Popular Science Monthly/Volume 5/June 1874/A New Process for the Preservation of Wood
THE question of the preservation of wood, applied to the sleepers of railroads, telegraph-poles, and wood for mechanical purposes, etc., becomes from day to day more urgent, in presence of the increase of railways. Of all the materials employed until now, there remain hardly two in use—sulphate of copper and creosote.
The sulphate of copper gives only imperfect and very variable results. This we can easily understand: this very soluble salt must be in part diluted by rain-water and the humidity of the soil, so that, at the end of a certain time, the preservative effect has disappeared. Besides, this process very often causes alteration in the wood from the impurity of the salt employed, or from its acid reaction—circumstances which it is very difficult to avoid, when we operate on a large scale with materials containing, in a state of combination, an energetic acid, having for its base a metalloid, such as chlorine, sulphur, nitrogen, etc.
As to creosote, it is a substance comparatively rare, of a high price, of an inflammable nature, and, in consequence, difficult to transport and handle. Besides, and this perhaps is the most important consideration, it is a product which, like those we extract from fossil-coal, may, some day or other, in consequence of a discovery analogous to that of aniline, acquire a high industrial value. Its employment would then become impossible for the preservation of wood.
Hence we may say that these two substances do but imperfectly comply with the necessary conditions, and the question arises, whether there is no other material that might be used in all our present yards, i e,, that might be injected equally well by Boucherie's process (gravitative force of a heavy column of liquid), and by the modified process of Bréant (successive action, in a close tank, of a vacuum and of a pressure of several atmospheres). I propose the acid tannate of protoxide of iron, and base my views on the following considerations:
Wood, as we know, consists of cellulose, or cells in which there is gradually deposited, as the timber matures, lignine, a hard concretion which makes up the greater part of the volume in hard woods, such as ebony, guaiacum, oak; in knots, the shells of nuts, etc. Wood, further, contains sap, which holds in suspension gummy materials, nitrogenous and albuminous substances, coloring-matters, etc.; these are the elements of decay in wood. Inasmuch as they offer to animal and plant-parasites an abundance of agreeable food, they undergo decomposition more or less rapidly, determining, by their own decay, the decay of all the other elements of the wood.
If we succeed in expelling these essentially putrescible materials, or fixing them in unalterable combinations, we thus prevent their decomposition, and, in consequence, that of the other more resistant organic substances, cellulose and lignine. A certain number of observed facts seem to demonstrate that the action of tannin upon vegetable tissues must be analogous to that which is exercised upon animal tissues—operating in the vegetable tissues a kind of tanning, which will have for result the formation of hard and imputrescible albuminous tannates, quite analogous to the gelatinous tannate products in the tanning of skins.
Thus, the sizing of wines is effected as well with the white of an egg (albuminous matter) as by isinglass (gelatinous matter). The tannic acid contained in the wine forms with either of these materials a solid net-work, which envelops and precipitates the lees to the bottom of the cask. An infusion of oak-bark preserves the skins of animals, and is also employed to protect from rotting the nets of hunters and fishermen. In fine, among exotic or indigenous woods, soft or hard, the most resistant are the richest in tannic acid, as among indigenous woods the oak and the chestnut; the first remarkably hard, the second soft enough, are both preserved during many years, and we cannot doubt that this is owing to the influence of the tannic acid with which they are impregnated, which, after the cutting of the wood, reacts upon the azotic and albuminous materials contained in their capillary vessels. We may therefore conclude that the injection of a solution of tannic acid into the various species of woods will assure their preservation, by putting them, in a chemical point of view, in conditions analogous to those in which we find the oak after it has been felled.
But it is not enough to protect soft wood from rotting; it must also be hardened; and though by the action of tannic acid we in some measure attain this end, the soft materials in the sap-vessels being transformed, still it is important that we should give to woods that are naturally soft a higher degree of hardness, in order to fit them for industrial uses.
I accomplish this object by the intervention of the remarkable property of tannate of iron, which, perfectly soluble, and even colorless, in a state of protoxide, is, under the influence of air, transformed into an insoluble salt, of an intense black color. Dissolved in tannic acid to the condition of a soluble salt, in proportions which vary according to the degree of hardness to be given to the wood, it is rapidly transformed under the influence of air, and is deposited in the cells of the wood in a solid state, which it petrifies, so to speak, thus increasing the preservative effects of the tannic acid.
Thus we find resolved, in a manner simple and practical, the question of the introduction of an insoluble salt into wood. We may operate by injecting successively tannic acid, and then a soluble salt of iron; or, by means of a single operation, inject, sheltered from the air, the tannate of protoxide of iron, prepared in advance.
Results of experiments demonstrate the efficacy of this system. In fact, we frequently find in ferruginous soils very old oaks of a black color, and in a state of perfect preservation. I will cite an example that is quite remarkable.
In 1830 there were found at Rouen pieces of oak-wood, from some piles of a bridge built in the year 1150. This wood resembles ebony, of which it has acquired the hardness and the color. Chemical analysis has demonstrated that this modification was owing to the presence of tannate of peroxide of iron (Berthier). Argument and experience, therefore, agree in deciding in favor of the process which I propose.
The bark of most trees, the young branches and leaves, especially of the oaks, birches, elms, sumacs, chestnuts, and walnuts, the roots of the tormentil, and snake-weed, the green shells of horse-chestnuts, and the extracts of exotic woods, contain a large proportion of tannin, to which they owe their astringent properties.
The First Cost of the Materials used for Injection.—In the present state of industry, we can procure tannin at one franc, at most, per kilogramme, in the form of extracts of wood that are employed in dyeing; but we may remark that these products, employed now only for dyeing and tanning, utilize only a very small quantity of the resources of the vegetable kingdom. There is no doubt that a considerable consumption of this product would lead to the establishment of numerous factories, principally in the poor regions, where chestnut and other suitable kinds of trees occur. The industrial wealth of the country would thus be augmented, and, at the same time, the price of this new product would be considerably reduced. Even at the present price, six hundred grammes being sufficient for the injection of one sleeper, the cost of the tannic acid would not exceed 60⁄100 of a franc per sleeper. As to the salts of iron, they are so low in price that it is hardly worth while to estimate their cost. The salts of the protoxide of iron, soluble in tannic acid, carbonate, sulphate, protochloride, and pyrolignite, are easily procured or prepared. The pyrolignite, which seems the most convenient, is worth twenty francs the hundred kilogrammes, and, at a standard of 20° Baumé, contains about seven per cent, of iron. Of this, tannic acid neutralizes twelve per cent. of its own weight, and, if we adopt, for the normal product to inject, 4⁄5 of tannic acid and 1⁄5 of tannate of protoxide of iron, the cost would amount to from 5⁄100 to 6⁄100 of a franc, making, in all, an expense of 66⁄100 of a franc per sleeper.
Trials of this process are at this moment in course of execution upon a grand scale, by the Railway Company of the East, and the Administration of the National Telegraphs, with the authority and coöperation of the Minister of the Interior.—Comptes Rendus.