This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Vertical, or nearly vertical cracks (as C, Fig. 128) are not objectionable, and do not weaken the timber. But horizontal cracks (as D, Fig. 129), are decidedly so, and should not be allowed.
Knots in timber are another element of weakness. They are the hearts, where branches grow out of the trunk. If they are of nearly the same color as the wood, and their rings gradually die out into it, they need not be seriously feared. If, however, they are very dark or black, they are sure to shrink and fall out in time, leaving, of course, a hole and weakness at that place. Dead knots, - that is, loose knots, - in a piece of timber, mean, as a rule, that the heart is decaying. Knots should be avoided at the centre of a beam, regularly loaded, and at the point of greatest bending moment, where the loads are irregular. The farther the knots (and cracks) are from these points the better.
Timber with "wind-shakes" should be entirely avoided, as it has no strength. These are caused by the wind shaking tall trees, loosening the rings from each other, so that when the timber is sawed, the wood is full of small, almost separate pieces or splinters at these points.
A timber with wind-shakes should be condemned as unsound.
A timber with the rings at the end showing nearly vertical (E Fig. 130) will be much stronger than one showing them nearly horizontal. (F Fig. 131.)
To tell sound timber, Lord Bacon recommended to speak through it to a friend from end to end. If the voice is distinctly heard at the other end it is sound. If the voice comes abruptly or indistinctly it is knotty, imperfect at the heart, or decayed. More recent authorities recommend listening to the ticking of a watch at the other end, or the scratching of a pin on its surface. If, in sawing across a piece it makes a clean cut, it is neither too green nor decayed. The same if the section looks bright and smells sweet. If the section is soft or splinters up badly it is decayed. If it wets the saw it is full of sap and green. If a blow on timber rings out clearly it is sound; if it sounds soft, subdued, or dull, it is very green or else decayed. The color at freshly-sawed spots should be uniform throughout; timbers of darker cross-section are generally stronger than those of lighter color (of the same kind of wood.)
The annular rings should be perfectly regular. The closer they are, the stronger the wood. Their direction should be parallel to the axis throughout the length of the timber, or it will surely twist in time, and is, besides, much weaker. Where the rings at both ends are not in the same direction the timber has either twisted in growing, or has a "wandering heart," - that is, a crooked one. Such timber should be condemned. Besides looking; at the rings at the end, a longitudinal cut near the heart will show whether it has grown regularly and straight, or whether it has twisted or wandered.
The weight of timber is important in judging its quality. If specimens of a wood are much heavier than the well-known weight of that wood, when seasoned, they may he condemned as green and full of sap. If they are much lighter than thoroughly seasoned specimens of the same wood, they are very probably decayed.
Tredgold claims that timber is "seasoned" when it has lost one-fifth of its original weight (when green); and " dry " when it has lost one-third. Some timbers, however, lose nearly one-half of their original weight in drying. Many methods are used to season or dry timber quickly.
The best method, however, is to stack the timber on dry ground (in as dry an atmosphere as possible) and in such a position that the air can circulate, as freely as possible around each piece. Sheds are built over the timber to protect it from the sun, rain, and also from severe winds as far as possible.
Timber dried slowly, in this manner, is the best. It will crack somewhat, but not so much so as hastily dried timber. Many processes are used to keep it from cracking, the most effective being to bore the timber from end to end, at the centre, where the loss of material does not weaken it much, while the hole greatly relieves the strain from shrinkage. Some authorities claim that two years' exposure is sufficient, though formerly timber was kept very much longer. But even two years is rarely granted with our modern conditions, and most of the seasoning is done after the timber is in the building. Hence its frequent decay. There are many artificial methods for drying timber, but they are expensive. The best known is to place it in a kiln and force a rapid current of heated air past it, this is known as " kiln-drying." It is very apt to badly " check " or crack the wood. To preserve timber, besides charring, the "creo-soting" process is most effective. The timber is placed in an iron chamber, from which the air is exhausted; after which creosote is forced in under a high pressure, filling, of course, all the pores which have been forced open by the suction of the departing air. Creo-soted wood, however, cannot be used in dwellings, as the least application of warmed air to it, causes a strong odor, and would render the building untenantable.
Methods of Seasoning.