354 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



proportion to the loss of water. When the moisture per cent falls below about HO the water comes from the cell wail, 

 ami the loss of water and weight is accompanied liy a loss of volume, so that both factors of the fraction 



Specific gravity = . ht 

 volume 



are afi'eetcd and the change in the specilic gravity no longer is simply proportional to the loss of water or weight. 

 The loss of weight and volume, however, being unequal and disproportionate, a marked reduction of the specific 

 gravity takes place, amounting in these pines to about 8 to 10 per cent of the specilic weight of the dry wood. 



SHKINKACK. 



ft 



The behavior of the wood of the southern pines in shrinkage docs not dilier materially. Generally the heavier 

 wood shrinks the most, and sap wood shrinks about one-fourth more than henrtwood of the same specilic weight. 

 Very resinous pieces ("light wood'') shrink much less than other wood. In keeping with these general facts, the 

 shrinkage of the wood of the upper logs is usually 15 to 20 per cent less than that of the butt pieces, and the 

 shrinkage of the heavy heartwood of old trees is greater than that of the lighter peripheral parts of the same, while 

 the shrinkage of the heavy wood of saplings is greatest of all. <>n the whole, the wood of these pines shrinks 

 about 10 per cent in its volume. 3 to 4 per cent along the radius, and 6 to 7 per rent along the tangent or along the 

 yearly rings. 



After leaving the kiln the wood at once begins to absorb moisture and lo swell. In an experiment with short 

 pieces of loblolly and shortleaf, representing ordinary flooring or Hiding si/.cs, these regained more than half the 

 water and underwent more than half the total swelling during the first 10 days after leaving the kiln (see tig. !M). 

 Even in this less than air-dry wood the changes in weight far excel the changes in volume (sum of radial and 

 tangential swelling), and therefore the specilic gravity, even at this low per cent of moisture, was decreased l>y 

 drying and increased by subsequent absorption of moisture. Immersion and, still more readily, boiling, cause the 

 wood to return to its original size, but temperatures even above the boiling point do not prevent the wood from 

 " working,'' or shrinking, and swelling. 



/O 20 7f 334. 



FIG. 94. Loss of water in kiln drying and reabworption in uir, shrinking, mid swelling. 



Iii tig. 94 are represented the results of experiments on the rate of loss of water in the dry kiln and the reali- 

 sorption of water in the air. The wood used was of loblolly and Khortleaf pine kept on a shelf in an ordinary room 

 before and after kiln-drying. The measurements were made with caliper. 



KKKKCT OK KILN-DRYING. 



Although kiln-drying h;is become quite universal, opinions are still divided as to its effects upon the strength 

 of the material and other qualities. Many objections and claims as to physical and chemical changes produced by 

 the treatment remain unsubstantiated. The method most widely used and most severely criticised is that of the 

 "blower" kiln, where hot air (180 F.) is forced into the drying room by means of powerful fans. Besides the 



