KILN DRYING HANDBOOK 39 
| KEY FOR DETERMINING FROM STRESS SECTIONS THE PROBABLE 
| CONDITIONS IN LUMBER DURING SEASONING 
1. When the prongs turn out on sawing.—The surface is in tension (attempting 
to shrink), and the center is in compression (opposing surface shrinkage), 
A. If the prongs turn in after room drying— 
| Indication of unequal moisture distribution, with the surface 
drier than the center. 
Occurrence: In the early stages of drying. 
Remarks: The lumber does not need steaming at this time. If a 
tendency to surface check is noticed, use a higher humidity to 
retard surface drying. 
B. If the prongs do not change after room drying— 
Indication of practically equal moisture distribution, with the sur- 
face in tension and the center in compression. 
Occurrence: After oversteaming at a low moisture content. 
Remarks: The lumber should have received less severe steaming 
treatment. 
2. When the prongs turn in on sawing—The center is in tension (attempting to 
| shrink) and the surface is in compression (opposing center shrinkage). 
A. If the prongs pinch tighter after room drying— 
Indication of unequal moisture distribution, with the surface drier 
, than the center. 
Remarks: An advantageous point to relieve stresses by steaming. 
B. If the prongs become straight or turn out after room drying— 
Indication of unequal moisture distribution, with the center drier 
than the surface. 
Occurrence: After steaming and before redrying. 
Remarks: After redrying the prongs should remain practically 
straight. 
3. When the prongs remain straight on sawing—The lumber is free from 
stresses. 
A. If the prongs remain straight after room drying— 
Indication of freedom from stresses, with equal moisture distri- 
bution. 
B. If the prongs turn in after room drying— 
Indication of unequal moisture distribution, with the surface drier 
than the center. 
Remarks: A short steaming treatment to balance the moisture 
content should relieve all stresses. 
C. If the prongs turn out after room drying— 
Indication of unequal moisture distribution, with the center drier 
than the surface. 
Occurrence; During some period of redrying after steaming. 
STRESS REMEDIES 
_ The prevention of stress troubles in a kiln charge, as far as that 
is possible, is even more desirable than remedying them. The condi- 
tion of stock entering the kiln should be carefully determined, so that 
its subsequent treatment may be suitable. 
STEAMING 
_ A high-humidity treatment of a kiln charge, which for convenience 
is ordinarily called a “steaming,” is a treatment at a humidity that 
corresponds at least to the moisture content of the surface of the lum- 
ber. The first steaming should, as a rule, be comparatively mild in or- 
der to avoid the possibility both of honeycomb during the treatment 
and of the opening up of old surface checks or the formation of new 
ones during subsequent drying, even when it is given only in order to 
warm the stock before drying commences. In fact, if the stock has 
