26 



HARDWOOD RECORD 



namely, in the cell walls, in tlie protoijlasmie contents of the cells, 

 and as free water in the cell cavities and spaces. It is only the water 

 within the cell walls that counts, for as long as the amount there 

 remains undisturbed the size and shape of the cells will remain the 

 same. The cell walls may be considered as made up of little particles 

 with water between them. When wood is dried the films of water be- 

 tween the particles become thinner and thinner until almost entirely 

 gone. As a result the cell walls grow thinner with loss of moisture — 

 in other wonls the cell shrinks. 



It is at nnce evident that if drying does not take jilace uniformly 

 thi'oughout an entire piece of timber the shrinkage as a whole can- 

 not be uniform. The jirocess of diying is from the outside inward. 

 Now if it can be brought -about that the loss of moisture at the sur- 

 face is met by a steady capillary current of water from the inside, 

 the shrinkage, so far as the degree of moisture affected, would be 

 uniform. In the best type of dry kilns this condition is approxi- 

 mated by first heating the wood thoroughly in a moist atmosphere be- 

 fore allowing drying to begin. In air-seasoning and in ordinary dry 

 kilns this condition too often is not attained and the result is that 

 a dry shell is formed which incloses a moist interior. Subsequent 

 drying out of the inner portion is rendered more difficult by this 

 case-hardened condition. If the outer dries and shrinks and the 

 inner does not, the stretching effect produced is sure to cause cheek- 

 ing at the surface. Later when the inner part dries it tends to 

 shrink away from the outer shell, and numerous cracks open up, 

 often producing a "honeycombed" condition. 



For a giveu surface area the loss of water from wood is always 

 greater from the ends than from the sides. This, of course, is due 

 to the fact that the vessels and other water carriers are cut across, 

 allowing ready entrance of drying air and outlet for the water vapor. , 

 Water does not flow out of boards and timbers of its own accord, 

 but must be evaporated, though it may be forced out of very sappy 

 specimens by heat. In drying a log or polo with the bark on, most 

 of the water must be evaporated through the ends; but in the case 

 of peeled timbers and sawn boards, the loss is greatest from the 

 auiface because the area exposed is so much greater. 



The more rapid drying of the ends causes a shrinkage and if the 

 material were sufficiently plastic the ends would become bluntly 

 tapering. The rigidity of the wood substance prevents this and the 

 wood pulls apart. Later, as the rest of the stick dries out, many 

 of the checks will come together. Some of the largest, however, will 

 remain and even increase in size as the drying ]iroceeds. These are 

 caused not by unequal drying, but by unequal shrinkage due to 

 structural peculiarities which will be explained shortly. 



When cells composing wood dry out, their walls become thinner. 

 The thicker the cell wall the greater will be the shrinkage upon 

 drying. Since in all our woods cells with thick walls and cells with 

 thin walls are more or less intermixed, and especially as the wood 

 first formed in a season (springwood) nearly .always differs from 

 that formed in summer and fall (summerwood) in this respect, 

 strains occur with loss of water, because the summerwood, being of 

 thicker-walled cells, shrinks more than springwood. Heavier wood 

 in general shrinks more than light wood of the same kind. It will 

 not do, of course, to compare the shrinkage of one kind of wood 

 with that of a different kind on a basis of weight or density, since 

 extra weight in some cases maj- be due to infiltrated substances such 

 as resins, gums, and pigments which tend to reduce shrinkage. Sap- 

 wood as a rule shrinks more than heartwood of the same tree, due 

 to its greater moisture content and the lack of infiltrated substances; 

 on this account sapwood is more liable to check than heartwood. This 

 tendency, however, may be offset by differences in density, since light, 

 porous sapwood would probably shrink less than liard, heavy heart- 

 wood. Moreover, there are often present in the hearts of trees va- 

 rious forms of shake or splits, such as star shake, cup shake, etc., 

 ■which will open up on subsequent drying. Such cracks can be dis- 

 tinguished from season checks by the slight discoloration of the sur- 

 faces, much as an old flaw in iron can be told from a fresh break. 



