Finishing the stock, polishing, engraving, hardening, &c., strictly speaking, involve no science of consequence, and as such it is scarcely necessary to occupy the attention of the reader respecting them. The best method of staining barrels is by the following recipe: but one material fact must not be overlooked. A considerable difficulty exists in staining barrels all steel; in such a case, therefore, the acid should not be so much diluted.
| 1 | oz. muriate tincture of steel. |
| 1 | oz. spirits of wine. |
| 1⁄4 | oz. muriate of mercury. |
| 1⁄4 | oz. strong nitric acid. |
| 1⁄8 | oz. blue stone. |
| 1 | quart of water. |
These are to be well mixed, and allowed to stand a month, to amalgamate. After the oil or grease has been removed from the barrels by lime, the mixture is laid on lightly with a sponge every two hours. It should be scratched off with a steel-wire brush night and morning, until the barrels are dark enough; and then the acid is destroyed by pouring on the barrels boiling water, and continuing to rub them till nearly cool.
The Birmingham people brown their barrels of inferior quality in the following way, to make them look equal to the best. They dissolve as much muriate of mercury as can be taken up in a dram-glassful of spirits of wine; this solution is mixed with one pint of water, or as much diluted as the person requires. A small quantity of the mixture is poured on a little whitening, and laid on the barrel with a sponge, rather lightly; as soon as dry, it is brushed off, and a fresh coat is laid on; and so on until the barrel is dark enough, which is generally about two days. The effect that the mercury has on every one of the joints of the fibres is wonderful: it never fails to make them, in two or three days at most, a beautiful brown; while the other parts, being harder, remain, comparatively speaking, quite light. The rust is killed by hot water, but after that, the barrels are suddenly immersed in cold water; which has the effect of heightening the brightness of both the colours. The appearance is beautiful, and equally as fine to the eye as stub-barrels browned in the same way; though this process is mostly used for the charcoal iron and the threepenny iron barrels. The only method in which there is no deception, is the smoke brown or stain; and, plainly speaking, this and no other is the reason the gun-makers condemn it. As the acid is decidedly weaker, and of course less liable to impart injury to the iron, no barrel can be browned by it, to look well and fine, but the best; or, in other words, none save those possessing steel in their composition.
The method of staining is this: the barrels are anointed with a little vitriolic acid, to cause the iron to receive the effect of the gas more readily; it is then washed off, and the barrels rubbed dry. The forge fire must then be lighted, and blown up with coal possessing as much hydrogen gas and as little sulphur as possible. When the coals are burnt till they give out a clear white flame with no black smoke around it, the barrels must be passed gradually through that flame backward and forward, until the whole are covered with a black sooty covering. Place them in as damp and cool a cellar as can be procured, and allow them to stand for eighteen hours; at that time, if the place is sufficiently damp, the iron parts will be found covered with a red rust, while the particles of steel still retain the original sooty coat. Scratch these off with a steel brush, the same as by any other method of staining; then take a piece of linen cloth, and wash or polish the barrels with water and a little washed emery; when the steel will be found of its original bright colour, and the iron a shade darker, with the outlines of both distinctly preserved. Rub the barrels dry, and again pass them through the flame precisely as before; but above all things be careful not to allow them to remain in the flame till they become hot enough to melt the solder. When you have once passed them through, do not be in a hurry to pass them again; but in both be guided by moderation: neither allow them, after the first time, to stand to rust more than twelve hours each time. Polish them as before, and you will find them a shade darker at every smoking. Persevere, until they become as dark as you wish to have them. The utmost you can obtain is a fine purple-black colour on the iron; and on the steel, a shade inclined to a copper colour: but if proper attention be paid to the polishing, it will not change much from its original colour.
The barrels are taken out of stain in the same way as in the other recipes, by hot water; but you must continue to scratch or brush them longer, for by that means you obtain a greater gloss. The principle of this stain is simply thus: the hydrogen gas contained in the coal acting on the iron (from being of a softer nature than the steel, which it does not affect), and the flame also possessing a quantity of tar, it is imperceptibly embodied by the iron during the action of the oxide; and, when finished, by filling up the spaces created, it becomes decidedly more impervious to damp or wet than the other stain, which is entirely composed of the oxide of iron.
The only objection to this brown has been found to arise from the discharge of the black colour from the softer parts of the barrels; as it being but coal tar, the sweat of the hand, hot water in washing, &c., invariably extract it in a comparatively short time.
The recipe, for the Birmingham imitations, is as follows:—
| 1 | oz. sweet nitre. |
| 1⁄2 | oz. tincture of steel. |
| 1⁄4 | oz. blue vitriol. |
| 6 | drops nitric acid. |
| 14 | grs. corrosive sublimate. |
| 1 | pint of water. |
When the barrels are dark enough, drop a few drops of muriatic acid in a basin of water, and wash the barrel slightly, to brighten the twists.