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Transactions of the Highland and Agricultural Society of Scotland
The Best Mode of Preparing Wood for Fencing with a View to its Preservation

By David Watson Wemyss of Newton Bank.
[Premium—The Minor Gold Medal.]

The very best quality of wood will begin to decay in a short time when exposed to the changes of the atmosphere. Timber, if kept constantly under water, will last a long time; and. on the other hand, if kept constantly dry, it will perhaps last nearly as long; but if exposed to the weather, where it becomes wet and dry alternately, it will soon decay. It therefore becomes a subject of great importance to know in what manner timber may be preserved in exposed situations.

It is considered that dry wood consists, on an average, of 96 per cent. of fibrous and 4 of soluble matter; but that the proportions vary somewhat with the seasons, the soils, and the plant.

If excluded from the contact of moist air, woody matter, like most of the other organised substances, may be preserved for an indefinite period. But if the woody matter be not protected against the action of air and moisture, the case is very different. By degrees its hydrogen and oxygen are disengaged, and the carbon predominates more and more. Thus the particles of the texture are disintegrated gradually, their white colour fades, and passes through all the shades till it becomes quite black. The cells of woody matter also contain different sorts of substances tending to disorganise, and these are mixed and modified in many different ways. Woody matter, being formed of one atom of carbon and one atom of water, as soon as it is submitted to the action of a somewhat elevated temperature, without the contact of air, experiences an internal reaction, which tends to separate the atom of water from the atom of carbon. The water is vaporised, and the carbon remains in the form of a black and granular residue. Now, if any means could be devised by which the substances in the cells of woody matter could be deprived of their tendency to disorganise when in contact with common air, wood may be rendered permanently durable.

All the varieties of timber grown in this country, as well as the Norway and American battens, which are imported, are those which are chiefly used for fencing purposes. Larch is the most durable, although well-grown Scotch fir, of good age and cut down in the proper season, is often used, and suits the purpose as well, although not so durable as the larch.

All conditions being equal, the most resinous timbers resist decomposition the longest; also, the older and more compact the grain of the timber, the longer it will last when exposed to atmospheric influences,—therefore, in the first case, pine and larch timber lasts longer than non-resinous trees. From the amount of resin in the Scotch pine, it will no doubt last longer than foreign timber creosoted.

The kinds of trees already mentioned last much better than the ash, poplar, beech, and other deciduous trees; and in this case the heart-wood of trees is more durable than the sap-wood. Taking these two things, therefore, into consideration, if we try to get these two qualities into the timber, we will succeed to a certain degree.

In the case of home-grown timber intended for fencing purposes, the first and most important matter is to have the trees cut down at a proper season, which is from November till January, when the leaves have faded and the sap down. When the wood is cut into rails, they ought to be laid flat in piles one above another on level ground, and kept as straight as possible, and from twisting, which they are liable to do, and which is very inconvenient when the fence is being put up.

Sometimes wood fences are made of the small or thinnings of plantations, the trees being split up the middle, but this is not a durable sort of fence, as the rails are often too heavy for the nails to hold any length of time; also, the bark being on the trees, prevents the effect of any substance or substances which may be applied for its preservation, besides being a harbour for wet and vermin.

When timber is decaying by dry-rot, it will be found to be connected with the growth of a small plant in the wood belonging to the tribe of fungi. It feeds upon the sap and grows very fast; and by its rapid growth, and by removing all the sap from the wood, the timber very soon becomes brittle. This plant also spreads rapidly, but a good free circulation of air often prevents it. What the true cause of dry-rot is, has never been determined, but it frequently shows itself by a species of mildew, which covers the timber, and the action of which apparently causes the wood to decay and crumble down into powder. The mildew, however, is neither the dry-rot nor its cause, but its effect. It is distinctly seen by the microscope to be a fungus; and as the fungus itself is so minute as to require the aid of the microscope to be distinctly seen, its seeds may be supposed to be so very minute as to be taken up by the spongeoles of trees. But, whatever may be the cause of dry-rot in timber, there is no doubt of the fact that simply steeping timber in a solution of corrosive sublimate preserves it from dry-rot. After it is subjected to this process, it is requisite that it should have free access of air, which it is sure to have when the timber is applied to fencing purposes.

