timbt-rf The toimnouly nocii>toJ answer is tlmt uh wood bccoinos 

 lirivr it bvcoiiu-s stroiigor niid stiffvr. Like nioHt ^■-'■"^''"'■'"■^■o"'* 

 iibout an orguniv product subject to the wliims of nature, this state- 

 ment is correct only within limits. A stick of timber may actually 

 lie weaker after dryinj; Uinii while iu n green conilition. 



The total effect of carefully drying small blocks or pieces of wood 

 is to make them sUffer and stronger. The action on tho wood sub- 

 stance is much tho same as tho loss of water from a piece of rawhide 

 or of paper. A small block of thoroughly dry clear spruce wood two 

 inches square will sustain a porniaiicut load four times as great as 

 ;i green block of the same size will support. This is an extreme 

 .•xample, but in the case of any wood in small sizes the effect of the 

 drying is to double or triple tho endwise crushing strength and the 

 !itrefs at elastic limit in bending. The modulus of elasticity or 

 measure of stiffness is only about one and one-half times as great 

 after complete drying. These ratios apply only to wood in a very 

 • Iry condition. For airdried wood tho ratios are considerably lower, 

 particularly in the case of ultimate strength and clastic limit. 



In spite of tho fact that drying can thus be shown to have a 

 strengthening effect, it is ne\'ertheleEs true that it also has a weak- 

 ening effect at the same time. In small pieces the weakening 

 effect is considerably less than the strengthening, so that the 

 algebraic sum of the two gives a positive result — an increase in 

 strength of the piece as a whole. In a large timber a negative result 

 may be obtained — the piece as a whole may be weakened. 



But how is it possible for the same drying action to strengthen 

 and weaken at the same time? To understand this it is necessary 

 to remember that wood is a structure, as much so as a bridge or any 

 other frame structure. The strength of any structure is dependent 

 not only on the strength of the material composing it but also upon 

 the manner in which it is fastened together. Imagine an intricate 

 structure buUt out of soft metal and that it is proposed to strengthen 

 it by tempering. The whole is heated and suddenly cooled. The 

 metal is now harcier than it was before, it may be several times as 

 stiff and as strong, and yet the whole structure may have been wrecked 

 by the treatment. Drying increases the stiffness and strength of 

 wood substance but at the same time it tends to pull apart and 

 break dovvTi the structure upon which the strength and stiffness of 

 the stick as a whole depends. 



If a piece of wood is stronger after drying than before, how do 

 we know that it has been weakened any by the process! This is 

 proved by the fact that when a wood is dried and then soaked it is 

 invariably weaker than it was before drying at all. Soaking green 

 or fresh wood in cold water has no effect on the strength even 

 though the soaking be prolonged. Steaming produces similar results. 

 Experiments by the United States Forest Service showed that the sub- 

 sequent immersion in water of samples that had been steamed showed 

 them weaker than unsteamed wood similarly dried and resoaked. 



When wood dries it shrinks. By the very nature of the structure of 

 wood this shrinkage cannot be uniform throughout the different parts. 

 Cells with thick walls shrink more than those with thin. Shrinkage is 

 very slight endwise, and about twice as much tangentially as ra- 

 dially. The cells were formed under conditions where there was 

 abundance of water — the walls were completely saturated until the 

 tree was cut and drying began. With loss of water from the walls 

 they began to get thinner and this caused the cells to try to pull 

 away from their neighbors. If they succeed checks and cracks are 

 formed, some large and conspicuous, but most of them so small that 

 they cannot be readily seen. Every one of these little fissures is 

 an element of weakness, and it is due to this breaking of the con- 

 tinuity of the wood substance that the structure as a whole is 

 weakened. The extent of the weakening depends upon the serious- 

 ness of the checking and cracking. 