Everjthing else being equal, the denser a wood is t%e more it 

 will shrink, and therefore the greater its tendency to check. In 

 ring porous woods, such as oak, ash, hickory, chestnut, black locust 



and others, the more rapid the growth, within certain limits, the 

 denser the wood. Hence woods from second-growth trees of this 

 class are more difficult to season without injury than slowly grown 

 materia! from virgin forests. The oak best suited for ties, because 

 of its strength and wear-resistance, is hard and heavy, and on that 

 account is rendered the more diflScult to dry without splitting to 

 pieces. In the case of conifers and of diffuse-porous woods, such 

 as beech, birch, maple, poplar and others, the relation of density 

 and therefore of checking to rate of growth cannot be definitely 

 stated, but here also the denser the wood, as indicated by the 

 greater proportion of darker summerwood, the more it will shrink 

 and the greater its liability to check. 



It is a curious fact that a cell shrinks very little lengthwise. A 

 dry wood cell is therefore practically of the same length as it was 

 in a green or saturated condition, but is smaller in cross section, 

 has thinner walls, and a larger cavity. It is at once apparent that 

 this fact makes shrinkage more irregular, for wherever cells cross each 

 other at a decided angle they will tend to pull apart upon drying. 

 This occurs where\er pith rays and wood fibers meet. A considerable 

 portion of every wood is made up of these rays which for the most 

 p;irt liave their cells lying in a radial direction instead of longitu- 

 dinally. In pine over 1,'5,000 of these occur on a square inch of a 

 tangential section, and even in oak the very large rays which are 

 readily visible to the eye as flakes on quarter-sawed material, repre- 

 sent scarcely one per cent of the number which the microscope re- 

 veals. 



A ))ith ray shrinks in height and width, that is, vertically and tan- 

 geutially as applied to the position in a standing tree, but very 

 little in length or radially. The other elements of the wood shrink 

 radially and tangentially but almost none lengthwise or verticall.v, 

 as applied to the tree. Here then, we find the shrinkage of the 

 rays tending to shorten a stick of wood while the other cells resist 

 it, and the tendency of a stick to get smaller in circumference is 

 resisted by the endwise reaction or thrust of the rays. Only in a 

 tangential direction or around the stick in direction of the annual 

 rings of growth do the two forces coincide. Another factor to the 

 same end is that the denser bands of summerwood are continuous 

 in a tangential direction while radially they are separated by alter- 

 nate zones of less dense springwood. Consequently the shrinkage 

 along the rings (tangential) is fully twice as much as toward the 

 center (radial). This explains why some cracks open more and more 

 as drying advances. 



Although actiml shrinkage in length is small, nevertheless the ten- 

 dency of the rays to shorten a stick produces strains which are re- 

 sponsible for some of the splitting open of tics, jiosts, and sawed 

 timbers with box heart. At the very center of a tree the wood is 

 light and weak, while farther out it becomes denser and stronger. 

 Longitudinal shrinkage is accordingly least at the center and greater 

 toward the outside, tending to become greatest in the sapwood for 

 the same reasons that shrinkage in other directions is greatest there. 

 When a round or a box heart timber dries fast it splits radially and 

 as drying continues, the cleft widens, partly on account of the greater 

 tangential shrinkage and also because the greater contraction of 

 the outer fibers warps the sections apart. If one will take a small 

 hardwood stem and while green split it for a short distance at the 

 end and place it where it can dry out rapidly he will find that the 

 sections have become bow-shaped with the concave sides out. These 

 various facts taken together explain why, for example, an oak tie, 

 pole or log may split open its entire length if drying proceeds rapidly 

 and far enough. 



If wood were perfectly rigid it would be absolutely impossible 

 to dry it without having it break into minute fragments. As it is 

 slightly plastic the cells can adjust or mold themselves to new con- 

 ditions if the change is not too rapid. When drying is hastened 

 not only is the loss of moisture uneven but the wood substance is not 

 given a chance to reshape itself. Summer-felled timber usually 

 checks worse than winter-felled because it dies faster, due to the 

 warmer weather. 



When one understands the causes underlying the checking of 

 wood he is. then in position to take the necessary precaution to 



