Although these principles constitute the basis of all drying problems and must, therefore, be continually carried in mind in the consideration of them, it is equally necessary to have a comprehensive understanding of the characteristics of the materials to be dried, and its action during the drying process. All failures in the past, in the drying of timber products, can be directly attributed to either the kiln designer's neglect of these things, or his failure to carry them fully in mind in the consideration of his problems. Wood has characteristics very much different from those of other materials, and what little knowledge we have of it and its properties has been taken from the accumulated records of experience. The reason for this imperfect knowledge lies in the fact that wood is not a homogeneous material like the metals, but a complicated structure, and so variable that one stick will behave in a manner widely different from that of another, although it may have been cut from the same tree. The great variety of woods often makes the mere distinction of the kind or species of the tree most difficult. It is not uncommon to find men of long experience disagree as to the kind of tree a certain piece of lumber was cut from, and, in some cases, there is even a wide difference in the appearance and evidently the structure of timber cut from the same tree. Objects of Kiln-drying The objects of kiln-drying wood may be placed under three main headings: (1) To reduce shipping expenses; (2) to reduce the quantity necessary to maintain in stock; and (3) to reduce losses in air-drying and to properly prepare the wood for subsequent use. Item number 2 naturally follows as a consequence of either 1 or 3. The reduction in weight on account of shipping expenses is of greatest significance with the Northwestern lumbermen in the case of Douglas fir, redwood, Western red cedar, sugar pine, bull pine, and other softwoods. Very rapid methods of rough drying are possible with some of these species, and are in use. High temperatures are used, and the water is sometimes boiled off from the wood by heating above 212 degrees Fahrenheit. These high-temperature methods will not apply to the majority of hardwoods, however, nor to many of the softwoods. It must first of all be recognized that the drying of lumber is a totally different operation from the drying of a fabric or of thin material. In the latter, it is largely a matter of evaporated moisture, but wood is not only hygroscopic and attracts moisture from the air, but its physical behavior is very complex and renders the extraction of moisture a very complicated process.