March 10, 1917 



HARDWOOD RECORD 



19 



weight of water dried out of the wood. JQivide-4he weight of water by 

 the dry weight of the wood and multiply by 100. The result is the 

 moisture content of the wood expressed in per cent of the dry weight. 

 In this connection, it might be well to observe that any lumber whoso 

 moisture is under six per cent jnay be considered thoroughly kUn-dried 

 while a moisture content of from ten to fifteen per cent indicates 

 thoroughly air-dried lumber. 



Test for Moistuke Distribution 

 The buyer is sometimes led to beUeve by a superficial examination 

 that lumber is thoroughly seasoned and in good condition when the 

 centers of the boards are still somewhat green. This condition is 

 brought about by subjecting the surfaces of the boards to rapid 

 drying and stopping the process before the centers are equally 

 dry. This state always indicates that the lumber is casehardened, 

 the seriousness of the condition denoting the degree of casehardening, 

 and if resawed such lumber will invariably cup either on the resaw 



u 



fin 



29 



AP(?; 



FIG. 2- 



16.2 11.2 9.0 6.6 



21-S 14.0 9.8 1.1 



13 S 10.0 1.1 6.1 



SCCTIOnS or oak SMOWIMC REVLRSAL of STRESStS uUPiNG ORTINi> 



-SECTIONS CUT FROM A CASEHARDENING BOARD AT DIF- 

 FERENT STAGES OF THE SEASONING PROCESS 



or on subsequent drying of the exposed interior. Therefore, a test 

 to determine the distribution of moisture in lumber would serve the 

 double purpose of "showing up" the drying process and indicating 

 the degree of casehardening. 



(Eefer to Fig. 1 — moisture distribution.) The method of testing 

 lumber for distribution of moisture is similar to that of the moisture 

 test already described except that the sections are resawed twice 

 as shown at 2 (Fig. 1 — moisture distribution). The two outside 

 portions (weighed together) and the center portion of each section 

 are labeled and weighed separately at once on a sensitive balance to 

 an accuracy of 1 per cent. These weights should be recorded on the 

 outside and center portions and on paper if the record is to be 

 permanent. The portions should now be completely dried on steam 

 pipes or in an oven until they cease losing weight when they should 

 be reweighed. The first weights minus the second weights equal the 

 weights of water evaporated. The weights of water times 100 and 

 divided by the weight of the dry wood gives the moisture contents of 

 the outer and inner portions of the sections in per cent of the dry 

 weights of the wood. 



Shrinkage 



Correctly spealdng, shrinkage is a property of wood and it is 

 dependent on the moisture condition, degree of casehardening, and 

 the method of seasoning. It is a fact that the moisture content of 

 most hardwoods may be reduced to about thirty, and that of the 

 conifers to about twenty-five per cent of -the dry weight of the wood 

 before shrinkage takes place. The reason for this is that no water 

 leaves the eeU walls above these valves. Drying up to this point 

 means a removal of water from the cell cavities only. As the 

 moisture content is reduced below the fiber "saturation point," how- 



ever, water is taken from the cell walls and they begin to shrink and 

 continue to do so until perfectly dry. In general the shrinkage varies 

 with the moisture content below the fiber saturation point. 



If we define casehardening in wood as a "setting" of the surface 

 in an expanded condition due to the stresses which existed in the wood 

 whUe drying, it is clear that the shrinkage of lumber varies inversely 

 with the degree of casehardening, or the more expanded the surfaces 

 are when they " set " the less will the shrinkage be. 



It is also known that wood which is dried in air at high temperature 

 and humidities shrinks more than it does when dried at lower temper- 

 atures and humidities. This effect is due to the fact that wood 

 becomes soft and plastic when hot and moist, and therefore offers 

 less resistance to shrinkage stresses. 



The amount of shrinkage and swelling, or the working of wood 

 with varying moisture content, is the determining factor affecting 

 the adaptibUity of woods for various special uses. Therefore, the 

 shrinkage of lumber is a point of vital interest to the lumberman. 



Measuring Shrinkage 

 (Eefer to Fig. 1 — shrinkage.) The method of measuring the 

 shrinkage of wood is as follows: (1) Draw a line, in pencil, across 

 the board, using a square as shown at 1, (Fig. 1 — shrinkage). 

 The edges of the board should be surfaced so as to present smooth 

 faces. Place a small hook scale, graduated in fiftieths or hundredths 

 of an inch, along the pencil mark as shown at 2, (Fig. 1 — 

 shrinkage) . Observe the width of the board on the scale to the nearest 

 fiftieth or hundredth of an inch and record the reading. If the 

 board is now placed over steam pipes or in an oven untU it ceases 



ELIMINATION OF CASE- 

 HARDENING IN KILN DRY 



RED GUM 

 1-NO FINAL STEAMING 



2&3-18 MIN 



4.S.&6-36 



7-3 MRS 



FIG. a— SECTIONS OF KILN-DRIED RED GDM CUT PROM BOARDS 

 BEFORE AND AFTER FINAL STEAMING 



to lose weight it may be remeasured in the manner described. The 

 first measurement minus the di'y measurement leaves the amount of 

 shrinkage. This difference times, 100 and divided by the original 

 measurement equals the shrinkage of the board in per cent of the 

 original measurement. This process may be continued around the 

 board and the cross-sections computed if one desires to know the 

 shrinkage in cross-section. 



Casehakdening 

 (Eefer to Fig. 1 — casehardening.) Casehardened lumber is in 

 a condition of stress. The cause of this stressed condition is a pro- 

 nounced tendency of the surface and interior of the board to shrink 

 unequally when the surface is dried faster than the interior. The 



