AIR AND HOT-AIR SEASONING. 



83 



or at an angle, as this only produces unequal drying ; but planks 

 may be stacked flat or on edge. Laslett gives the following 

 table of the times required for seasoning Oak and Fir in a shed : 



Months. Months. 

 Pieces 24 ins. and upwards square, Oak about 26, Fir, 13 



Under 24 ins. to 20 ins. 



20 16 



.. 16 12 



12 8 



8 , 4 , 



22 



18 



14 



10 



6 



11 

 9 

 7 

 5 

 3 



For planks half or two thirds of these times would be requisite, 

 according to their thickness. Too prolonged seasoning will cause 

 an undue widening and deepening of the shakes that open at the 

 surface during drying. 



Kiln-drying or hot-air seasoning is a more rapid, but more 

 expensive process, suited only to boards or other small material. 

 It is a common practice to first steam the timber, which reduces 



FIG. 44." Honeycombed" board, splitting along the pith-rays. (After Roth.) 



its hygroscopicity and, therefore, its warping. This, however, is 

 said to reduce the strength, if not also the durability of the wood. 

 If not steamed, the ends of boards should be clamped before kiln- 

 drying to prevent splitting and warping. Neither hygroscopicity 

 nor shrinkage of wood can be altogether overcome by drying at 

 temperatures below 200 F. ; but as a rule only the first shrinking 

 is likely to cause splitting, so that any timber which has had 

 from three to six months air-drying may be safely kiln-dried. 

 Too rapid kiln drying, however, is apt to produce " case harden- 

 ing " in Oak and other hard woods, the drying and shrinking, 

 that is, of an outside shell followed by "honeycombing," or 

 splitting of the interior along the pith-rays (Fig. 44). Previous 



