Defects in Timber.
Fig. 1.—Heart shake.
Fig. 2.—Star shake.
Fig. 3.—Cup shake.
Perhaps the most common and prolific defect encountered with the use of timber is the presence of cracks or shakes of different character, which are due to different causes. [Fig. 1] indicates a very common form, known as a “heart shake,” dividing the timber at the centre; while [Fig. 2], a “star shake,” is really a number of heart shakes diverging from the centre. The process of seasoning sometimes results in the separation of the annual rings, forming cup shakes, as shown in [Fig. 3]. It should be understood that the presence of shakes may render useless an otherwise perfect specimen of timber, as it frequently happens that in the conversion of timber so affected the usable portions do not permit of the sizes necessary for such items as wing spars and struts. The
Fig. 4.—Twisted grain.
defect of twisted grain ([Fig. 4]) is often found in ash, and is caused by the action of the wind when the tree is growing, and renders such wood of limited utility. Shrinkage affects all timber in varying degrees, and its effect on boards due to their position in the log is shown by [Fig. 5], while [Fig. 6] indicates the effect of drying on a squared-up section. Incidentally one may point out that the annual rings, viewed from the end of the section, should be as straight as possible, which would obviate to an extent the distortion due to drying in a component subsequent to its finishing. Another defect, and one somewhat difficult to detect, is the presence of a brownish speckled tint in the grain. Any evidence of this in a specimen indicates the beginning of decay, and is caused by insufficient seasoning and lengthy exposure in a stagnant situation.
Fig. 5.—Shrinkage of boards
due to position in log.
Fig. 6.—Effect of drying
on a squared-up section.