10 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 



of specific gravity figures given in this table is based on the weight of 

 the wood when oven dry and on its volume when green or at a 

 specified stage of drying. 



SPECIFIC GRAVITY BASED ON VOLUME WHEN GREEN. 



In the determination of the figures for specific gravity based on 

 volume when green the test specimens are weighed and measured 

 when green. Their oven-dry weight is then computed by dividing 

 the weight when green by 1 plus the proportion of moisture, moisture 

 being determined as described in previous paragraphs. The specific- 

 gravity data based on green volume are more reliable than the data 

 based on air-dry or oven-dry volume because they are based on the 

 largest number of determinations, and these determinations are 

 unaffected by the shrinkage of the wood. Specific gravity so deter- 

 mined is, aside from actual strength data, the best criterion of the 

 strength of clear wood of any species. 



It has been found that in oak, more than in any other species or 

 group of closely related species, pieces of the same density may vary 

 widely in mechanical properties. Occasional very dense pieces of oak 

 are for some unknown reason low in strength; but in all species 

 specimens of low density are invariably weak. 



SPECIFIC GRAVITY BASED ON AIR-DRY VOLUME. 



Specific gravity based on air-dry volume is obtained in the same 

 manner as that. based on volume when green, except that the volume 

 measurements are made after the material has been air dried. The 

 data in the tables are less reliable than those for the specific gravity on 

 green volume because they are based on fewer determinations and are 

 affected by variations in the shrinkage which has taken place. 



SPECIFIC GRAVITY BASED ON OVEN-DRY VOLUME. 



In determining the specific gravity based on oven-dry volume, the 

 volume as well as the weight is taken after the specimens are dried 

 to a practically constant weight in air at 100° C. The difference 

 between specific gravity based on green volume and that on oven-dry 

 volume is due to the shrinkage, and one may be determined from the 

 other if the shrinkage in volume is known. Specific gravity on oven- 

 dry volume = specific gravity based on volume when green ■+■ (1 — 

 the shrinkage.) 



The specific gravity based on volume when oven dry and the shrink- 

 age in volume (see columns 8 and 10, Table 1) determinations were 

 made on the same specimens, of which there were usually four from 

 each tree. The specific gravity based on green volume was deter- 

 mined from a much larger number of specimens and is consequently 

 somewhat more reliable. Because these two specific gravities were 



