There are several methods recommended to preserve iron-work from rusting. The use of paint and varnish for this purpose is familiar. In Sweden, they cover iron-work with a mixture of pitch, tar, and wood soot, which acquires a gloss, similar to that of varnish, and is said to prevent the oxidizement of the metal very effectually. Fat-oil varnish, mixed with four-fifths of rectified spirit of turpentine, has been recommended. It is applied with a brush, or sponge. Articles, varnished with this preparation, are said to retain their metallic brilliancy, and never contract any spots of rust. Another composition, for the same purpose, is highly recommended. It consists in applying a mixture of one pound of hogs' lard, free from salt, one ounce of camphor, two drachms of black lead in powder, and two drachms of dragon's blood. At Sheffield and Birmingham, sundry articles, made of steel and iron, are preserved from rust, when sent to foreign markets, by wrapping them in coarse brown paper, prepared first with oil.
Among the different preparations, recommended at various times to prevent iron from contracting rust, we may mention one, which has been used with success, and which gives a lead colour. It is nothing more than taking some litharge, and heating it in an iron pot, and scattering over it some sulphur. The litharge will change its colour, forming a kind of sulphuret of lead, which is then ground with drying oil, and applied like paint. We are told, that this preparation gets remarkably hard, and resists the weather more effectually than any other lead colour.
Sec. VI. Of the making of Wheels and other Works incombustible.
It is usual to give a coat of paint to the wood-work of wheels, &c. which are designed to carry a number of cases. To prevent their taking fire, paint, in some measure, has the effect. The following composition is recommended: Take brick dust, coal-ashes, and iron filings, of each an equal quantity, and mix them with a double size, made hot. Apply this to the wood, and when dry, give it another coat.
Several methods have been adopted for the same purpose; but wood may be made to resist, in a great degree, the action of fire, and rendered almost incombustible, by soaking it in a solution of the supersulphate of potassa and alumina, (alum), in sulphate of iron, (green vitriol), and in other salts, which are incombustible.
With respect to the use of alum as a preservative against fire, it is certain, that, although its use in this way is very ancient, it was not often recommended; for writers on the art of war, such, for example, as Anas, mentions the use of vinegar, in the following quotation from his Poliorcet. cap. 24: "Majus juverit, si prius ligna aceto linantur; nam a materia aceto illita, ignis abstinet."
The use of alum, to prevent substances from taking fire, is not a new invention; notwithstanding we find it recommended in modern works, not only for wood, but also for paper, and linen and cotton dresses, &c. Aulus Gellius relates, that Archelaus, one of the generals of Mithridates, washed over a wooden tower with a solution of alum, and thereby rendered it so much proof against fire, that all Sylla's attempts to set it in flames proved abortive.
A writer in the Anthologia Hibernica, vol. iii, for 1794, observes, that the use of alum to prevent the action of fire, on wood, or other combustible bodies, is not new, and those, who lay claim, are not entitled to originality on that head.
Another mode to prevent wood from taking fire may also be adopted. It consists in mixing together, one ounce of sulphur, one ounce of red ochre, and six ounces of green vitriol. The wood work is covered with joiners' glue, and the mixture is then put over it. This process is to be repeated three or four times, allowing the glue to dry before a new coat is applied.
There are several other preparations for the same purpose, not only for the covering of wood, but also paper. But M. Ruggeri is of opinion, that they cannot be depended upon, when used on paper; for the paper will, in part, be consumed. The formula for one of these compositions, is thus given by that gentleman: To a pound of flour, mix a handful of powdered alum, and add to it strong glue-water, and bring it to a proper consistence with clay. Flour and glue-water, mixed together, with the addition of a small quantity of muriate of soda, (common salt), is also recommended for the same purpose. Wood, steeped in a solution of common salt, so as to be thoroughly impregnated with it, is very difficult of combustion. In Persia, salt is used to prevent timber from the attack of worms. The practice of salting ship timber is highly recommended.