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Watchmakers' Hand Book

Part II,
Page 11


      139. To bronze copper. The following are two methods recommended for bronzing objects of this metal, for example, a medal.

      Dissolve two parts of verdigris (acetate of copper) and one part of sal-ammoniac in vinegar. Boil the solution, skim it, and dilute with water until it no longer possesses a feebly metallic smell, nor produces a whitish precipitate on the addition of water. Then let it boil again in an earthenware or porcelain vessel and transfer it, while boiling, into another vessel containing the perfectly clean medals, etc., and place the whole on the fire. As soon as the medals assume the required color, remove them, and wash carefully in clean water.

      The objects must not be left too long in the acid bath over the fire, because the layer of oxide would become too thick, and would easily scale off the surface; whereas, if the operation is properly conducted, the coating adheres so firmly that it cannot be separated even by scraping. Of course, it is only after a certain number of trials, and with experience, that the exact moment can be ascertained for removing the objects from the bath. It is very necessary that the bath be not too concentrated, as the superficial oxide becomes proportionately less adherent: moreover, a whitish powder is deposited on the medal, which turns green on exposure to the air and spoils the appearance of the bronzing.

      140. Chinese bronzing. The Chinese employ the following mixture for bronzing copper, the several constituents being powdered before being incorporated together: 2 parts of verdigris, 2 parts of cinnabar, 5 parts of sal-ammoniac, 5 of alum, and 2 parts of the beak and of the liver of a duck. A past having been made, with vinegar, it is spread over the perfectly clean surface of the copper, and the whole exposed for an instant to the fire, then allowed to cool, washed, and the operation repeated as often as may be needed in order to obtain the desired tint.

      By adding sulfate of copper to the mixture a browner shade will be obtained, and it may be made yellower by adding borax. Copper thus treated is said to present a beautiful appearance, and to be so permanent that neither air nor water has any influence against it.

      141. To bronze brass. Dissolve copper turnings in nitric nitric acid until it is completely saturated. Immerse the brass objects to be bronzed in this solution after they have been cleaned, smoothed with water of Ayr stone, and heated to such a temperature as the hand can just support; on being placed over charcoal fire they will assume a green color; rub them over with rags, repeat the immersion and heating over charcoal until the required tint is obtained. The shade may be improved by oiling the finished surfaces.

      It is asserted that by immersing copper articles in molten sulpher containing lamp-black in suspension, they assume the appearance of bronze; and that they may even be polished without losing their color.


      142. Gold gilding without the aid of mercury. Prepare the gold in fine powder, as explained in the following paragraph, or procure it from the dealers in chemical products, who manufacture it for various tints. Make a mixture of this powder with pure rock salt and cream of tartar (bi-tartrate of potash), pulverized in the same manner as described in speaking of silver-plating and take the same precautions in its application.

      The gold surface will present a dull appearance; acid cannot be used to improve its color when operating, for example, on a wheel with attached pinion, but the same result may be attained by a very simple method. Rub the object after plating with cream of tartar, mixed with a large portion of warm water; then immediately wash in an abundance of warm water at not less than 40° C. (104° F.); soap it thoroughly, so as to neutralize any acid that may remain, and finally pass through alcohol to dissolve any remaining soap.

      The surface will still be improved by rubbing with a very hard piece of pith, such as is occasionally met with.

      M. Robert, in describing the above method, adds: "In this manner I have gilded cocks, domes, compensation balance weights, and even their brass rims. When skillfully and expeditiously performed, the pinion need not be dis-colored; but, if it is at any time slightly marked, it may be restored by at once rubbing the surface with a soft stick and fine rouge.

      143. Preparation of the gold powder. As already observed this can be obtained of any desired color from the dealers in chemical products, but the following method is given for the benefit of any one who desires to prepare it for himself:

      Place some gold in thin leaves in a dish, and add a little honey, thoroughly intermixing the two by the aid of a glass rod flattened at one end; then place the paste so obtained in a glass of water containing a little alcohol, washing it and allowing the powder to settle. decant the liquid and again wash the residue, repeating the operation until a fine brilliant powder is obtained. This powder is mixed as required with rock salt and powdered cream of tartar in the manner already described.

      144. Second method. Dissolve one part by weight (say about ten grains) of pure gold, rolled very thin, in aqua regia (155) contained in a porcelain dish, which may be gently heated on a sand-bath, and evaporate the acid until it assumes a blood-red color. Add about 30 parts, by weight, of warm distilled water, in which 4 parts of crystallized cyanide of potassium have been previously dissolved; thoroughly stir the mixture with a glass rod, and filter it through a glass funnel.

