23. Length of time lumber should be left in the kiln.—No one should undertake to operate a kiln unless he understands perfectly the particular make of the kiln that he is handling, for if the ventilation is not correctly regulated, the entire charge of the kiln may become mildewed, casehardened, checked, discolored, or dried unevenly. No rule can be given for the time which lumber should be left in the kiln, as it depends upon the condition of the lumber, temperature, kind of lumber, dimensions, and ventilation. Generally speaking, if the kiln is properly constructed and operated, from two to four days for each inch in thickness of soft wood, and from two to three times as long, at a lower temperature, for hard wood, is usually enough to extract the moisture. It is, however, best to allow the lumber to stay in the kiln, at a moderate temperature, from three days to two weeks after the moisture is extracted, in order to harden and cook the solids of the sap, as by so doing the lumber is not so liable to be influenced by moisture in the future; this is the effect that long weather-drying accomplishes.
24. The care of kiln-dried lumber.—It is a common mistake to allow lumber to lie in an open shed or other place where it will absorb moisture from the atmosphere, and still call it kiln-dried. Lumber of this sort should be kept in a place where heat can be applied in damp weather, and should be stacked in a close, compact pile, so as to prevent the air from coming in contact with it.
25. Steaming wood.—This process makes wood pliable, and adds to its durability by destroying the germs which may cause decay; it also neutralizes, to a great extent, the effect of the presence of sap. Steaming or immersing wood in boiling water minimizes its tendency to shrink and swell, and wood thus treated is not so apt to check in seasoning. Steamed wood loses some of its original strength on account of the effect of the high temperature upon the fibers.
26. Preserving wood.—In order to preserve wood, it is sometimes treated with creosote or other chemicals, which are forced into the wood at a sufficient pressure to cause them to permeate the wood thoroughly. This treatment enables the wood to resist better the elements and to keep away insects, which do a great deal of damage, frequently honeycombing the wood with holes, with little or no evidence of their presence upon the outside.
Suggestive Exercises
14. What are some of the results of piling lumber carelessly? Should the back and the front of the lumber pile be upon the same level? Why?
15. How should lumber ways be built? What kind of places should be avoided in seeking a location for lumber piles?
16. Should the piles be placed close to each other? How wide should the piles be made? What is the objection to a square pile? How thick should the lumber sticks be? How should they be placed? What is the result if they are not carefully placed? How should the sticks and the ends of the boards be placed at the front of the pile? Why? What causes lumber to warp? Describe methods of piling lumber to minimize warping. Should a lumber pile be allowed to stand indefinitely? What is the proper time to cut lumber? Does it injure lumber to allow a little rain to beat into the pile?
17. What is meant by weather-dried lumber? Why is it not suitable for furniture and for inside finish? How is this remedied? For what purposes is weather-dried lumber the best?
18. What is the chief objection to kiln-drying lumber?