The Oak (Ward)/Chapter X
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Oak Timber―Its Structure and Technological Peculiarities
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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 other directions. In the radial direction—i.e., in the direction of the medullary rays—it may shrink from 1 to 7·5 per cent of its measurement when first felled; and in the direction vertical to this—i.e., parallel to a tangent to the cylindrical stem—the variation is from 0·8 to 10·6 per cent. Of course, green oak shrinks much more than seasoned and older wood, the process of seasoning being, in point of fact, the period of chief shrinkage. It is said that wood from the variety sessiliflora shrinks more than that of the variety pedunculata, but it may be doubted how far the difference would hold if sufficiently numerous comparisons were made.
(5) Swelling may be regarded as complementary to shrinkage. It has been found that if oak wood is allowed to absorb water until thoroughly saturated it will increase from 0·13 to 0·4 per cent in length, and be distended radially from 2·66 to 3·9 per cent, or tangentially 5·59 to 7·55 per cent, according to age and condition, young wood swelling more than old. It has also been found that the total volume increased from 5·5 to 7·9 per cent, and the weight from 60 to 91 per cent, on complete saturation.
(6) Elasticity and Tenacity.—Oak is very elastic, and easily bent if steamed, and it does not readily splinter. When pulled in a direction parallel to the length of the structure the absolute tenacity = 2·23 to 14·51 kgr.—i.e., it took a pull equal to this weight per 1 sq. mm. of section to pull the wood asunder.
The limit of elasticity corresponds to a load of 2·72 to 3·5 kgr., according to various authorities, the specimen lengthening th in the former case.
The modulus of elasticity is given as 826 to 1,030 kgr., and the breaking limit as 4·66 to 6·85.
When the pull is in a direction across the length of the fibers, the results differ according as the load is applied so as to act radially or tangentially.
When acting radially the modulus of elasticity is given as 188·7 kgr., and the breaking limit as 0·582 kgr.
When acting parallel to a tangent the modulus of elasticity = 129·8 kgr., and the breaking limit 0·406 kgr.
The absolute tenacity in the transverse direction is given as 0·44 to 0·61 kgr.
In the case where pressures are applied in the direction of the length of the fibers the limit of elasticity = 2·09 to 2·22 kgr.; the modulus of elasticity, 933 to 1,250 kgr.; and the absolute resistance, 2·58 to 3·64 kgr.
Flexibility.—The limit of elasticity = 1·77 to 2·71 kgr.; modulus of elasticity, 620 to 735 kgr.; resistance to bending, 4·53 to 6·18 kgr.
Torsion.—Oak warps considerably unless carefully seasoned. Limit of elasticity = 0·4 to 0·54 kgr.; modulus of elasticity, 612·5 to 785 kgr.; resistance to torsion, 0·75 to 0·97 kgr.
Resistance to shearing-stress, in the direction of the fibers = 0·61 to 0·97 kgr.; perpendicular to them, 1·9 to 3·49 kgr.
(7) Resistance to Splitting.—Oak is easily split into tolerably smooth and even staves, and is much employed for this purpose.
(8) Hardness.—Oak is neither the hardest and heaviest nor the most supple and toughest of woods, but it combines in a useful manner the average of these qualities. Good oak is hard, firm, and compact, and with a glossy surface, and varies much; young oak is often tougher, more cross-grained, and harder to work than older wood. According to Gayer, if we call the resistance which the beech offers to the saw, applied transverse to the fibers, 1, then that of freshly felled oak = 1·09.
(9) Durability.—A mild climate and open situation produces the most durable oak, and it is extraordinarily durable under water, in the earth, or exposed to wind and weather, or under shelter; in the latter case it becomes more and more brittle as years roll by.
The alburnum becomes rotten usually in a few years if exposed, and is the prey of insects if under cover. The heart, if sound, may last for centuries under cover and well ventilated, and even in earth or water will endure for several generations. There are, for instance, in the museum at Kew, a portion of a pile from old London Bridge which was taken up in 1827, after having been in use for about 650 years, and a piece of a beam from the Tower of London, of which it is stated that it was "probably coeval with the building of the Tower by William Rufus"; and many other specimens of very old oak are known.
