The Oak (Ward)/Chapter X
OAK TIMBER—ITS STRUCTURE AND TECHNOLOGICAL PECULIARITIES.
It is now time to look at the timber of the oak as a material, and to examine its technical properties from the various points of view of those who employ such material. Oak timber may be described as follows:
(1) Appearance and Structure.—Pith pentangular, 1 to 4 mm. diameter, whitish at first, and then browner, formed of small, thick-walled cells.
Sap-wood narrow and yellowish-white; heart-wood varies in shades of grayish or yellow brown (fawn color) to reddish or very dark brown. It darkens on exposure, and works to a glossy surface if healthy.
Annual rings well marked by the one to four lines of large vessels in the spring wood, whence radiate outward tongue-like and branched groups of smaller and smaller vessels, tracheids, and cells, in a groundwork of darker fibers. Indistinct peripheral lines of parenchyma are also visible, especially in the broader annual rings. The annual rings are slightly undulating, bending outward between the large medullary rays (Fig. 38).
Medullary rays of two kinds, a smaller number of very broad, shining ones, from ½ to 1 mm., or even a centimetre or more apart, and very numerous (about
Fig. 38.—Transverse section of wood of oak (magnified five diameters), showing five annual rings, as denoted by the large vessels of the spring wood; the vessels become smaller in the summer and autumn wood, and are arranged in tongue-like groups. Nine broad medullary rays are shown, the rest are very narrow (cf. Fig. 27). The rest of the section is filled with tracheids, fibers, and wood-parenchyma. (Müller.)
twelve per mm.) fine ones between them, which undulate between the vessels. In slowly-grown close wood there is no vestige of radial arrangement left.
In the tangential section the small medullary rays are seen to consist each of a vertical row of a few cells, the large ones having numerous cells (see Fig. 27).
Wood-parenchyma cells broader than small medullary rays, and the color is chiefly due to pigment in these wood- and ray-cells. The wood-cells are pitted with oblique, slit-shaped, simple pits.
The vessels have bordered pits, and the septa are perforated each by one large circular opening. The smaller vessels have delicate spirals on their walls as well as bordered pits.
Nördlinger says that pith-flecks occur occasionally.
It is impossible to distinguish between the wood of the varieties pedunculata and sessiliflora.
(2) Its density varies considerably. Taking the weight of a given volume of water as unity, the weight of an equal volume of oak timber may weigh from 0·633 when air-dry to 1·280 when fresh cut. We may take the average density of green—i.e., newly-felled—oak with all its sap present, as about 1·075, and that of the seasoned wood as about 0·78.
It must be borne in mind, however, that these weights refer to the wood as a structure—that is, a complex of vessels and cells, etc., containing air and liquids—and do not give the specific gravity of the wood substance itself. The latter may be obtained by driving off all the air and water from the wood, and is found to be 1·56, compared with an equal volume of water taken as unity. It is the varying quantities of this wood substance, and of air and water in the cavities, which make the density of different pieces of oak vary so much.
(3) The proportion of sap contained in the cavities of the vessels, cells, etc., of course differs at different times. In the spring, just as the buds are opening, the quantity of water increases more and more up to about July, when the maximum is attained; the proportion of water to solids then sinks until October, when the leaves fall; it increases again up to Christmas-tide, and then sinks to the minimum in the coldest part of the winter. The proportion of water to the total weight of the felled wood may vary from 23 to 39 per cent.
(4) Obviously the loss of water on drying causes shrinkage of the wood, and although oak shrinks very little in the direction of its length (0·028 to 0·435 per cent), the effect is very marked in