It is evident that if drying does not take place uniformly through- 

 out an entire piece of timber, the shrinkage as a whole cannot be 

 uniform. The process of drying is from the outside inward, and if 



— se- 



ll. !■ loss i>i ini.isiiirc Iriiiii ihc ^nrl:i.-,- is iiiri |,y :i »l->:i.|y capillurT 

 current of water from tho inside the shrinkage, so far a-t tho degree 

 of moisture affected it would Im- uuifurm. In the Iwst type of dry 

 kilns this condition is npproxunatetl by heating the wood thoroughly 

 in a moist atmosphoro before allowing drying to begin. 



In air-seasoning and ordinary kiln-drying this condition is too 

 often not attained, and tho result is that a dry shell is formed 

 which encloses a moist interior. Subsequent drying out of the outer 

 portion is rendereil moro difllcult by this case-hardened condition. 

 .\s tho outer dries it is prevented from shrinking by the wet interior, 

 which is still at its greatest volume. This outer portion must cither 

 check open or the fibers become strained in tension. If this outer 

 shell dries while tho fibers are thus strained they become get in this 

 condition, and are no longer in tension. Later when the inner part 

 dries it temls to shrink away wrom the hardened outer shell, so that 

 tho inner fibers are now strained in tension and the outer in cor 

 pression. If the stress exceeds the cohesion numerous cracks open up, 

 producing a honeycombed condition or hollow-horning as it is some- 

 times called. If such a case hardened stick bo resawed the two 

 halves will cup from the internal tension and external compression, 

 with the concave surfaces inward. 



The weakening effect of such cracks and splits is especially notice- 

 able in large air-seasoned beams, where failure is often by hori- 

 zontal shear. This tendency to shear, that is for tho upper half 

 of the beam to slide on the lower part, is always present, but usually 

 is not serious in green beams or in small clear specimens, unless the 

 ratio of the height of the beam to the span is quite large. Since 

 checks are so much more likely to occur in largo than in small 

 timbers because of the difficulty of drying them as uniformly and as 

 well, it is common to have the season checks reduce the actual area, 

 reducing the shearing action considerably below the calculated area 

 used in tho formula for horizontal shear. Tho effect of a failure 

 in horizontal shear is to divide a beam into two or more beams, the 

 combined strength of which is much less than that of the original 

 beam. Consequently with largo timbers in commercial use it is 

 unsafe to count upon any greater strength after seasoning than that 

 of the green condition. 



There is another factor that enters into the drying effect. The 

 fact that a piece of wood grows smaller as it dries modifies to some 

 extent the figures given in tables for increase in strength when these 

 are applied to the same stick. Figures showing such increase are 

 calculated in terms of pounds per square inch of actual area, and 

 do not apply directly to the selfsame block of wood; for if the area 

 of the end of the block is one square inch when green this area will 

 be less than a square inch when the block is dry. Ratios of increase 

 in strength given in tables must therefore be multiplied by a shrink- 

 age factor if applied to the selfsame stick of wood. The shrink- 

 age factor for compression strength is simply the ratio of the area 

 of the dry block to its original area when green. With beams the 

 change in the resisting moments produced by the change in dimen- 

 sions must also be taken into consideration. The loss in strength due 

 to reduced dimensions of a stick varies from eight to thirteen per 

 cent when drying from a moist condition to kiln dry. 



In the foregoing discussion of the effect of drying it must be 

 understood that it is actual loss of water from the cell walls that 

 is referred to. In drying wood no increase in strength results until 

 the free water is evaporated and the cell, walls begin to dry. This 

 general statement haa some apparent exceptions. Tiemann says 

 that the wood of blue gum (Eucalyptus globulus) begins to shrink 

 immediately from the green condition, even at seventy to ninety 

 per cent moisture content, instead of from thirty to twenty-five per 

 cent, as in other species of hardwoods. Certain kinds of oak may 

 behave in somewhat the same way. 



It is important that the lumber producer and the lumber user 

 realize that proper seasoning of lumber means more than merely 

 getting out the water. Drying both strengthens and weakens. The 

 final resiilt depends on the care taken in the drying process. — R. 



