SEASONING WOOD. 31 



and if left many months all pieces sink. Yet even after a year's immer- 

 sion a piece of oak 2 by 2 inches and only C inches long still contains 

 air, i. e., it has not taken up all the water it can. By rafting, or pro- 

 longed immersion, wood loses some of its weight, soluble materials 

 being leached out, but it is not impaired either as fuel or as building 

 material. Immersion and, still more, boiling and steaming reduce the 

 hygroscopicity of wood and, therefore, also the troublesome "working" 

 or shrinking and swelling. 



Exposure in dry air to a temperature of 300° F. for a short time 

 reduces, but does not destroy, the hygroscopicity and with it the tend- 

 ency to shrink and swell. A piece of red oak, which has been sub- 

 jected to a temperature of over 300° F., still swells in hot water and 

 shrinks in the kiln. 



In artificial drying, temperatures of from 158° F. to 180° F. are 

 usually employed. Pine, spruce, cypress, cedar, etc., are dried fresh 

 from the saw, allowing four days for 1-inch boards; hard woods, espe- 

 cially oak, ash, maple, birch, sycamore, etc., are air-seasoned for three to 

 six months, to allow the first shrinkage to take place more gradually, 

 and are then exposed to the above temperatures in the kiln for about 

 six to ten days for 1-inch lumber. Freshly cut poplar and cotton wood 

 are often dried directly in kilns. 



By employing lower temperatures, 100° to 120° F., green oak, ash, 

 etc., can be seasoned in dry kilns without danger to the material. 

 Steaming the lumber is commonly resorted to in order to prevent check- 

 ing and "casehardeuing," but not, as has frequently been asserted, to 

 enable the board to dry. Yard -dried lumber is not dry, and its moist- 

 ure is too unevenly distributed to insure good behavior after manufac- 

 ture. Careful piling of the lumber, both in the yard and kiln, is essen- 

 tial to good drying. Piling boards on edge or standing them on end 

 is believed to hasten drying. This is true only because in either case 

 the air can circulate more freely around them than when they are piled 

 in the ordinary way. Boards on end dry unequally; the upper half 

 dries much faster than the lower half and horizontal piling is, therefore, 

 preferable. 



Since the proportion of sap and heart wood varies with size, age, 

 species, and individual, the following figures must be regarded as mere 

 approximations: 



Pounds of water lost in drying 100 pounds of green wood in the kiln. 



Heartwood 

 or interior. 



(1) Pines, cedars, spruces, and firs 45-65 16-25 



(2) Cypress, extremely variable 50-65 18-60 



(3) Poplar, cottonwodd, basswood 60-65 40-60 



(4) Oak, beech, ash, elm, maple, birch, hickory, chestnut, walnut, and syca- 

 more 40-50 30-40 



The lighter kinds have the most water in the sapwood, thus sycamore has more than hickory. 



