November 10. lOlS. 



HARDWOOD RECORD 



25 



on the resaw. It is quite generally understood that wood does not 

 commence to shrink until it has dried to the fiber saturation point 

 which corresponds to a moisture content of about 25 to 30 per cent 

 of the dry wood weight. Some species as eucalyptus globulus and 

 willow oak commence to shrink almost as soon as they begin to dry, 

 liowever. The cell walls of tlio conifers and hardwoods become satu- 

 rated when the moisture contents reach 25 and .30 per cent respec- 

 tively of the dry wood weight so that tlie moisture removed above 

 these points must be ' ' free water ' ' from tlie cell cavities. In view 

 of these facts the reason why no shrinkage can occur during the 

 period of "free water" removal is evident. As has been previously 

 stated, the tendency to caseharden invariably results from too rapid 

 surface drying. If the humidity and circulation of the air sur- 

 rounding the lumber are such that the moisture is evaporated from 

 the surface more rapidly than it naturally transfuses from the center, 

 the surface fibers dry to the fiber saturation point first and there- 

 fore, tend to shrink, unsuccessfully attempting to compress the center 

 fibers which are still 

 above the fiber satu- 

 ration point. In or- 

 der to bring com- 

 pression stresses to 

 bear on the moist in- 

 ner fibers, simulta- 

 neous tension stress- 

 es must occur in the 

 shrinking surface fi- 

 bers. If this sur- 

 face tension becomes 

 great enough and 

 the wood is not plas- 

 tic the outer cells 

 are torn apart, form- 

 i n g the surface 

 checks already men- 

 tioned. In case the 

 surface tension is 

 not relieved by sur- 

 face checking at this 

 stage of the drying 

 process, the surface 

 fibers are not per- 

 mitted to shrink nor- 

 mally, with the re- 

 sult that the ulti- 

 mate shrinkage of 

 the surface of the 

 wood is less than nor- 

 mal. Another way of 



expressing this fact is that the surface of the wood "sets" in an 

 expanded condition. As the inner fibers dry past the fiber -saturation 

 point, they in turn tend to shrink. They, therefore, tend to pull 

 away from the "set" surface fibers, exactly reversing the conditions 

 of stress in the wood. At this stage in the process of seasoning, the 

 center fibers are in tension thereby forcing the surface fibers to under- 

 go compression. This explains why so many surface checks close up 

 as the drying progresses. If the internal tension stresses become great 

 enough, the inner fibers tear themselves away from the outer fibers, 

 forming the oval-shaped interior checks already referred to as honey- 

 combing or hoUowhorning. These checks, of course, relieve both the 

 internal tension stresses and the external compression stresses to a 

 greater or less degree. With this in mind, it is quite clear that sur- 

 face checked and honeycombed wood may not necessarily be badly 

 casehardened when dry. It is equally evident, however, that case- 

 hardening was the direct cause of these defects. In many instances 

 the stresses are not entirely relieved by surface checking and honey- 

 combing, allowing a part or all of stresses to persist to the close of the 

 seasoning period. If material in this condition is resawed, it always 

 cups on the saw, the freshly cut sides being concave. No loss is expe- 

 rienced, however, if the boards are not resawed or surfaced. 



PLATE IV. SECTIONS OF WESTERN LARCH SHOWING THE EFFECT OF THE METHOD OF 

 DRYING ON THE SHRINK.\GE. FROM LEFT TO RIGHT IN EACH GROUP THE SECTIONS ARE 

 KILN-DRIED. OVEN-DRIED AND GREEN. THE ILLUSTRATION FURNISHES A PLAIN PIC- 

 TURE OF THE RESULTS ATTAINED BY THE METHODS OF HANDLING THE WOOD. 



It is a fact that if a partially dried board is resawed just after 

 the outside has begun to shrink and is in tension, the inside being 

 still above the fiber-saturation point and in compression, the halves 

 will be convex toward the saw. At first glance this would appear 

 to indicate the reverse of casehardening. If the halves are allowed 

 to continue drying, however, they rapidly straighten out flat and 

 finally cup to an excessive degree concave on the resawed surfaces. 

 Plate III illustrates this reversal of stresses during the drying. It 

 follows necessarily that the amount of cupping of resawed stock 

 depends to a marked extent on the amount and distribution of 

 moisture in the material at the time it is resawed. Casehard- 

 ening or cupping after resawing, which is due to unequal moisture 

 distribution, and that partially or altogether disappears if the stock 

 is allowed to come to normal moisture equilibrium, we shall call 

 "temporary casehardening" due to moisture conditions. The cup- 

 ping on the saw which occurs when thoroughly kUn-dried (uniform 

 moisture distribution) stock is resawed, let us term "permanent 



caseharden- 

 ing ' ' caused by a 

 difference in the con- 

 dition of stress. 



The object here is 

 1 distinguish b e- 

 tween the ' ' tempo- 

 rary" and "perma- 

 nent" forms of case- 

 hardening. 



So far we have 

 dealt only with those 

 phases of the sub- 

 ject which are more 

 or less manifest. Let 

 us turn now to a 

 discussion of the un- 

 derlying causes of 

 the various condi- 

 tions which exist in 

 casehardened wood 

 in the light of facts 

 brought forward dur- 

 ing the recent ex- 

 periments to deter- 

 mine the cause of 

 the casehardening of 

 western larch, and 

 see some of the re- 

 sults brought about 

 by the experiments 

 performed. 



Some Experiments and Their Results 

 A very simple experiment was performed to determine the effect 

 of the method of drying on the shrinkage of wood. Three adja- 

 cent sections were cut from several green western larch boards. 

 One was placed in water to retain the green dimension, the second 

 was dried slowly in the dry kiln, and finally oven dried, and the 

 third was dried very rapidly in the oven. Plates IV and V show 

 the results of this test. Without exception the sections dried slowly 

 in the warm, moist kiln air shrank much more than those dried 

 rapidly in the hot, dry oven air. The general conclusion arrived at 

 was that the faster wood is dried the less it shrinks and vice versa. 



Sections were cut from several permanently casehardened larch 

 boards. Quite severe inward cupping occurred on the resaw. When 

 these resawed sections were placed in the drying oven, the prongs 

 first curved outward quite unexpectedly to a very noticeable degree. 

 Close observation showed that this outward cupping reached a maxi- 

 mum in a very short time, after which the prongs commenced to 

 bend inward quite slowly, eventually curving inward slightly more than 

 the original amount. Similarly, if one placed these sections in water 

 the prongs curved inward at first but finally cupped outward again 

 so that the ultimate amount of cupping was slightly reduced from 



