317 



The above bulletins may be obtained free from the Secretary 

 of Agriculture, Washington, D. C. 



EFFECT OF MOISTURE ON WOOD. 



The effect of water in softening organic tissue, as in wetting 

 a piece of paper or a sponge, is well known, and so is the 

 stiffening effect of drying. The same law applies to wood. By 

 different methods of seasoning two pieces of the same stick may 

 be given very different degrees of strength. 



Wood in its green state contains moisture in the pores of the 

 cells, like honey in a comb, and also in the substance of the cell 

 walls. As seasoning begins, the moisture in the pores is first 

 evaporated. This lessens the weight of the wood, but does not 

 affect its strength. It is not until the moisture in the substance 

 of the cell walls is drawn upon that the strength of the wood 

 begins to increase. Scientifically, this point is known as the 

 "fiber-saturation point." From this condition to that of absolute 

 dryness the gain in the strength of wood is somewhat remark- 

 able. In the case of spruce the strength is multiplied four times ; 

 indeed, spruce, in small sizes, thoroughly dried in an oven, is as 

 strong, weight for weight, as steel. Even after the reabsorption 

 of moisture, when the wood is again exposed to the air the 

 strength of the sticks is still from 50 to 150 per cent, greater 

 than wdien it was green. When, in drying, the fiber-saturation 

 point is passed, the strength of wood increases as drying pro- 

 gresses, in accordance with a definite law, and this law can be 

 used to calculate from the strength of a stick at one degree of 

 moisture what its strength will be at any other degree. 



Manufacturers, engineers, and builders need to know not only 

 the strength but the weakness of the material they use, and for 

 this reason they are quite as much interested in knowing how 

 timbers are affected" by moisture as they are in knowing how they 

 are weakened by knots, checks, cross-grain, and other defects. 

 It is obvious that where timbers are certain to be weakened by 

 excessive moisture they will have to be used in larger sizes, for 

 safety. So far, engineers of timber tests, while showing that 

 small pieces gained greatly in strength, do not advise counting 

 on the same results in the seasoning of large timbers, owing to 

 the fact that the large timbers usually found in the market have 

 defects which are sure to counterbalance the gain from seasoning. 



