As lumber changes its size and shape in giving up its moisture or sap, it must not be used in any sort of fine construction until it has been sufficiently dried to make sure that it will hold its shape when completed. In order to make sure that lumber is properly dried, or cured, as it is often called, great attention must be given to this matter. After coming from the sawmill, it is usually dried in the open air for a while. To do this the lumber is piled on strips in such a way that the air can circulate freely on all sides. Green material, fresh from the log, should be left piled in this way for a considerable time. To produce an excellent quality of cabinet material it is well to have the lumber air-dried for many months. It is then taken to a dry kiln, which is merely a room prepared for the purpose of continuing the drying process. Different methods of drying are used in different kilns. Steam is sometimes turned into the dry kiln, after which the steam is turned off, and dry air is introduced and the temperature is continually raised until it reaches as much as 175 and 180 degrees, where it is left for a number of days. A number of methods have been devised for the purpose of hastening the drying process. It is not desirable, however, to dry a board too rapidly, for there is danger of the outside surface becoming dry while the inner portion of the board still contains moisture which, upon drying, will cause the board to crack. A point not generally understood by people who are inexperienced in handling lumber is the fact that a board may be properly kiln dried, and yet not remain so if it is improperly treated. If lumber is stored in a damp room it will absorb more or less moisture, and will become unfit for cabinet work. This makes it very necessary that any stock of material that you may have on hand for your manual training work should be kept in a room which is perfectly dry. A very excellent plan for the storage of cabinet lumber is to provide racks or hangers from the ceiling. This keeps it up where it will be out of the way, and at the same time free from moisture. You cannot hope to get good results, however perfect your material may be furnished, if you store it in a damp basement room. This tendency to absorb moisture from the atmsophere makes it necessary that any piece of cabinet construction should have its surface properly protected by being well finished, with filler, varnish or some other suitable protection, as explained in Chapter IV.