12 BULLETIN" 556, U. S. DEPARTMENT OF AGRICULTURE. 



ordinarily have more than 12 per cent moisture and average from 

 10 to 15 per cent heavier than the listed weights. 



SHRINKAGE FROM GREEN TO OVEN DRY. 



When wood is dried below the fiber saturation point (see glossary, 

 p. 21), shrinkage begins and continues until the moisture is all driven 

 off. Shrinkage along the length of timber is very small. Shrinkage 

 in directions at right angles to the grain is very much greater and 

 varies from 2 or 3 per cent to about 20 per cent. Radial shrinkage 

 is about three-fifths as great as tangential shrinkage (see glossary, 

 pp. 22 and 23.) Shrinkage in volume is of course the resultant of 

 shrinkages along the fibers and in the radial and tangential direc- 

 tions. However, shrinkage in volume and radial and tangential 

 shrinkages were independently determined in the present series of 

 tests. The first was determined from four specimens, and each of the 

 others from one specimen from each tree. 



All shrinkages given are expressed in percentages of the original 

 or green dimensions, and are total shrinkages to zero moisture 

 Shrinkage to an air dry condition of about 12 per cent moisture is 

 sometimes more and sometimes less than half the total shrinkage. 

 At about 12 per cent moisture the volume changes by about one- 

 half of 1 per cent for each moisture content change of 1 per cent. 

 Shrinkage in volume is important in measuring cordwood. 



Radial shrinkage is the measure of the change in width of a quarter- 

 sawed or edge-grain board. In most species at about 12 per cent 

 moisture a moisture content change of 1 per cent may be expected 

 to cause a change of about three-sixteenths of 1 per cent in the 

 width of such a board. This is equivalent to three thirty-seconds of 

 an inch change in the width of a 10-inch board for a 5 per cent 

 change in moisture (5X^6 per cent of 10 inches = ■£% of an inch). 



Tangential shrinkage is the measure of the change in width of a 

 flat sawed board. At about 12 per cent moisture a moisture content 

 change of 1 per cent may be expected to cause a change of about 

 five-sixteenths of 1 per cent in the width of such a board, which is 

 equivalent to five thirty-seconds of an inch change in the width of 

 a 10-inch board for 5 per cent change in moisture. 



Both radial and tangential shrinkages are important in flooring, 

 fixtures, and any construction which is to remain well joined under 

 changing atmospheric conditions. 



STATIC BENDING. 



In the static bending test a 2 by 2 by 30 inch beam is supported 

 over a 2S-inch span. Loading is applied to its center and at a 

 constant rate of deflection until the beam fails. Readings of load 

 and deflection are taken simultaneously. 



