The permanence of the form and dimensions of the woods requires particular consideration, even more than their compara-tive degrees of ornament, especially as concerns those works which consist of various parts, for unless they are combined with a due regard to the strength of the pieces in different directions, and to the manner and degree in which they are likely to be influenced by the atmosphere, the works will split or warp, and may probably be rendered entirely useless.
The piece of dried wood is materially smaller than in its first or wet state, and as it is at all times liable to re-absorb moisture from a damp atmosphere, and to give it off to a dry one, even after having been thoroughly seasoned, the alterations of size again occur, although in a less degree.
The change in the direction of the length of the fibres is in general very inconsiderable.* It is so little in those of straight grain, that a rod split out of clean fir or deal is sometimes employed as the pendulum of a clock, for which use it is only inferior to some of the compensating pendulums; whereas a piece of the same wood taken diametrically out of the center of a tree, or the crossway of the grain, forms an excellent hygronmeter, and indicates by its change of length the comparative degree of moisture of the atmosphere. The important difference in the general circumstances of the woods, in the two directions of the grain, I propose to notice, first as regards the purposes of turning, and afterwards those of joinery-work, which will render it necessary to revert to the wood in its original, or unseasoned state.
* Good box-wood and lance-wood are approved by the Tithe Commissioners as materials for the verified scales to be employed in laying down the plans for the recent Parliamentary surrey, as being next in accuracy to those of metal; whereas scales of ivory are entirely rejected by thorn, owing to their material variation in length under hygrometrical influence. See their printed papers.
Mr. Fincham says he has found a remarkable variation in the New Zealand pine the Kowrie or Cowrio, corrupted into Cowdie, which expands so much as to cause the strips constituting the inside mouldings of ships to expand and buckle, probably from the comparative moisture of our atmosphere: and Colonel Lloyd says he found the teak timbers used by him in constructing a large room in the Mauritius to have shrunk three quarters of an inch in length in thirty-eight feet, although this wood is by many considered to shrink sideways least of all others.
The turner commonly employs the transverse section of the wood, and we may suppose the annual rings then exhibited, to consist of circular rows of fibres of uniform size, each of which, for the sake of explanation, I will suppose to be the one-hundredth of an inch in diameter.
When the log of green wood is exposed to a dry atmosphere, the outer fibres contract both at the sides and ends, whereas those within, are in a measure shielded from the immediate effect of the atmosphere, and nearly retain their original dimensions. Supposing all the outside fibres to be reduced to the one hundred and tenth, or the one hundred and twentieth of an inch, as the external series can no longer fill out the original extent of the annual ring, the same as they did before they were dried; they divide, not singly, but into groups, as the unyielding center, or the incompressible mass within the arch, causes the parts of which the latter is composed to separate, and the divisions occur in preference at the natural indentations of the margin, which appear to indicate the places where the splits are likely to commence.
The ends being the most exposed to the air are the first attacked, and there the splits are principally radial with occasional diversions dicentric with the layers of fibres, as in fig. and on the side of the log the splits become gradually extended in the direction of its length. The air penetrates the cracks, and extends both cause and effect, and an exposure of a few-weeks, days, or even one day, to a hot dry atmosphere, will, sometimes, spoil the entire log, and the more rapidly the harder the wood, from its smaller penetrability to the air. This effect is in part stayed by covering the ends of the wood with grease, wax, glue or paper, to dafend them, but the best plan is to transfer the pieces very gradually from the one atmosphere to the other, to expose them equally to the air at all parts, and to avoid the influence of the sun and hot dry air.
The horizontal slice or block of the entire tree, is the most proper for the works of the lathe, as it is presented by nature the most nearly prepared to our hand, and its appearance, strength, grain, and shrinking, are the most uniform. The annual rings, if any be visible, are, as in fig. 13, nearly concentric with the object, the fibres around the circumference are alike, and the contraction occurs without causing any sensible depar-ture from the circular form. Although thin transverse slices arc necessarily weak from the inconsiderable length of the fibres of which they arc composed, (equal only in length to the thick-ness of the plate,) they are strengthened in the generality of turned works by the margin, such as we find in the rim of a snuff-box, which supports the bottom like the hoop of a drum or tambarine.
The entire circular section is therefore most appropriate for turning, next to it the quartering,fig. 14, should be chosen, but its appearance is less favourable; and a worse effect happens, as the shrinking causes a sensible departure from the circle, the contraction being invariably greater upon the circular arcs of fibres, than the radial lines or medullary rays. If such works be turned before the materials are thoroughly prepared, they will become considerably oval; so much so, that a manufacturer who is in the habit of working up large quantities of pear-tree, informs me that hollowed pieces rough turned to the circle, alter so much and so unequally in the drying, that works of three inches will sometimes shrink half an inch more on the one diameter than the other, and become quite oval; it is therefore necessary to leave them half an inch larger than the intended size. Even in woods that were comparatively dry, a small difference may in general be detected by the callipers, when they have been turned some time, from their unequal contraction.