30 TIMBER. 



The rapidity with which water is evaporated, that is, the rate of 

 drying, depends on the size and shape of the piece and on the struc- 

 ture of the wood. An inch board dries more than four times as fast 

 as a 4-inch plank and more than twenty times as fast as a 10 inch 

 timber. White pine dries faster than oak. A very moist piece of pine 

 or oak will, during one hour, lose more than four times as much water 

 per square inch from the cross section, but only one-half as much from 

 the tangential, as from the radial section. 



In a long timber, where the end or cross sections form but a small 

 part of the drying surface, this difference is not so evident. Never- 

 theless, the ends dry and shrink first, and being opposed in this shrink- 

 ing by the more moist adjoining parts, they check, the cracks largely 

 disappearing as seasoning progresses. 



High temperatures are very effective in evaporating the water from 

 wood, no matter how humid the air. A fresh piece of sapwood may 

 lose weight in boding water, and can be dried to quite an extent in 

 hot steam. 



Kept on a shelf in an ordinary dwelling wood still retains 8 to 10 

 per cent of its weight of water, and always contains more water per 

 pound than the surrounding air. Nor is this amount of water constant; 

 the weight of a pan full of shavings varies with the time of day, being 

 on a summer day greatest in the morning and least in the afternoon. 



Desiccating the air with chemicals will cause the wood to dry, but 

 wood thus dried at 80° F. will still lose water in the kiln. Wood dried 

 at 120° F. loses water still if dried at 200° F., and this again will lose 

 more water if the temperature is raised. So that absolutely dry wood 

 can not be obtained, and chemical destruction sets in before all the 

 water is driven off. 



On removal from the kiln the wood at once takes up water from the 

 air, even in the driest weather. At first the absorption is quite rapid; 

 at the end of a week a short piece of pine, li inches thick, has regained 

 two-thirds of, and, in a few months, all the moisture which it had when 

 air dry, 8 to 10 per cent, and also its former dimensions. 



In thin boards all parts soon attain the same degree of dryness; in 

 heavy timbers the interior remains moister for many months, and even 

 years, than the exterior parts. Finally an equilibrium is reached, and 

 then only the outer parts change with the weather. 



With kiln-dried wood all parts are equally dry, and when exposed 

 the moisture coming from the air must pass in through the outer parts, 

 and thus the order is reversed. Ordinary timber requires months 

 before it is at its best; kiln-dry timber, if properly handled, is prime 

 at once. 



Dry wood, when soaked in water, soon regains its original volume, 

 and in the heartwood portion it may even surpass it; that is to say, 

 swell to a larger dimension than it had when green. With the soaking 

 it continues to increase in weight, the cell cavities filling with water, 



