Chloride of antimony has been much used for browning gun-barrels, is excellent in its operation, and has been called, in consequence, browning salt. It is mixed to a thin creamy consistence with olive oil; the iron is slightly heated, dressed evenly upon its surface with this mixture, and left until the requisite degree of browning is produced. The sharpening of the chloride of antimony can be affected by adding a little nitric acid to the paste of olive oil and chloride of antimony, so as to hasten the operation.
Another formula is - Aquafortis, 1/2 oz.; sweet spirit of nitre, 1/2 oz.; spirit of wine, 1 oz.; blue vitriol, 2 oz.; tincture of chloride of iron, 1 oz.; water, 40 oz. Dissolve the blue vitriol in the water, then add the other materials, and the water is warmed to dissolve the blue vitriol; let it get cold before adding the other materials. The burnishing and marking can be effected with the burnisher and scratch brush. The polishing is best effected by rubbing with a piece of smooth, hard wood, called polishing wood. It is lastly varnished with shellac varnish, and again polished with the hard wood polisher.
Some prefer the tone of brown produced by blue vitriol, 1 oz.; sweet spirit of nitre, 1 oz.; water, 20 oz.
In any case, the surface of the iron must be well cleaned, and rendered quite bright; it is then freed from grease by rubbing with whiting and water, or better, with powdered quicklime and water. The browning composition is then placed on, and allowed to remain twenty-four hours. It is then rubbed off with a stiff brush. If not sufficiently browned, repeat the last process after browning. Clean the surface well with hot water containing a little soda or potash, and, lastly, with boiling water, and dry it. The surface can be burnished and polished.
Varnish with tinsmith's lacquer, or with gum shellac, 2 oz.; dragon's blood, 3 dr.; methylated spirits of wine, 4 pints. The metal should be made hot before applying this varnish, and will present an excellent appearance. If the varnish is not required to colour, but only to preserve the actual tint produced on the metal surface by the browning fluid, leave out the dragon's blood.