54 ECONOMIC WOODS OF THE UNITED STATES 



wood. A piece of green spruce timber, for example, may become 

 four times stronger when thoroughly dried.* This is an extreme 

 case, however, and does not apply to large timbers where check- 

 ing, which always occurs to some extent, may counterbalance 

 partially or- even entirely the gain in strength due to drying. 



In small forms of hardwood material, as implement and 

 carriage stock, and in coniferous timber in some forms, as cross- 

 arms for telegraph poles, thorough and xmiform reduction of the 

 moisture content produces a large increase in strength. In fact 

 a comparatively weak wood may, when perfectly dry, be much 

 stronger than a strong wood in a green condition. Consequently 

 tests to determine the mechanical properties of wood must, to 

 be comparable, take into consideration the moisture content of 

 the specimens. By means of a great many tests the relation of 

 the moisture degree to the mechanical properties can be approx- 

 imated and coefficients or correction factors determined by which 

 the strength value at any given water content can be reduced to 

 a standard (usually 12 per cent) or other desired moisture degree. f 



Loss of water from cell lumina alone does not affect the mechan- 

 ical properties of wood. It is only when the cell walls begin to 

 give up their water that increase in strength, stiffness, hardness, 

 and resilience occur. Conversely, the absorption of water weakens 

 wood only to the point where the cell walls become completely 

 saturated. This critical point has been termed by Tiemann 

 (Joe. cit.) the fibre-saturation point. It varies with different treat- 

 ments of the wood and imder different conditions. The water 

 content at this point is greater in wood previously dried and 

 especially in wood which has been subjected to high temperature 

 than it is in green wood. The amount of moisture at the fibre- 

 saturation point in green wood of various species has been found 

 by Tiemaim {loc. cit.) to be between 20 and 30 (average about 27) 

 per cent. 



The water content of wood materially affects durability. 

 Since decay is produced by fungi, and to a less extent by bacteria, 

 both of which require considerable water for their development, 



* In comparing the strength and stiffness of wood in green and dry condi- 

 tions, the fact should be borne in mind that, owing to shrinkage, dry wood is 

 more compact and contains a greater amount of wood substance per unit of 

 volume than green wood. 



t Such tables have been prepared for several of the commercial woods of 

 the United States. (See Bui. 70 and Cir. 108, U. S. Forest Service.) 



