The remedy consists in adding to the bath only enough silver salt and no more, so that a piece of copper will not become sensibly silvered in it, without the aid of electricity. The cold electro-silvering baths generally employed for electroplating such articles as tablespoons or forks are contained in large rectangular wooden troughs lined with gutta-percha, or made of riveted wrought iron. They are sufficiently high to allow about 4 in. of liquid above the immersed object, whose distance from the bottom and sides should be nearly the same, to give a regular deposit of metal at both extremities of the object. The upper ledge of the trough carries two brass rods all round, which do not touch one another, one above the other, so that other metallic rods, being put across, will rest upon the higher or the lower rod, but not both at the same time. Each rod is connected with one of the poles of the battery by conducting wires, the points of contact of which should be perfectly clean. The rod which supports the articles to be silvered is connected with the negative pole represented by zinc in most batteries; and the other, supporting the anodes, is attached to the positive pole, which is carbon with Bunsen's elements, copper for Daniell's, and platinum with Grove's cells.
A certain number of spoons and forks fixed to a rod, by means of copper wires, are cleansed at the same time, and the rod is placed upon the negative conducting rod of the trough. Then, facing these articles, hang upon the positive conducting wire of the trough another metallic rod to which the soluble silver anode is attached like a flag. Next comes another series of spoons and forks, faced by another soluble anode, in such a manner that each row of spoons and forks is between two anodes. The articles to be silvered all rest upon the negative conducting rod, and the soluble anodes upon the positive one. This disposition is for obtaining an equal deposit upon all the pieces. The objects require turning upside down during the operation, in order to prevent a thicker deposit on the lower parts, as the richest part of the solution is the densest, and therefore lies near the bottom of the trough. The denser layers, being richer in metal, deposit it more abundantly upon the direction which they follow, and form grooves which cannot be filled by the lighter and poorer currents. It is, therefore, advantageous to keep the objects in constant motion.
In this case the frame supporting the articles does not rest upon the trough, but is suspended above the bath, and receives its motion from a small eccentric, or other motive power. The silver deposit will adhere strongly if the articles have been fully amalgamated in the solution of nitrate of binoxide of mercury, and have remained in the silver bath 12-15 hours, according to the intensity of the current. The silvering will be the better and finer as the intensity of the current is weaker, up to a certain limit. A sufficient quantity of silver may be deposited in 3-4 hours, but the result -is not satisfactory, and the burnishing is very difficult. When the articles have acquired a film of silver, they are sometimes removed from the bath and thoroughly scratch-brushed, cleansed in alcohol, or, preferably, in a hot silvering bath, thence again passed through the mercurial solution, and finished in the former cold electro-bath. This first scratch-brushing; which is not always necessary, obviates the tendency of certain alloys to assume a crystalline appearance, and corrects imperfections of the cleansing process. Electro-silvering baths do not generally work so well when freshly prepared as when they have been used for a certain time; the deposit is often granulated, bluish, or yellowish.
It is therefore desirable to mix a portion of old liquors with those recently prepared, or new baths may acquire an artificial age by boiling them for a few hours, or adding to them one or two thousandths of aqua ammonia.
By reserves, certain parts of a metallic article, which may be already covered with an electro-deposit on its whole surface, are coated with another metal. To gild the parts in relief of an object of which the body is silvered; make a gold reserve, and use a silver reserve for silvering 6f certain parts of a body already gilt. This requires a little practice and care, and a firm hand to make thin lines with the hair pencil. Thoroughly scratch-brush and wipe the object; the parts intended to have the primitive colour must be covered by a brush with a resist varnish; dry in the air, or in a store, or upon a gentle fire until it no longer feels sticky. Place in the bath; the galvanic deposit will only coat those parts unprotected by the varnish. The temperature of the bath should be low, and the current weak, for fear of having rough lines where the deposit touches the varnish, from the latter becoming softened, or from bubbles which are disengaged at the negative pole under the action of a strong electric current.
When the deposit is completed, remove the resist varnish with warm essence of turpentine, and afterwards with tepid alcohol; gaseline or benzole are preferable, as they rapidly dissolve in the cold nearly all resinous and fatty bodies, or the varnish may be destroyed by a brief immersion in concentrated sulphuric acid when cold. It often happens that several colours and metals have to be placed upon the same object, such as silver with both a bright and a dead lustre, and yellow, green, red, white, or pink golds, or platinum. Varnishes are also employed for avoiding the deposit of the precious metals upon those parts which do not need them.
Dissolve in boiled linseed oil or essence of turpentine, rosin, or copal; these varnishes are not sufficiently coloured to distinguish the places where they have been laid on, mix with them therefore a certain proportion of red-lead, chrome yellow, or Prussian blue, which at the same time facilitates their drying.
Should the anodes become black during the passage of the electric current, the solution contains too little potassium cyanide and too much silver. In this case the deposit is adherent, but too slow, and the bath loses, more silver than it can gain from the anodes. Care* fully add sufficient potassium cyanide. If the anodes remain white during the current, the proportion of potassium cyanide is too great, the deposited silver is often without adherence, and the anodes lose more metal than is deposited J add silver salt until it dissolves with difficulty. When in good working order" the soluble anodes become grey during the passage of the electricity, and white when the circuit is broken. The specific gravity of the bath may vary from 5o to 15° B, for salts, and still furnish good results. There is a simple and rapid process for ascertaining the state of the bath, and establishing the proper ratio between the silver and the cyanide. About 1/2 pint of the liquor is put into a tall glass, and a solution of 1/3 oz. silver nitrate in 3 oz. distilled water is poured into the former, drop by drop.
If the white precipitate produced is rapidly dissolved by stirring, the liquor is too rich in cyanide, or too poor in silver; should the precipitate remain undissolved after long stirring, the liquor is too rich in silver and too poor in potassium cyanide. When the precipitate is dissolved but slowly, the liquor is in the best condition.
Dip the article in a solution of nitric acid and water, half and half, for a few minutes, then wash well in clean water, and dry in hot sawdust. When thoroughly dry, brush the sawdust away with a soft brush, and burnish the parts required to be bright.
Carbon bisulphide, in small proportion, imparts a bright lustre to electroplated articles. Put 1 oz. carbon bisulphide into a pint bottle containing a strong silver solution with cyanide in excess. The bottle should be repeatedly shaken, and the mixture is ready for use in a few days. A few drops of this solution may be poured into the plating bath occasionally, until the work appears sufficiently bright. The bisulphide solution, however, must be added with care, for an excess is apt to spoil the solution. In plating surfaces which cannot easily be scratch-brushed, this brightening process is very serviceable. Care must be taken never to add too much at a time.
Use first, prepared ox-gall; next, isinglass; then, alum, to kill the former; finish with hard white lac.
This change of colour is due to the deposit, by galvanic action, of pure silver and of subcyanide, which is rapidly decomposed and darkened by light. It is therefore necessary to remove the subcyanide by one of the following methods: - (a) The articles are left immersed in the bath for some time after the electric current has been interrupted, when the silver subcyanide is dissolved by the potassium cyanide.
(6) Having smeared the objects with a paste of borax, they are heated in a muffle until the salt fuses and dissolves the subcyanide. This process anneals and softens the metal.
(c) The poles of the battery are inverted for a few seconds, that is to say, the articles become soluble anodes, and the electric current carries away the silver subcyanide in preference to the metal; this operation should be very short, otherwise the silver will entirely abandon the objects and will coat the silver sheets.