HARDWOOD RECORD 



the wood-user's failure to appreciate the need for such rules. 

 With decrease in the Supply of virgin timber, the quality of timber 

 on the market steadily decreases, making revision of grading rules 

 necessary. The time seems sure to come when timber will be 

 i;raded not on defects alone but according to its strength, or with 

 s|ii'cial reference to the use to which it is to be put. 



What are the factors upon which such rules could be based? If 

 strength is the feature most desired, it will be necessary to grade 

 mateiials with this end in view. How can the comparative 

 strength of woods be told by inspection? The answer is found in 

 the exhaustive tests that have been and still are being made on 

 woods of various kinds. Take two pieces of wood of tlie same 

 species and of equal weight, dryness and freedom from defects, 

 :iiid the stronger of the two is the one that contains the most wood 

 substance. Cottonwood is light because it contains so much air 

 space; hickory is heavy because it contains little air and much 

 wood substance. If a wood were solid, it would sink in water like 

 a stone. 



If one piece of hickory, say, is stronger than another piece of 

 hickory because it contains more wood substance, and the more 

 wood substance it contains the heavier it will be, then the com- 

 parative strength of the two pieces can be inferred from their 

 respective weights. Within the same species, the heavier a wood 

 is the stronger it is. This, of course, can apply only when woods 

 are equally sound and equally dry. Green wood is heavier than 

 seasoned wood, but it is also weaker, unless as a result of improper 

 sca.soning the occurrence of checks and splits offsets the gain in 

 strength due to dryness. So often is the latter the ease that 

 strength values for structural timbers are usually based on green 

 material. 



If kiln-dried or thoroughly airscasoued material of the same 

 kind is being graded, it is impossible to judge the relative strength 

 by the weight. With many different sized pieces this would be 

 more diflScult; with green wood it would be practically impossible. 

 Increase in the amount of wood substance means increased density 

 .ind this can, in a great many cases, be judged by the eye. This 

 i~ particularly the case in pines and other conifers. If you 



• xamine a piece of southern pine, for example, you will note that 

 it is composed of alternate layers of light and dark portions. Try 

 them with your knife and see how much easier the light eoloreil 

 ]portion cuts than the other. Try indenting them with your thumb 

 nail and note how much harder the dark colored band is. It 

 follows that the greater the proportion of this dark colored or 

 ^nmmerwood a piece contains, the heavier, harder, and stronger 

 It will be, and rice versa. Hence in choosing a coniferous wood 

 for strength jdck material that shows to the eye to be the densest. 

 Wood that contains one-third summerwood may be classed as 

 dense wood. 



Another way to determine the density and therefore the strength 



• if material is by its resilience and resonance. Stril<e a piece of 

 ■.vood a blow with a hammer or similar blunt tool. If the hammer 

 shows a marked tendency to rebound without indenting the wood 

 it indicates resilience, which is a criterion of strength. Dense 

 wooils give out a clear, sharji tone when struck, thus affording 

 still another means of judging quality. 



Some grading rules go farther than others and attcnijit to 

 classify woods according to the number of rings per inch as seen 

 on Cross section — in other words, upon the rate of growth. There 

 is no question but that rate of growth does affect the properties 

 of a wood, but it is not alwaj-s possible to state just what the 

 effect will be. It is quite frequently the case that in conifers 

 slow growth produces harder and stronger wood than rapid 

 -rowth. < 'onse(|uently wood showing not less than eight rings to 

 the inch is considered strongest. Such a rule is included in most 

 paving block specifications. In the case of wood cut from the 

 outer portions of large overmature trees the rate of growth is 

 slow, but the materia! is soft and comparatively weak. .Sapwood 

 tioni large western yellow pine trees resembles white pine and is 

 .sometimes marketed as such, Jt is evident, then, that iu conifers 

 ring width is by no means a criterion of thr quality of the wood. 



With equal proportions of deuse summerwood, however, narrow- 

 ringed woods are usually stronger than wide-ringed, and are more 

 even-textured. 



In the case of hardwoods the same rule holds that the greater 

 the pro[iortion of summerwood, the harder, heavier, and stronger 

 the material will be. It is also true that wood from the outside of 

 large, overmature trees is lighter, softer and weaker than that 

 from more thriftily growing specimens. Hardwoods may be 

 divided into two groups — one in which the large pores are col- 

 lected in well-defined rings or bands — the ring-porous woods like 

 oak, ash, (hestnut and hickory; the other in which the pores are 

 of much the same size and scattered rather uniformly throughout 

 the annual ring — the diffuse-porous woods of which poplar, gum, 

 maple and birch are common examples. In the diffuse-porous woods 

 there appears to be no definite relation whatever between the 

 width of ring ami the strength of the material, though a medium 

 rate of growth is usually stronger than very fast or very slow 

 growth. Here density of material is best evidenced by the appear- 

 ance, by the weight, and by the response to a blow of a hammer. 



In the case of ring-porous woods wide rings are closely associated 

 with strength. Hence, if one is choosing oak or ash or hickory for 

 such exacting purposes as handles, spokes, tongues, shafts, and 

 such portions of furniture as withstand hard usage, he should 

 choose wood with wide rings. If, however, one wishes to use such 

 woods where ease of working, lightness, and reduced shrinking 

 and swelling are more important than strength and weight he 

 should select material showing as many rings to the inch as he can 

 find. Slowly grown, ring-porous woods contain much less hard- 

 wood fiber than rapidly grown woods of the same group, and 

 consequently have greater strength and hardness, but are also more 

 likely to cheek in seasoning and are more difficult to work. This 

 explains why "second growth" ring-porous woods are often pre- 

 ferred to forest grown material, as is the case in hickory for 

 handles. 



In the hickory trade "second growth" has come to mean the 

 same as sapwood, the grades being based largely on the color of 

 the wood. It is quite easy to see how this condition may have 

 come about. The term "second growth" is usually applied to 

 young timber growing iu rather open stands, therefore growing 

 rapidly. Such trees contain a very large proportion of sapwood, 

 in some eases may be all sap. The wood being harder, stronger 

 and tougher than that from old trees growing slowly in the forest, 

 a natural preference arose for the second growth timber, and as it 

 is white in color, the desirable properties came to be associated 

 with the color and not with the ring width and density of the 

 wood, as it should. 



Other things being the same, sapwood and heartwood are equal 

 in strength and toughness. Very often, however, other things are 

 not equal. Heartwood, being at the center of the tree, is more 

 subject to defects such as knots, shake, worm holes and decay, 

 which may materially weaken it. On the other hand the sapwood 

 occupies the outer layers of the tree and in large specimens is of 

 slow growth and, for reasons explained above, often lighter, softer, 

 and weaker than the heart. It is evident, then, that strength is 

 not depenilent upon whether a wood is heart or sap, but upon the 

 structure and condition of the wood without reference to that 

 question. 



There are various ways, however, in which heart and sap differ. 

 One is durability or resistance to decay. The sapwood of no 

 species is durable in contact with the ground, the same being 

 just as true of cedar or cypress as of any other wood. Neither 

 is all heartwood durable, though in any case it is usually n-.ore so 

 than the sajnvoo<l. In selecting woods in which natural durability 

 is the essential <'onsideration one should discriminate against 

 sapwood and in favor of heart, and especially in favor of dark 

 colored heartwood, if wood is to be impregnated with preserva- 

 tives, sai)woo<l will be found to afford much easier penetrance than 

 heart in all cases where a distinct heart is present. 



Not only is sa|>vvood more likely to decay and stain, but is also, 

 <'spe<-ially in the nNinufai-tnrcil form, more liable tii insect injury. 



