336 



SELLING LUMBER 



Determining 

 the Factor 

 of Density 



Choosing an 

 Area for 

 Measurement 



Color in 

 Summer and 

 Spring Wood 



of material properly belonging in the grade, from the standpoint 

 of strength. 



It was found from extensive tests that the strength of wood 

 depends upon its density, or rate of growth, defects and moisture 

 content, and as the specific gravity or dry weight increases, the 

 strength also increases in a fairly uniform manner. 



The factor of density is determined by making use of the 

 varying proportions of summer wood and spring wood, or hard 

 and dark rings as compared with the light and soft rings, of the 

 different timbers. It was therefore decided to establish, as the 

 requirements necessary to comply with the rules, six rings per 

 inch, with one-third summer wood; pieces having less than six: 

 rings per inch may be accepted, provided the summer wood is 

 one-half or more. 



It would be entirely impractical, from standpoint of time, for 

 an inspector to consider a whole cross section in estimating the 

 density of summer wood in the larger size timber. It would also 

 be impossible, in a large number of cases, for him to accurately 

 estimate the summer wood in the whole portion of the piece, on 

 account of the fineness of the rings in that portion of the timber 

 lying nearest the bark. In order that this difficulty might be 

 overcome it was decided to use for estimating the summer wood, 

 the zone including the 3, 4 and 5 inches from the pith center. 

 This portion is the most easily measured or estimated and fairly 

 represents the entire cross section. Measurements were made of 

 200 trees selected by the forest service from various places in 

 the South. The proportion of summer wood in the 3, 4 and 5 

 inches average about the same as the proportion of summer wood 

 in the entire cross section, very few trees showing any marked 

 difference, so it is apparently entirely just to use the 3, 4 and 5 

 inches from the pith center as the criterion of the entire cross 

 section of the stick. 



Contrast in color of summer wood and spring wood is an- 

 other prime factor in the determination of strength. It was found 

 from numerous tests that in the heavier and stronger material 

 the contrast between the summer and spring wood increased in 

 proportion to its strength. The contrast may be classified as 

 sharp, medium and poor. Material of medium or poor contrast of 

 color between summer and spring wood should not be admitted 

 where greatest strength is required, unless the proportion of sum 