Whatever other causes may combine to promote the decomposition of wood by dry-rot, or other forms of decay, there can be no doubt that imperfect seasoning, by leaving in the pores of the timber a large proportion of the fermentable juices always found in recently felled timber, is one of the most important, and therefore that good seasoning is as essential in promoting the durability of wood as it is in lessening the tendency to those changes of form and bulk which so greatly increase the difficulties of the carpenter and joiner.

The process of seasoning usually consists simply in the exposure of the timber to the action of air in a dry situation, in stacks or piles so constructed as to allow the free circulation of air in contact with as much as possible of the surface of each piece of timber, until the sap or vegetable juices shall have dried up so far as to offer no facility for the germination of the fungi which constitute various kinds of dry-rot. In order to the success of this operation, it is important that the pile of timber be so far elevated from the ground as to allow the circulation of air beneath as well as through and around it; and also that, if exposure to rain be not entirely avoided, care be taken to prevent the lodgment of moisture in any place where it would be likely to remain long.

Dead oil or pitch is a first class article in filling up the pores of timber, as it possesses the qualities of resin. It first coagulates albuminous substances; second, absorbs and appropriates the oxygen in the pores, and so protects from cremacausis; third, resinifies in the pores of the wood, and thus shuts out both air and moisture; and fourth, acts as a poison to the lower form of animal and vegetable life, and so protects the wood from all parasites. All these properties specially fit it for impregnating timber exposed to alternate states from wet to dry, as indeed some of them do for situations constantly wet.

The cheapest and best composition for coating over wood fencing is coal-tar, lime, and resin,—in the proportion of ╝ bushel slaked lime and Żlb. resin to 4 gallons coal-tar,—then boiled in a pot for an hour, and kept well stirred while on the fire. This applied to fences, either posts or rails, in a hot state, with brushes made for the purpose, will be found an effectual coating for preserving the wood against all weathers and seasons. If this coating is applied every two or three years, the fences will last for many years.

Coal-tar is allowed to be very superior to vegetable tar, and its efficacy in resisting the worm is attributed to its containing sulphocyanic acid, which is highly destructive to animal and vegetable life. It is necessary, however, to observe that the coal be deprived of its ammonia, which would produce immediate decay if thrown into the timber.

The protecting power of metallic oxides, when applied to the surface of wood in the form of paint, is well known; and many abortive schemes for the preservation of timber have been devised to act upon the same principle, which is that of excluding such external influences as might promote decay. Wood, when painted, is not preserved from the effects of the weather, as the ingredients in paint—namely, white lead and oil—have no power to do so. Still ornamental, upright fences—that is, wood cut to measurement and dressed—are generally painted all over, and when this is attended to, and renewed annually, it will remain fresh for many years. To imperfectly seasoned timber, however, such applications are worse than useless, because by filling up the pores they impede the natural drying of the vegetable juices, and therefore rather promote than check natural decay. Far more efficient than these are the numerous modes of protection which involve the impregnation of the timber with some antiseptic substance, or with such matters as, by pre-occupying the pores, may render the reception and germination of destructive fungi mechanically impossible.

The creosoting of railway sleepers has of late, years been generally adopted by railway companies in this country, with more or less success.

The system of creosoting is to subject the timber, along with dead oil or pitch, to a pressure, varying from 100 to 200 lbs. per square inch, for about ten or twelve hours. This is done in large iron tanks, and from 10 to 12 lbs. of the oil is thus pressed into each cubic foot of the timber. The cost of the process is about 4d. per cubic foot.

There is very good reason to think that the state of the timber previous to its being creosoted has much to do in its after preservation; for example, timber creosoted in a green state cannot take in the oil so effectually as that which has been previously properly seasoned.

Timber used for fencing purposes, when creosoted, has been found to last a long time; some years after posts have been inserted in the ground, they have been found to be as fresh as when first inserted.

Of plans for protecting timber by impregnation, one in particular has attained general celebrity. The preservative agent in this process is bichloride of zinc, commonly called "corrosive sublimate," which is dissolved in water, and forced into the pores of the timber in closed tanks, by means of forcing-pumps, and which combines with the albumen of the wood, and converts it into a compound capable of resisting the ordinary chemical changes of vegetable matter. Chloride of zinc, creosote, obtained from the distillation of tar, oil of tar, and other bituminous matters containing creosote and pyrolignite of iron, have also been successfully used.

Another system of preserving timber is to dissolve 1 lb. of blue vitriol in boiling water, and then mix it with five gallons of water, and have the timber steeped in the mixture for a few days.

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