(10) Burning Properties.—The calorific power of oak wood is high, in accordance with its density, but it splutters and crackles and blackens too much. Nevertheless, it produces a valuable charcoal. Hartig says that if we call the cooking-power of a given volume of beech 1, that of an equal volume of oak = 0·92 to 0·96.
(11) Peculiarities.—Oak timber is apt to suffer from various diseases, and from frost-cracks and star-shakes, cup-shakes, etc., as we shall see in the next chapter. It often presents brittle wood, red-rot (foxiness), white-rot, spottiness of various kinds, and is sometimes twisted. At the roots it is very often affected with burrs. It contains gallic acid, and so corrodes iron nails, clamps, etc.
(13) Uses.—Owing to its high price and great specific weight, oak has suffered in competition with spruce, larch, and pine so far as building is concerned; but its uses are very various and widespread nevertheless, and it is invaluable to the engineer and builder wherever strength and durability are aimed at.
As already said, its great value depends on its marvelous combinations of several average properties; and considerable variations in the density, durability, ease of working, and beauty when worked, and so forth, are met with according to the situation and climate in which the oak grows. Generally speaking, it is found that when the oak grows isolated in plains, in rich soil and a mild climate (habitat of Q. pedunculata), it grows rapidly, and produces a wood of very tough and horny consistency, which is regarded as the best for naval and hydraulic work, cartwrights, etc., and wherever strength, tenacity, and solidity are required in high degree (Fig. 39, top). The best should have broad and equal rings, but not broader than 7 to 8 mm., with narrow vascular zone and the smallest possible vessels, and with a pale, rather than dark, and even color on the fresh section. It should also have long fibers and a strong, fresh smell.In close, high forest, on poor soil, and in a rougher climate, it may take 300 years to reach 0·6 metre diameter, and the wood is then softer and more porous,
Fig. 39.—Three specimens of oak grown under different conditions.
beautifully speckled, and shrinking little (Fig. 39, middle). Such wood is excellent for sculpture and carving, and is very pretty; it is also well adapted for cooperage.
In deep soil of moderate quality, in hilly country, and growing as coppice under standards, we have a wood of irregular growth and not very valuable, but useful in an all-round way for sawing and splitting (Fig. 39, bottom).
Speaking generally, it is found that, other things being equal, the most resistant, closest, and toughest timber comes from isolated trees growing in the open: straight and long timber, less marked for the above qualities, comes, on the contrary, from trees grown in close, high forest. This is the conclusion arrived at by the naval authorities in France and England, and may be accepted as according with the facts of structure, etc. Some differences may be put down to the varieties, but probably Boppe is right in concluding that rate of growth, etc., due to differences in the soil and climate, are the determining causes.
The builder employs oak for sills, staircase treads,
Description of Fig. 39.—The upper one is from a rapidly-grown tree, in the open, and at a low altitude; the wood is very strong, hard, and heavy (density 0·827), because there is a preponderance of fibers in the broad rings. The middle specimen comes from a tree growing slowly in a forest at a considerable altitude; the narrow rings have too large a proportion of vessels, whence the wood is soft (density 0·691), porous, and weak. The lower section is from a tree which has grown very irregularly on poor soil, as shown by the variable rings; only the parts with broad rings are good—hence bad wood predominates (density 0·742). (Nanquette-Boppe.)
keys, wedges and treenails, gate-posts and doors, and superior joinery.
Railway-sleepers are best made of young oak, as it is denser, and the Austrians say such sleepers last from seven to ten years if not treated, and for as long as sixteen years if treated with zinc chloride and other preservatives.
On the Continent heavy oak is used in machines, for axletrees, spokes, stamps of mills, anvil-stocks, hammer-handles, etc.
Oak is much used for carving of all kinds, large furniture, paneling, parquetry, for the felloes, spokes, and axles of wheels, and for other parts of wagons, etc. In cooperage it is much used for the staves, etc., of casks, measures, sieves.
Split oak makes excellent palings and shingles, and oak vine-props are only second to those of chestnut. Walking-sticks are also made of oak, and even water-pipes have been used, but they taint the water.