﻿28 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



itv of the surrounding atmosphere; and the changes in the moisture 

 ent of the seel ion Avill be portrayed by changes in the length 

 and curvature of the individual prongs. Loss of moisture on one 

 side of a thick section will usually be most plainly indicated by 

 a change of shape, the prong bending toward the side that has been 

 drying. If the prong be in the center and there is an equal loss of 

 moisture from both sides, the only indication will be a shrinkage 

 in the length. In thin sections this is apt to be the case anyway, 

 because their very thinness precludes much difference in moisture 

 between the two sides. Under ordinary circumstances, except after 

 special treatments, the drying of stress sections will cause a con- 

 traction or an inward turning, or both, to take place in all the 

 prongs, the amount being proportional to the amount of moisture 

 lost from each prong-. The final shape of the section, then, is a 

 criterion by which to judge the condition of the stock in the kiln 

 after the drying has been completed. Caution must be used, how- 

 ever, since the sections dry without further stress and the stock in 

 the kiln probably does not. The more nearly dry the stock is when 

 the stress section is cut, the more reliable an indicator will it be in 

 this respect. 



Now that the meaning and function of stress sections have been 

 explained, it is necessary to understand the significance of the story 

 they tell and to learn how to correct matters if they are in need of 

 correction. 



STRESS REMEDIES. 



RELIEF OF SURFACE TENSION. 



The first evidence of stress in green stock in the kiln is a tension 

 in the outer shell. This is shown in the stress section by an outward 

 turning of the prongs, and may be considered a normal condition of 

 affairs, more or less unavoidable. If this tension becomes too severe, 

 surface checks will result. As it is easier to watch for surface checks 

 than to cut stress sections, the condition of the stock in the early part 

 of the run is usually judged by the presence or absence of surface 

 checks. Excessive tension in the surface and surface checks are 

 caused by too steep a moisture gradient ; in other words, the moisture 

 is being removed from the surface more rapidly than the rate of 

 transfusion from the center to the surface. The remedy is to slow 

 down the rate of evaporation by increasing the relative humidity of 

 the air in the kiln. The effect of a definite increase in humidity will 

 be apparent from a study of stress sections cut before and after the 

 change in humidity. 



PRELIMINARY STEAMING OF AIR-DRIED STOCK. 



Air-dried or partially air-dried stock is frequently put into kilns 

 for further drying. Its condition upon entering the kiln should be 

 carefully determined so that suitable subsequent treatment may be 

 accorded. If deep surface checks are present, the fact should be 

 noted and recorded and the drying carried on with unusual care. 

 Casehardening is frequently present in air-dried stock, and the sur- 

 face is apt to be so dry that the transfusion of the moisture is badly 

 hampered. For these reasons, and also to warm the stock through 



