34 



HARDWOOD RECORD 



particularly the sides (longitudinal faces) offers to the entrance 

 of a blunt body, such as a hammer. The test in hardness is one 

 of transverse compression of the fibers, and therefore depends on 

 their resistance to collapse. In a single fiber this resistance 

 depends on that of the material (presumabh' alike in all wood), 

 on the shape of the fiber, and the relative thickness of its walls. 

 Fibers like those of hardwoods (fibers proper), with a hexagonal 

 cross section and commonly scarcely any cell lumen or hollow, 

 naturally, behave almost like solid wood substance. They offer 

 great resistance, so that if the outer surface of a stick is formed 

 by such fibers its hardness is very great. If, on the other hand, 

 the surface layer is composed of thin walled vessels or of tracheids, 

 like those of the spriugwood in conifers, the wood is soft. In the 

 usual test the indentation extends but a short ditance ahead of thr- 

 intrument (as, for instance, when a timber is struck with a ham- 

 mer); but if the test is continued long enough the compression 

 results in destruction of all the thin walled and much of the thick 

 walled tissue of the wood, so that timbers, such as those some- 

 times buried in collapsed portions of deep mining shafts, are 

 destroyed throughout. Such a crushed stick continues to resist 

 further crushing, becomes compacted, dense, and heavy, but loses 

 nearly all its bending strength; it takes up water rapidly, and 

 when soaked crumbles like wood in the later stages of decay. 

 Closer examination shows that all thin walled fibers have collapsed 

 just like crushed pasteboard tubes, the break running along two 

 or more lines the length of the fiber, the form of the cross section 

 being changed from a hexagon to an S shape, or an approach to 

 this form. 



The hardness of wood in the sense as noted is quite variable, 

 even in wood of the same species, varj-ing on different sides and 

 also according to the portion of the annual ring exposed at the 

 surface, the extent of compression, and other circumstances. 



In nearly all wood used for construction, whether a bridge tim- 

 ber, the studding or joists of a house, or merely a table or chair 

 leg, the stiffness of the wood is an essential quality, and in many 

 if not most cases it is far more important than the ultimate 

 strength. , Thus, a rafter or joist need not be very strong, but it 

 must bend but little under its assigned load, and even in furniture 

 and smaller objects the piece must not only be sufficiently large 

 to hold up its weight without l)reaking, but to hold it without 

 being distorted to an unsightly or troublesome degree. In this 

 case ultimate strength is not considered, but stiffness or elasticity 

 rather, and in the majority of cases a "strong wood" is, witli 

 the artisan, really a stiff wood. The stiffness of a piece of wood 

 depends upon its weight and on its structure. If a single fiber of 

 pine and one of hickory, both of the same size and thickness of 

 wall, could be tested, they would probably be found alike in 

 stiffness, strength and degree of extensibility, for both are practi- 

 cally alike chemically and physically. The great difference be- 

 tween these woods must therefore be in the combination in whicli 

 the fibers o«cur in the wood structure, and it is in this that we 

 find a ready and plausible explanation for differences. An exami- 

 nation of a piece of typical hardwood and a piece of coniferous 

 wood shows that, — 



1 — The elements of structure are alike in conifers, unlike in 

 hardwood. 



2 — They are all large (comparatively) in conifers, while in hard- 

 wood extremely small elements (fibers proper) form scattered 

 bodies among larger ones (parenchyma) and very large ones (the 

 vessels). 



3 — These bodies of small fibers, the strongest part of the wood, 

 have extremely thick Walls, compared to their size, in the hard- 

 woods, but much less so in the conifers. 



