As we use four billions of lath a year, this is an important item.
The process varies with the kind of lumber and its future purpose, but a great deal is wasted in many mills. The refuse is used for fuel, and in some cases burned in stacks built specially for the purpose of getting rid of it. This is one of the forms of waste which will undoubtedly be done away with in the future, and already many lumbermen are at work on the problem. The sawdust is conveyed directly to the furnaces under the boiler and used in the generation of steam.
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LUMBER: NO. 3
Having finally reached the commercial stage, the lumber is shipped away from the mill either by water or by rail to the lumber yards of the country.
Here it should be seasoned. In the past this process consisted of piling the planks in the open air in such a way that air could circulate freely through the pile, allowing the sap to evaporate and the wood to dry evenly. This was a sure but slow process, and in the hurry of modern life quicker methods have been tried.
One of these is known as kiln drying, by which the time is reduced to a few weeks. It consists of piling the wood in a room like a kiln and drying it by artificial heat. The result is not so satisfactory as the natural method, because the sap near the surface hardens and prevents the inner moisture from escaping, so that kiln-dried lumber while dry at the surface is "green" inside. When planed till part of the surface is removed the green wood is brought near to the air again, and warping is liable to occur.
Other methods have been tried, such as steaming to vaporize the sap, and soaking in hot water for the same purpose. Of course these processes all add to the cost of lumber, yet so valuable is time that it is difficult to obtain good old-fashioned seasoned wood unless it has lain for some time in a local yard.
In order to understand the phenomena of warping, shrinkage, checking, shakes, etc., it is necessary to know something of how the tree grows. Like all living organisms, it is made up of minute cells. The new cells are formed on the outside of the tree under the bark, and here the sap is most active. The cause of the flow of sap is not very clearly understood, but it corresponds to blood in the human body, in that it carries the nourishment that forms the cells. As a new mass or layer of soft new cells forms each season, the layers may be distinctly seen and counted, but the line of separation is not a sharply drawn one, as we find by examining a cross section of wood with the microscope. However, the layers or annual rings are distinct enough to be counted, so that the age of the tree at the time it was cut down may be readily discovered.