Part 301

METAL PROTECTIVES: See Rust Preventives.

METAL VARNISHES: See Varnishes.

METALS, HOW TO ATTACH TO RUBBER: See Adhesives, under Rubber Cements.

METALS, SECURING WOOD TO: See Adhesives.

«METALS, BRIGHTENING AND DEADENING, BY DIPPING:»

«Brightening Pickle.»—To brighten articles by dipping, the dipping liquid must not be too hot, otherwise the pickled surface turns dull; neither must it be prepared too thin, nor must wet articles be entered, else only tarnished surfaces will be obtained.

For a burnish-dip any aqua fortis over 33° Bé., i. e., possessing a specific gravity of 1.30, may be employed. It is advisable not to use highly concentrated aqua fortis, to reduce the danger of obtaining matt work. It is important that the quantity of oil of vitriol, which is added, {470} is correct. It is added because the action of the aqua fortis is very uncertain. Within a short time it becomes so heated in acting on the metals that it turns out only dull work, and pores or even holes are apt to be the result of the violent chemical action. If the aqua fortis is diluted with water the articles do not become bright, but tarnish. For this reason sulphuric acid should be used. This does not attack the metals; it only dilutes the aqua fortis and distributes the heat generated in pickling over a larger space. It is also much cheaper, and it absorbs water from the aqua fortis and, therefore, keeps it in a concentrated state and yet distributed over the space.

In the case of too much oil of vitriol the dilution becomes too great and the goods are tarnished; if too little is added, the mixture soon ceases to turn out bright articles, because of overheating. On this experience are based the formulas given below.

Dip the articles, which must be free from grease, into the pickle, after they have been either annealed and quenched in diluted sulphuric acid or washed out with benzine. Leave them in the dipping mixture until they become covered with a greenish froth. Then quickly immerse them in a vessel containing plenty of water, and wash them out well with running water. Before entering the dipped articles in the baths it is well to remove all traces of acid, by passing them through a weak soda or potassium cyanide solution and washing them out again. If the brightly dipped goods are to remain bright they must be coated with a thin spirit or zapon lacquer.

Following are two formulas for the pickle:

I.—Aqua fortis, 36° Bé., by weight 100 parts Oil of vitriol (sulphuric acid), 66° Bé., by weight 70 parts Cooking salt, by volume 1 1⁠/⁠2 parts Shining soot (lampblack), by volume 1 1⁠/⁠2 parts

II.—Aqua fortis, 40° Bé., by weight 100 parts Oil of vitriol, 66° Bé., by weight 100 parts Cooking salt, by volume 2 parts Shining soot, by volume 2 parts

«Matting or Deadening Pickle.»—When, instead of brilliancy, a matted appearance is desired for metals, the article is corroded either mechanically or chemically. In the first case it is pierced with fine holes near together, rubbed with emery powder or pumice stone and tamponned. In the other case the corrosion is effected in acid baths thus composed:

Nitric acid of 36° Bé., 200 parts, by volume; sulphuric acid of 56° Bé., 200 parts, by volume; sea salt, 1 part, by volume; zinc sulphate, 1 to 5 parts, by volume.

With this proportion of acids the articles can remain from 5 to 20 minutes in the mixture cold; the prominence of the matt depends on the length of time of the immersion. The pieces on being taken from the bath have an earthy appearance which is lightened by dipping them quickly in a brightening acid. If left too long the matted appearance is destroyed.

«Cotton Matt.»—This matt, thus called on account of its soft shade, is rarely employed except for articles of stamped brass, statuettes, or small objects. As much zinc is dissolved in the bath as it will take. The pieces are left in it from 15 to 30 minutes. On coming from the bath they are dull, and to brighten them somewhat they are generally dipped into acids as before described.

«Silver Matt.»—Articles of value for which gilding is desired are matted by covering them with a light coating of silver by the battery. It is known that this deposit is always matt, unless the bath contains too large a quantity of potassium cyanide. A brilliant silvering can be regularly obtained with electric baths only by adding carbon sulphide. Four drachms are put in an emery flask containing a quart of the bath fluid and allowed to rest for 24 hours, at the end of which a blackish precipitate is formed. After decanting, a quart is poured into the electric bath for each quart before every operation of silvering.

«Dangers of Dipping.»—The operation of dipping should be carried out only in a place where the escaping fumes of hyponitric acid and chlorine can pass off without molesting the workmen, e. g., under a well-drawing chimney, preferably in a vapor chamber. If such an arrangement is not present the operator should choose a draughty place and protect himself from the fumes by tying a wet sponge under his nose. The vapors are liable to produce very violent and dangerous inflammations of the respiratory organs, coming on in a surprisingly {471} quick manner after one has felt no previous injurious effect at all.

«COLORING METALS:»

See also Plating.

«Processes by Oxidation.»—By heat:—Coloration of Steel.—The steel, heated uniformly, is covered in the air with a pellicle of oxide and has successively the following colors: Straw yellow, blue (480° to 570° F.), violet, purple, water-green, disappearance of the color; lastly the steel reddens. For producing the blue readily, plunge the object into a bath of 25 parts of lead and 1 part of tin; its temperature is sufficient for bluing small pieces.

Bronzing of Steel.—I.—The piece to be bronzed is wet by the use of a sponge with a solution formed of iron perchloride, cupric sulphate, and a nitric acid. It is dried in a stove at 86° F., then kept for 20 minutes over boiling water. It is dried again at 86° F., and rubbed with a scratch brush.

This operation is repeated several times.

Bronzing of Steel.—II.—Rust and grease are removed from the objects with a paste of whiting and soda. They are immersed in a bath of dilute sulphuric acid, and rubbed with very fine pumice-stone powder. They are then exposed from 2 to 3 minutes to the vapor of a mixture of equal parts of concentrated chlorhydric and nitric acids.

The object is heated to 570° to 660° F. until the bronze color appears. When cooled, it is covered with paraffine or vaseline while rubbing, and heated a second time until the vaseline or paraffine commences to decompose. The operation is repeated. The shades obtained are beautiful, and the bronzing is not changeable. By subjecting the object to the vapors of the mixture of chlorhydric and nitric acids, shades of a light reddish brown are obtained. By adding to these two acids acetic acid, beautiful yellow bronze tints are procured. By varying the proportion of these three acids, all the colors from light reddish brown to deep brown, or from light yellow bronze to deep yellow bronze, are produced at will.

Bronzing.—III.—Under the name of Tuker bronze, a colored metal is found in trade which imitates ornamental bronze perfectly. It is obtained by deoxidizing or, if preferred, by burnishing cast iron. A thin layer of linseed oil or of linseed-oil varnish is spread on. It is heated at a temperature sufficient for producing in the open air the oxidation of the metal. The temperature is raised more or less, according as a simple yellow coloration or a deep brown is desired.

Lustrous Black.—In a quantity of oil of turpentine, sulphuric acid is poured drop by drop, stirring continually until a precipitate is no longer formed. Then the whole is poured into water, shaken, decanted, and the washing of the precipitate commenced again until blue litmus paper immersed in the water is no longer reddened. The precipitate will thus be completely freed from acid. After having drained it on a cloth, it is ready for use. It is spread on the iron and burned at the fire.

If the precipitate spreads with difficulty over the metal, a little turpentine can be added. It is afterwards rubbed with a linen rag, soaked with linseed oil, until the surface assumes a beautiful lustrous black. This covering is not liable to be detached.