The quantity of sap-wood is various in different plants, and the line of division is usually most distinctly marked; in some, as boxwood, the sap-wood is very inconsiderable, and together with the bark is on the average only about the thickness of a stout card, whereas in others, as the snake-wood, it constitutes fully two-thirds of the diameter, so that a large tree yields but an inconsiderable stick of wood, of one third or fourth the external diameter.
It may be presumed that in the same variety of wood, about an average number of the layers exist as sap-wood, as in cutting up a number of pieces of the same kind, such as the black Botany-Bay wood, and others, it is found that in those measuring about two inches diameter, the piece of heart-wood is only about as large as the finger, but in pieces one, two, or three inches larger, the heart-wood is also respectively one, two, or three inches larger, or nearly to the full extent of the increase of the diameter.
The sap-wood may be therefore, in general, considered as of about an average thickness in each kind of wood: it is mostly softer, lighter, more even in colour, and more disposed to decay than the heart-wood, which prove it to be in a less matured or useful state, whether for mechanical or chemical purposes.
At the time the tree is separated from its root, its organic life ceases, and then commences the gradual evaporation of the sap, and the drying and contracting of the tubes, or tissues, previously distended by its presence.
The woods are in general felled during the cold months, when the vegetative powers of the plant are nearly dormant, and when they are the most free from sap; but none of the woods are fit for use in the state in which they arc cut down, for although no distinct circulation is going on within the heart-wood, still the capillary vessels keep the trees continually moist throughout their substance, in which state they should not be employed.
It the green or wet woods are placed in confined situations, the tree or plank, first becomes stained or doated, and this speedily leads to its decomposition or decay - effects that are averted by careful drying with free access of air.*
Other mischiefs almost as fatal as decay also occur to unseasoned woods; round blocks cut out of the entire circular stem of green wood, or the same pieces divided into quarterings, split in the direction of the medullary rays, or radially, also though less frequently upon the annual rings. Such of the round blocks as consist of the entire section contract pretty equally, and nearly retain their circular form, but those from the quarterings become oval from their unequal shrinking.
As a general observation, it may be said the woods do not alter in any material degree in respect to length. Boards and flat pieces contract however in width, they warp and twist, and when they arc fitted as panels into loose grooves, they shrink away from that edge which happens to be the most slightly held; but when restrained by nails, mortices or other unyielding attach-ments, which do not allow them the power of contraction, they split with irresistibe force, and the materials and labour thus improperly employed will render no useful service.
* On this account the timbers tor ships are usually cut out to their shape and dimensions for about a year before they are framed together, and they are commonly left a twelvemonth longer in the skeleton state, to complete the seasoning; as in that condition they are more favourably situated as regards exposure to the air than when they are closely covered in with the planking.
Mr. Fincham considers that the destruction of timber by the decay commonly known as dry-rot, cannot occur unless air, moisture, and heat, are all present, and that the entire exclusion of any of the three stays the mischief. By way of experiment, he bored a hole in one of the timbers of an old ship built of oak, whose wood was at the time perfectly sound; the admission of air, the third element, to the central part of the wood, (the two others being to a certain degree present,) caused the hole to be filled up in the course of twenty-four hours with mouldi-ness, a well-known vegetation, which very speedily became so compact a fungus as to admit of being withdrawn like a stick. He considers the shakes or splits in timber to predispose it to decay in damp and confined situations, from admitting the air in the same manner.
The woods differ amazingly in their resistance to decay; some perish in one or two years, whereas others are very durable, and even preserve their fragrance when they are opened after many years, or almost centuries.
Mr. Q. Loddiges says, the oak boxes, for the plants in his green-houses, decay in two or three years, whereas he has found those of teak to last fully six or seven times as long: the situation is one of severe trial for the wood.
There are two quarto works on dry-rot; the one by Mr. M'William, 1818; the other by Mr. John Knowles, Surveyor of Her Majesty's Navy, 1821.
The process of Kyanizing is intended to prevent the re-vegetation of timber, by infusing into its pores an antiseptic salt: the corrosive sublimate is generally employed, other metallic salts are also considered to be applicable, but the general utility of the process, especially in thick timbers, or those exposed to much wet, is still unsettled amongst practical men.
The Kyanizing is sometimes done in open tanks, at others,(by Timperley's process, Hull and Selby Railway.) in close vessels from which the air is first exhausted to the utmost, and the fluid is then admitted under a pressure of about 100 pounds on the inch. - See Minutes of Proceedings, Inst. Civ. Eng., p. 83, 1841. See also note H in Appendix to Vol. II., page 953, in which Payne's more recent preservative process is described.
In general, the softest woods shrink the most in width, but no correct observations on this subject have been published. Mr. Fincham considers the rock-elm to shrink as much as any wood, namely, about half an inch in the foot, whereas the teak scarcely shrinks at all; in the "Tortoise;" store-ship, when fifty years old, no openings were fouud to exist between the boards.
In the woods that have been partially dried, some of these effects are lessened when they are defended by paint or varnish, but they do not then cease, and with dry wood, every time a new surface is exposed to the air, even should the work have been made for many years, these perplexing alterations will in a degree recommence, even independently of the changes of the atmosphere, the fluctuations of which the woods are at all times too freely disposed to obey.
The disposition to shrink and warp from atmospheric influences, appears indeed to be never entirely subdued; some bog-oak, supposed to have been buried in the island of Sheppy, not leas than a thousand years, was dried for many months, and ultimately made into chairs and furniture; it was still found to shrink and cast, when divided into the small pieces required for the work.