4 — The fibers in conifers are arranged in perfect rows (or really 

 sheets, for the cells of each row are practically conterminous), 

 those of hardwoods are found in divided bodies, and appear like 

 separate strands of specially strong material. In addition, the 

 fibers (tracheids) in conifers are usually much longer than those 

 in hardwoods. On account of these structural conditions the fibers 

 in the conifer act much more perfectly together and allow less 



"give" than the heterogeneous elements and especially the sepa- 

 rated strands of fibers in hardwoods, which arrangement permits 

 more "give," and this "give" lessens the stiffness or elasticity of 

 the hardwood. For if we return to our single cell we would have 

 the upper part compressed when the fiber is bent, the lower ex- 

 tended, and the behavior would simply depend on the shape of the 

 fiber and the material of its wall, but if we have a set of fibers 

 and vessels grown together and tested the behavior depends not 

 only on their shape and the material, but also on the relative 

 position of the fibers and other elements. Those which are crooked 

 or oblique on the upper side of the stick will have their unfavor- 

 able attitude increased, those on the lower side will merely be 

 straightened or but partly strained, while the main part of the 

 load applied at first is borne by only a part of the fibers, that is, 

 those straightest in their position. Here the large fibers of the 

 conifer with their regularity of arrangement all fall in line at 

 once, they are "straight grain," the "give" is small, and the 

 timber is stiff. Moreover, when the load is removed the case is 

 exactly reversed. The fibers of coniferous wood, all being strained, 

 exert the same power to return, while many of the fibers in the 

 hardwood, on the other hand, are really under but little strain, 

 they make little effort to return, the timber does not "spring 

 back," and thus is neither very stiff nor springy or resilient; 

 it is not very elastic. Thus, it is that conifers are, as far as known 

 at present, generally stiffer than hardwoods of the same weight, 

 the difference often being very considerable. The finer and more 

 even the structure of the hardwood, the straighter the grain, the 

 greater the weight of the wood', and the more perfectly it is sea- 

 soned, the stiffer it is. In conifers this quality seems to vary 

 directly with their weight. In hardwoods the matter is too little 

 known to warrant any general statement, though here, too, heavy 

 woods like oak and ash are stiffer than light woods, such as poplar. 



PHYSICAL PROPEETIES 



Weight is an important indicator of the mechanical qualities of 

 wood and a -direct measure of its value as fuel or material for 

 coaling and dry distillation, and often determines the choice of 

 woods for a particular purpose. Thus, panels and other surface 

 lumber in vehicles, threshers, and other movable articles, which 

 should be no heavier than necessary to perform their function, and 

 all lumber for shipping crates and boxes, especially where these 

 must be tight and stiff", are invariably selected from the lightest 

 wood obtainable. 



Generally speaking our conifers are lighter than the hardwoods, 

 but there are light and heavy kinds in both. 



Shrinking, swelling, warping, and cheeking are the greatest 

 drawbacks to the use of wood, and are all expressions of the same 

 property of wood material, namely, its hygroscopicity, or capacity 

 to absorb or give off water and thereby change its volume. All the 

 walls of the cells grow thicker if a dry piece is moistened. This 

 Uureases the size of the cells and thereby the size of the piece. 

 The larger the single cell elements the more rapidly the water can 

 get to or from all parts, and the nearer all cells are alike in size 

 the more nearly they slirink and swell alike. 



This explains why pine or other coniferous wood shrinks and 

 swells much more evenly than hardwoods, and also why it is 

 more susceptible to moisture. It also accounts for the fact that 

 the lighter hardwoods give so much less trouble in shrinking and 

 swelling than the heavier ones. 



CHEMICAL PROPERTIES 



Since the chemical composition of the cell wall of all woods is 

 quite similar, the value of wood as fuel and in dry distillation 

 merely depends on its weight. Of the chemical properties impor- 

 tant in construction, it is chiefly durability — resistance to decay 

 when placed in the ground or otherwise unfavorably exposed — 

 and color which enter into the selection of materials, both depend- 

 ent on chemical combinations. What the substances are which 

 make the heart of cedar and white oak durable and what the 

 processes are which lead to their formation are as yet but little 

 understooti. It is certain that these bodies are present only in 



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