AIR AND HOT-AIR SEASONmG 69 



however, the same shrinkage causes the centre plank to contract 

 in thickness at its edge, whilst planks cut from the outside will 

 shrink in breadth, their edges curving away from the centre of the 

 tree. 



The many methods of wood-preservation may be classified as 

 seasoning methods, either ^' natural " — i,e., slow or accelerated — 

 surface carbonization, or impregnation methods. Of these it is 

 generally believed that natural or air seasoning gives the best 

 results. Firewood should be dried rapidly ; but in other cases slow 

 drying in cool air and in the shade — a process difficult to effect in 

 the tropics — ^is most desirable in order to reduce the amount of 

 cracking. The timber should be squared as soon as cut, and even 

 halved or quartered, for the rate of drying depends largely on the 

 shape and size of the piece, an inch board drying more than four 

 times as fast as a 4-inch plank, and more than twenty times as fast 

 as a 10-inch timber. The wood is then piled in the seasoning yard 

 so as to be protected as far as possible from the sun and rain, but 

 with air circulating freely on all sides of each log. Bad ventfla- 

 tion is sure to cause rot ; but at the same time exposure to high 

 wind is Hkely to cause unequal drying, and is, therefore, to be 

 avoided. One of the most fertile causes of decay is the leaving of 

 logs to sink into soft ground where they are felled, often in the 

 immediate neighbourhood of rotting stumps or dead twigs, the 

 most fertUe source of infection by fungus - spores that can be 

 imagined. Timber should therefore be stacked, or at least skidded 

 a foot off the ground, as soon as possible and protected by a roof. 

 Experience is against the stacking of timber vertically 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 

 Under 24 ms. to 20 ms. 



20 „ 



16 



16 „ 



12 



12 „ 



8 



8 „ 



4 



26 



Fir, 13 



22 



„ 11 



18 



„ 9 



14 



„ 7 



10 



„ 5 



6 



,. 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. 



The chief methods of accelerated seasoning are kiln drying, or 

 hot-air seasoning, and steam-drying processes. Of these the 

 former is a rapid but expensive method. It is a common practice 



