The characteristics of good and the defects of bad timber may be dealt with simultaneously. Good timber should be cut from the heart wood of a well-matured tree, free from sap (of either white or blue colour), as well as from large loose or dead knots, shakes (whether they be cup, heart, or star shakes), rind-galls, and unevenness of colour. It should be regular, straight, and close in the grain, and when planed its shavings should be long and stringy; and it should shine when worked, smell sweet, and, when struck, give forth a clear sound; the fibres should be so straight and continuous that, when converted, very few of them would be cut through. The heart itself of a tree is much given to "shakes."
Wood is a conductor of sound, and the soundness of a log is often ascertained by the way in which it transmits a "tap" from one end to the other. Failure as a sound conductor, or the transmission of a dull sound, indicates decay.
Sap is the most common defect in the timber of to-day, and is the result of the converter's desire to get as much bulk of timber as possible out of his tree, and being thus obliged to cut outside the circle of the hard or heart wood into the sap. Sap is, therefore, found at the sides or edges of the wood. Sap can either be of a blue or white colour; the one is as bad in the end as the other. They both lead to decay, and in varnished work cause a dark stain. There is no cure for it, and it must be cut away.
Knots, of whatsoever kind, whether hard, loose, or dead, are a source of weakness, as they break the continuity of the fibres; but they are not so serious a defect when hard and sound, except in the centre of a beam, or where great tensile strength is required. The presence of a large quantity of knots is always a serious defect in timber.
Cup shakes are those which encircle the pith and separate the annual rings in a curved line, and, unless they are long and separate a large portion, they are not dangerous, vide fig. 261.
Heart shakes, the most common of shakes, occur at the centre of the tree, and radiate towards the bark, as fig. 263. These, of course, weaken the wood, and render it impossible to convert the tree into large-sized stuff.
Star shakes are similar to the last-named, but more numerous from the centre of the tree.
Rind galls are the result of the growth of the tree over a place where a branch has been improperly cut off. They usually take the form of irregular swellings.
This defect includes both "foxiness " and "doati-ness," the former being a red or yellow stain, which betokens decay, and the latter a speckled stain foreboding the same thing, which is met with in American oak and beech.