354 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



proportiou to the loss of water. When the moisture per cent falls below about 30 the water comes from the cell wall, 

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



Specific gravity =■ 



weight 

 volume 



are affected and the change in the speciiic 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 speciiic 

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



SHRINKAGE. 



The behavior of the wood of the southern pines in shrinkage does not differ materially. Generally the heavier 

 wood shrinks the most, and sapwood shrinks about one-fourth more than heartwood of the same specific weight. 

 Very resinous pioces ("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 13 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. On 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 cent along the tangent or along the 



yearly Tings. 



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

 pieces of loblolly and shortleaf, representing ordinary Jlooring or siding sizes, 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 fig. 94). 

 Even in this less than air-dry wood the changes in weight far e\:cel the changes in volume (sum of radial and 

 tangential swelling), and therefore the specific gravity, even at this low per cent of moisture, was decreased by 

 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/ 7 or shrinking, and swelling. 



Fig. 94.— Loss of ■water in kiln drying and leabboiption in air, shi inking, and swelling. 



In fig. 94 are represented the results of experiments on the rate of loss of water in the dry kiln and the reab- 

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

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



EFFECT OF KILN-DRYING. 



Although kiln-drying has 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 



