In pieces cut lengthways, such as fig. 15, circumstances are still less favourable; there being no perceptible contraction in the length of the fibres, the whole of the shrinking takes place laterally, at right angles to them, and the work becomes oval to the full extent of the contraction that occurs in the fibres.

The plank-wood is almost solely employed for large discs which would be too weak if cut out transversely; and in some cases for objects made of those ornamental woods which are best displayed in that section, as the tulip, rose, king, zebra, partridge, and satin woods. Specimens of oak from ancient buildings are sometimes thus worked, but in all such cases the wood should be exceedingly well dried beforehand; otherwise in addition to the inconvenience arising from the greater departure from the circle, the pieces will warp and twist, an effect that more generally concerns the joiner's art, and to the consideration of which we will now proceed.

When the green wood is cut up into planks, boards, and veneers, the splitting which occurs in the transverse section is less to be feared than distortion or warping, from the unequal contraction of the fibres. Thick planks are partially stayed from splitting and opening, by cleets nailed upon each end; boards are left unprotected, and veneers are protected from accidental violence by slips of cloth glued upon each end.

One plank only in each tree can be exactly diametrical, the others are parallel therewith, and, as shown in fig. 12, the two sides of all the boards, but that from the center, are differently circumstanced as regards the arrangement of the fibres, and contract differently It will be generally found that the boards exposed to similar conditions on both sides, become, from the simple effect of drying, convex on the side towards the center of the tree; this will be explained by a reference to the diagram, fig. 16, which shows that the longest continuous line of fibres is concentric with the axis of the tree. Thus let a, b, c, d, e,f, represent the section of a board, the line b, e, of which is supposed to contain five fibres, and the arc d, b,f, thirty: therefore supposing every fibre to shrink alike in general dimensions, the contraction on the are. will be six times that upon the short radial line, and the new margin of the board will be the dotted line which proceeds from g to h, the departure of which from the original straight line will be five times as much at d as at e.

16.

Manence Of Form And Combination Of Tbi Woods Secti 10014

This is not imaginary, as it is in all cases borne out by observation, where the pieces are exposed to similar circumstances on both sides. "When a true flat board is wanted, it is a cornmon practice to saw the wide plank in two or four pieces, to change sides with them alternately, and glue them together again, as in fig. 17, so that the pieces, 1, 3, 5, may present the sides towards the axis of the tree, and 2, 4, 6, those towards its circumference; the curvature from shrinking will then become a serpentine line consisting of six arcs, instead of one continuous circular sweep When the opposite sides of a board are exposed to unequal conditions, the moisture will swell the fibres on the one side and make, that convex, and in the opposite manner that exposed to the dry air or heat will contract and become concave; from these circumstances, when several pieces of wood are placed around the room or before the fire, "to air," the sides should be continually changed, that both may have equal treatment, so as to lessen the tendency to curvature. To remedy the defect when it may have occurred, the joiner exposes the convex side to the fire, but it is obviously better to be sparing of these sudden changes.

Fig. 17.

Manence Of Form And Combination Of Tbi Woods Secti 10015

Any unequal treatment of the two sides is almost sure to curl the board; if, for instance, we paste a sheet of paper upon one side of a board, it will in the first instance swell the surface and make it convex; as the paper dries it contracts, it forces the wood to accompany it, and the papered side becomes hollow; when two equal papers are pasted on opposite sides, this change does not generally occur. A similar effect is often observed when a veneer is glued on a piece of wood; hence it is usual to swell the surface on which the veneer is to be laid, by wetting it with a sponge dipped in thin size, so as to make it moderately round; in this case, the wetted surface of the board, and the glued surface of the veneer, are expanded nearly alike by the moisture, and in drying they also contract alike, so that under favourable management the board recovers its true flat figure.

The woods are much less disposed to become curved in the direction of their length, than crossways; but another evil equally or more untractable is now met with, as the general figure of the board is more or less disposed to twist and warp, so that when it is laid upon a flat surface it touches only at the two diagonal corners, and is said to be "in winding." This error is the less experienced in the straight-grained pines and mahogany, which are therefore selected for works in which constancy of figure is a matter of primary importance, as in models for the foundry, and objects exposed to great vicissitudes of climate.

The warping may arise from the curved direction of the fibres in respect to the length of the plank, and also from the spiral direction in which many trees grow; in some, for example, the furrows of the bark are frequently twisted as much as fifteen or twenty degrees from the perpendicular, and sometimes even thirty and forty. The woods themselves when split through the center of the tree differ materially; they sometimes present a tolerably flat surface, at others they are much in winding or twisted, a further corroboration of the "spiral growth " we cannot be therefore much surprised that the planks cut out from such woods, should in a degree pursue the paths thus early impressed upon them.

Boxwood it often very much twisted in this manner. I have a block, the diameter of which is nine inches; its surface is split at five parts, with spiral grooves, at an angle of nearly thirty degrees with the axis; these make exactly one complete revolution, or one turn of a screw in the length of the piece, which is just three feet

On the other hand, the Alerce, a pine growing in the island of Chiloe in South America, to the diameter of about four feet, and whose wood resembles the cedar of Lebanon in colour, is so remarkably straight in the grain, that it is the custom of the country to split it into planks about eight feet long and seven inches wide, which are almost as true as if they were cut with the saw, although of course not quite so smooth.

To correct the errors of winding and curvature in length, the joiner in working upon rigid pieces, first planes off the higher points so as to produce the true form by reduction. But when the objects are long and thin, they are corrected by the hands, just as we should straighten a cane, or a walking stick, except that the one angle of the board is rested upon the bench or floor the other is held in the hand, and the pressure is applied between them.

Broad thin pieces are sometimes warmed on both sides before the lire to lessen their rigidity; they are then fixed between two stout flat boards by means of several hand-screws, and allowed to remain until they are quite cold; this is just the reverse of the mode of bending timber for ship-building and other purposes, but applied in a less elaborate manner.

In concluding this division of the subject, I may observe that the shrinking and contracting of the straight-grained woods, especially deal and mahogany, cause but little distortion of their general shape after they have been properly dried; but the diversity of grain, a principal cause of beauty of figure in the ornamental woods, is at the same time a source of confusion in their shrinking, which being called on to pursue many paths, (which are parallel with the fibres, however tortuous,) gives rise to a greater disturbance from the original shape, or in extreme cases, even causes them to split when the contraction is restrained by the peculiarity of growth. In the handsome furniture woods the economy of manufacture corrects this evil from their great value they are cut into very thin slices or veneers, and glued upon a stout fabric of straight-grained wood, commonly inferior mahogany, cedar, or deal, by which the opposite characters, of beauty of appearance and permanence of form, are combined at a moderate expense; these processes will be explained.