Other authorities believe that these defects are caused by severe frost, and their idea receives support from the fact that in timber from warmer climates this fault is less often seen. This may be so for the reason that it would not pay traders to ship inferior timber from a great distance.
Loose hearts: in the less resinous woods, as those from the White Sea, the pith or medulla gradually dries and detaches itself, becoming what is known as a loose heart. In the more resinous woods, such as the Baltic and Memel, this defect is rare.
Rind galls are caused by the imperfect lopping of the branches, and show as curved swellings under the bark.
Upsets are the result of a well-defined injury to the fibres caused by crushing during the growth. This defect is most noticeable in hard woods.
It will be of no practical use to follow the living tree further in its steady progress towards decay, but take it when in its prime, and study the processes which fit it for the purpose of scaffolding.
Felling.—The best time to fell timber, according to Tredgold, is mid-winter, as the vegetative powers of the tree are then at rest, the result being that the sapwood is harder and more durable; the fermentable matters which tend to decay having been used up in the yearly vegetation. Evelyn, in his ‘Silva,’ states that: ‘To make excellent boards and planks it is the advice of some, that you should bark your trees in a fit season, and so let them stand naked a full year before felling.’ It is questionable if this is true of all trees, but it is often done in the case of the oak. The consensus of opinion is that trees should be felled in the winter, during the months of December, January and February, or if in summer, during July. Winter felling is probably the better, as the timber, drying more slowly, seasons better.
The spruce from Norway and the northern fir are generally cut when between 70 and 100 years old. When required for poles, spruce is cut earlier, it having the advantage of being equally durable at all ages. Ash, larch and elm are cut when between 50 and 100 years old.
Conversion.—By this is meant cutting the log to form balks, planks, deals, &c. It is generally carried out before shipment. A log is the trunk (sometimes called the stem or bole) of a tree with the branches cut off.
A balk is a log squared. Masts are the straight trunks of trees with a circumference of more than 24 inches. When of less circumference they are called poles.
According to size, timbers are classed as follow:—