The induced draft dry kiln requires quite an expensive equipment, as the blower and the appliance which drive it are necessary in addition to the equipment of the kiln itself, which would be similar in either of the types of kiln described. Lumber to be dried in this form of kiln must be well weather-dried before it is exposed to the high temperature of the kiln.

21. Results of the two systems. - While it is not the province of this book to pass judgment upon the results of the different methods or forms of dry kilns, it is obvious that the induced draft kiln is the more expensive to oper ate, as the expense of running the blower is avoided in the moist air system. In this latter type of kiln the steam simply passes through the pipes, the condensation being returned to the boiler to be reheated, so the only expense is that of maintaining the fire to keep up a low pressure. In the daytime, or while the engine which furnishes the power for the plant is running, the kilns of either type may be heated by exhaust steam.

Many users of one or both systems seem satisfied with the results obtained from either, while others are decided in their preference.

22. Filling a kiln. - In doing this, care should be used that there is plenty of room for the air to circulate freely around and through the pile - not less than 3' between the edges of the boards horizontally and vertically, and onefoot between the lumber and the wall or adjacent pile. Each course of boards should be so planned as to bring the same width over those of the course below, if possible, in order to keep a vertical air space through the pile. In some cases the kiln is filled by placing the boards edgeways.

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 regu lated, 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 arc 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 suit able 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?

19. What are the two methods of kiln-drying? Describe the prin ciple of the moist air kiln. What is claimed of it? How should hard wood lumber be treated before being kiln-dried?

20. Describe the induced draft system. What devices are used to extract the moisture from the heated air? What are the main points of difference between the two systems?

21. What is the difference in the condition of lumber which may be put in the two forms of kilns ? Which is the more expensive system to install and operate? How do users of the two systems compare them?

22. How should lumber be stacked in the kiln?

23. How long should lumber generally remain in the kiln to allow the moisture to be extracted? How long to insure most permanent results ?

24. How should kiln-dried lumber be cared for?

25. What is the effect of steaming wood?

26. How is wood sometimes treated to preserve it from the elements and from insects?