woods. Common ring-porous woods are ash, oak, elm, and hickory. Among the diffuseporous woods are birch, beech, basswood, maple, and walnut.