70 



OF WOOD IN GENERAL 



to first steam the timber, wMch. reduces 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° P. ; 

 but as a rule only the first shrinking is hkely 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-hardening " in Oak and other hard woods, the 

 drying and shrinking, that is, of an outside sheU followed by " honey- 

 combing," or sphtting of the interior along the pith-rays (Fig. 43). 

 Previous air-drying or steaming will obviate this. Various tem- 

 peratures are employed in kiln-drying ; but it is stated that at 100° 

 to 120° F., Oak, Ash, and other hardwoods can be seasoned in dry 

 kilns without any of the loss of strength often alleged to result 

 from artificial heat. Poplar planks are dried in kilns in America 

 at 158° P. to 180° P. ; but Oak, Ash, Maple, Birch, Sycamore, etc., 



Fig. 43-—" Honeycombed " tooard, splitting along the pith-rays. (After Roth ) 



are first air-seasoned for three to six months, and are then exposed 

 to these temperatures for six to ten days for 1-inch stuff. Pine, 

 Spruce, Cypress, and Cedar of the same dimensions are dried for 

 four days immediately after being felled and sawn up. Such 

 temperatures are more than sufficient to kill and prevent fungus 

 growth, and the fact that well- ventilated seasoned wood is seldom 

 attacked shows that the amount of moisture then left in the wood 

 is insufficient to support fungus growth. Walnut gun-stocks are 

 desiccated in the rough by a current of air at 90° or 100° P., passing 

 over them at such a rate as to change the whole volume of air 

 every three minutes, and it is found possible in this way to save a 

 year of seasoning. Temperatures of 250° to 300° P. are almost 

 certainly detrimental to the wood. Such desiccated timber must 

 not be exposed to damp before being used or it will re-absorb 

 moisture, and coloured woods are said to lose colour and lustre 

 under this treatment. 



Seasoning by passing the smoke-laden products of combustion 

 from the furnace through the timber pile has been found successful. 



