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Checking of Wood in Seasoning 



It is a matter of the most common observation that wood cheeks 

 or cracks upon drying. In some cases the extent of the checking 

 is much greater than in others; for instance one piece of wood may 

 split all to pieces while another will show only very small cracks. 

 It is noted that large timbers, particularlj' those in the round, are 

 more damaged than small pieces of the same material ; that hardwoods 

 usually check more than conifers; dense woods more than soft ones. 

 It is also observed that cheeking usually begins at the ends of a 

 stick, is more pronounced in some portions than in others, though the 

 largest cracks are mostly along a radius. Again, it will be found 

 that while some crevices open wider and wider as drying proceeds, 

 others tend to close up and be- 

 come imperceptible. 



This tendency of wood to check 

 is a matter of great economic 

 concern, for vast quantities of 

 timber are annually rendered un- 

 fit for use or lowered in grade 

 on this account. It is estimated 

 that the waste due to checking is 

 ten per cent in hardwoods and 

 three per cent in conifers, making 

 the value of material wasted in 

 drying nearly $22,000,000 an- 

 nually. Wlien wood once splits 

 it never knits together again; 

 consequently the presence of 

 checks, no matter how impercep- 

 tible, always impairs the strength 

 and stiffness of wood. For a 

 great many purposes for which 

 wood is employed, strength and 

 stiffness are highly important 



considerations, and these mechanical properties depend largely upon 

 the degree to which wood is seasoned. As seasoning progresses the 

 mechanical properties are improved so that by carefully drying a 

 stick of green wood it is possible to double or in exceptional cases 

 to quadruple its strength. In the ordinary processes of seasoning, 

 however, checking, which always occurs to some extent when wood 

 is dried, may partially or even entirely counterbalance the gain in 

 strength due to drying. Consequently with large timbers in commer- 

 cial use it is not safe to count upon any greater strength, even after 

 seasoning, than that of the green condition. 



What causes wood to check? The answer is simple; — unequal or 



CHECKING 



irregular shrinkage. Why is shrinkage not uniform in all directions 

 or throughout the entire piece? To answer this requires consideration 

 of a number of different things. If wood were a homogeneous sub- 

 stance, as much so, for example, as concrete or metal, loss of water 

 would be accompanied only by a reduction in volume. But it is far 

 from being a homogeneous substance — in fact, it is a structure so 

 intricate in design that there is nothing of man's construction with 

 which to compare it. This is particularly true of the hardwoods 

 which are more Qomplex than the conifers. 



Wood is composed of cells of various kinds. Some are fibrous and 

 strong, others blunt and weak; some have thick walls, some have 



thin ; some extend up and down 

 the tree, others are disposed at 

 right angles to them; some have 

 continuous walls while others 

 have walls with many thin places 

 or pits in them to allow com- 

 munication with neighboring cells. 

 With a possible variation in 

 length, in direction, in arrange- 

 ment, and in the thickness and 

 character of the cell wall, it is 

 readUy understood why no two 

 pieces of wood or any two por- 

 tions of the same wood can be 

 exactly alike. With the differ- 

 ences in structure come differ- 

 ences in behavior which make it 

 next to impossible to formulate 

 rules which can be applied to 

 specific cases. One of the big- 

 gest problems today before the 

 wood preservers is to determine 

 in advance how timbers are going to act when placed in the retort, 

 but for the most part a tie has to be opened up before the extent 

 of penetration of the liquid can even be approximated. Much the 

 same is true in seasoning lumber, for no hard and fast rules can 

 be applied with success. One must have a knowledge of the funda- 

 meulal principles involved and vary the procedure in a given case to 

 fit the actual conditions as they arise. 



In green wood the cells are all intimately joined together and are 

 their natural or normal size when saturated with water. As long 

 as this condition k maintained the wood is practically free from in- 

 ternal strains. Water occurs in living wood in three conditions. 



.\ND SPLITTING CONTROLLED BY THE USE OF S- 

 UiO.NS. 



SPLITTING OF POLE DUE TO GREATER SHRINKAGE OF THE 

 SAPWOOD THAN OF THE HEART. 



CHARACTERISTIC CHECKING AND SPLITTING OF EUCALYPTUS 



LOGS. 



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