      145. Third method. Roseleur recommends the following solution for gilding by simple immersion. Distilled water, 17 pints; pyrophosphate of soda (in crystals) 28 ounces; hydrocyanic acid, 1.3ounces; crystallized perchloride of gold, 2.3 ounces. The pyrophosphate is added in small quantities at a time, to 16 pints of water, in a porcelain vessel, stirring with a glass rod and applying gentle heat; then filter and cool. The gold salt is dissolved in a small amount of water; filter and add the hydrocyanic acid and the solution, heated to the boiling point, is ready for use.

      The articles to be dipped must be thoroughly cleansed and passed through a very dilute solution of nitrate of binoxide of mercury; they must be constantly agitated while in the bath and the best coating is obtained by dipping the articles in a nearly exhausted solution of the same kind immediately after the mercury solution.

      146. Electro gilding. But the method most usually adopted is that in which a battery is employed. It is, however, impossible, within the limits of this work, to explain the precautions that are necessary in conducting he process, managing the battery, etc., and the reader must be referred to works on electro-metallurgy for these details.

      147. To prepare the pieces to be plated. After the surface has been stoned, boil the object a few minutes in a solution of soda and potash, and rinse in clean water.

      Roseleur, in the articles already referred to, gives very full instructions, of which the following is an outline. The reader who desires to obtain more complete information can consult his works.

      Attach the pieces to a cork and brush with a clean charged with water and pumice-stone powder and thoroughly rinse. Place them in a solution consisting of: water, 2 1/4 gal.; nitrate of binoxide of mercury,1.14 oz.; sulphuric acid 1.7 oz. Then rinse again.

      148. Graining. Mix thoroughly with the application of moderate heat, silver powder, 1 ounce; pure common salt, finely powdered, 13 ounces; cream of tartar, 4-5 ounces. Make a thin paste of this mixture with water and spread with a spatula on the pieces; having mounted them on a cork to which a rotary motion is given, rub them in all directions with a brush with close bristles, adding fresh paste from time to time. When the desired grain is obtained, wash and scratch-brush with revolving wire brushes. Three of these are often used of varying degrees of hardness and a decoration of liqorice, weak size or stale beer is liberally applied to the surface.

      149. Resist. This is a composition for covering steel parts in order to protect them from the action of acids, itc., in the various processes of cleaning, graining and gilding. It consists of yellow wax, 2 ounces; clear resin, 3 1/3 ounces; very fine red sealing-wax, 1 1/2 ounces; finest rouge, 1 ounce. Melt the resin and sealing-wax in a porcelain dish, then add the yellow wax, and when the whole is thoroughly liquid, gradually add the rouge, stirring with a glass rod. The parts to be coated are slightly heated and covered with the mixture.

      To remove the resist after the gilding process is completed, place the pieces in warm oil or turpentine, then in a very hot soapy or alkaline solution and lastly in fresh water.

      150. When prepared as above explained, the object may be gilt by one of the preceding methods; of course a hot solution cannot be resorted to when the resist has been applied.

      151. To clean objects that are of gold or gilt. The following method is equally applicable to pieces that are gilt, such as cocks, domes, etc., the frames and parts of time-pieces and to either gold or gilt jewelry.

      To about a tumbler of water add 20 drops of strong ammonia. Immerse the object several times in this mixture and brush it with a soft brush; as soon as the operation appears to be completed (which experience will soon enable the workman to ascertain), wash in pure water, then in alcohol, and dry with a fine linen rag. The original brilliancy will then be restored.

      When the coating is thin and has been galvanically deposited, only very soft brushes must be used.

      Gilders, instead of dipping in alcohol and drying with a linen rag, usually immerse the pieces in boxwood sawdust, leaving them long enough to become thoroughly dry; after this treatment they merely require to be shaken and lightly rubbed with a fine brush.

      The sawdust must be perfectly dry; indeed it is a good plan to slightly warm it by placing the wooden box containing it for a few minutes on a hot oven or stove in the winter and exposing it to a hot sun in the summer.

      Instead of ammonia, alum (156) is sometimes boiled in water and the objects dipped two or three times in this solution, subsequently brushing as in the previous case.

      152. To restore the dead surface of gold or gilt objects. Place them for two or three minutes in chlorine water, rinse them in clean water, soap them and finally dry them in sawdust. It is advisable that parts that are polished be prevented from actual contact with the liquid as it would produce a somewhat deadened surface.

      153. To clean gold jewelry after soldering. Particles of binding wire are often left adhering to the surface of jewelry after soldering, and, on dipping the object into dipping liquid, a layer of oxide may be formed. This can be removed without detriment to the polished surface by plunging the object for a few seconds in nitric acid (155).

Submitted by: Samuel Kirk (##)

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