Most of the hard woods are fine grained.
Defects in Wood. The fact that timber is not a manufactured material like iron or cement but is a natural product which has been formed by years of growth in the open where it has been all the while exposed to various ad verse conditions of wind and weather, make it peculiarly liable to defects of different kinds, most of which can not be corrected and which render much of it unsuitable for use in construction. Moreover timber is not homogeneous like iron and steel products, in other words, it can not be safely assumed that several pieces of timber, even if they are cut from the same log, will have similar characteristics or will act in nearly the same way under the same conditions. Each piece of timber must be judged by itself and must be subjected to a very careful inspection if it is to be used in an important position with satisfactory results. Such inspection will often reveal some hidden weakness or blemish which is sufficient to warrant the rejection of the piece as not good enough for the particular purpose for which it is intended, and such weaknesses or blemishes are known as defects.
Most of the defects which render timber unsuitable for building purposes are due to irregularities in the growth of the tree from which the timber has been taken. These defects are known by various names as "heartshakes," "windshakes," "starshakes," and "knots." Other defects are due to deterioration of the timber after it has been in place for some time or even before the tree has been felled, among which are "dry rot" and "wet rot." The defects of the first class are defects of structure; those of the second class are defects of the material itself. It may also be said that the defects of the first class are per manent and are definitely defined, being caused by outside forces or conditions, thus the timber affected can be cut out and discarded leaving the rest of the piece perfectly sound and good, as the defect does not influence the timber near to it and does not spread. On the other hand the defects of the second class are in the nature of a disease which spreads from one part of a piece of timber to another and can even be carried from one piece of timber to an other by contact.
Heartshake. As indicated by the name, heartshake is a defect which shows itself at the heart of the tree in the center of the trunk. The appearance of a cross section of a log affected by heartshake is shown in Fig. 4. There is first a small cavity at the center caused by decay, and flaws or cracks extend from this cavity outward toward the bark. The heart shake is most often found in those trees which are old, rather than in young, vig orous saplings; it is especially to be feared in hemlock timber.
Windshake. The defect known as a windshake is so-called on account of the belief that it is caused by the rack ing and wrenching to which the growing tree is subjected by high winds. It is also claimed that it is produced by the expansion of the sapwood which causes a sep aration of the annual rings from each other, thus leaving a hollow space in the body of the trunk and following around between two of the annual rings. Fig. 5 shows the appearance of a windshake on the cross section of a log, and this appearance has given rise to the term rupshahe which is sometimes used instead of windshake. The hollow space may extend for a considerable distance up the trunk of the tree. Windshakes are very frequently found in pine timber.
Starshake. A starshake is not readily distinguished from a heartshake, as the appearance of a log of wood affected by one is very similar to that of a log affected by the other, but the difference between the two is that while the center of a log affected by a heart shake is decayed so as to leave a large round cavity at this point, a log affected by a starshake shows no such decay at the center, but the cracks forming the star extend right across the cross section of the log, becoming wider as they approach the center and narrowing down to nothing near the bark, while all of the wood has the appear ance of being sound.