ish on exposure to the air. This phenomenon is due to the simultaneous deposit, by galvanic action, of pure silver and of a subsalt, the subcyanide of silver, which is rapidly decomposed and darkened by light. It is therefore expedient to eliminate the latter compound, in order that this inconvenience shall disappear. This may be accomplished by the following methods: 1. The articles are left immersed in the bath for some time after the electric current has been interrupted, when the subcyanide of silver is dissolved by the cyanide of potassium. 2. After 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. 3. The electrodes are inverted for a few seconds, after plating, thus transforming the articles into anodes, and the electric current carries away the subcyanide of silver in preference to the metal. It is evident that this operation should not be prolonged, otherwise the silver will be entirely removed from the objects, and will be deposited on the silver plates. Bright Silvering. As deposited from the cyanide bath just described, the surface of the precipitated silver has a mat or dead appearance, which is well described as "chalky." To brighten this deposit it is necessary to resort to burnishing, which, especially in the case of objects of very or nate design, or with those of hollow shape, which require to be bright inside, is a tedious and troublesome operation. It is found, however, that the addition of a small quantity of bisulphide of carbon to the cyanide plating solu tion will cause the silver to be deposited bright. This observation was made by accident, and became the subject of a British patent, issued in 1847, to Lyons and Milward. Since that time the process devised by these inventors has been, with unimportant modifications, extensively used by platers. Lyons and Milward describe their process as being intended for producing bright deposited silver by adding to the cyanide of silver solution "compounds of sulphur or carbon," of which they name bisulphuret of carbon, terchloride or other chloride of carbon, sesquichloride of sulphur, or hyposulphite of sodium or potassium. The bisulphide of carbon, which they consider preferable, may be used either alone or dissolved in sulphuric ether, or in conjunction with any of the substances above named. The following procedure they find best adapted for their purposes: "We take the bisulphuret of carbon and combine it with the metal to be deposited, previously dissolved in a solution of the cyanide of potassium or sodium in the manner well known, and proceed as follows: We place six ounces of bisulphuret of carbon in a stoppered bottle and pour over it a gallon of the cyanide solution, from which we are about to deposit. We allow this to remain twenty-four hours. We then decant carefully a portion of the solution, which will have combined with a part of the bisulphuret of carbon, and for every twenty gallons of solution to be acted upon we add about two ounces of the solution containing the bisulphuret of carbon and stir it well with the twenty gallons, when it is ready for use; but we find it necessary to add fresh portions of the solution containing the bisulphuret of carbon every day when in use in about the same proportion; but in case the solution has remained many days upon the bisulphuret of carbon a smaller portion than two ounces will suffice to produce the effect. We have found the other named ingredients when used in larger proportions to produce similar results, but not so advantageously as the bisulphuret of carbon; and we have found that a much larger proportion of bisulphuret of carbon produces an improved dead deposit in place of the bright effect previously described; and we would state that the bisulphuret of carbon may be combined with solutions of metals by other means than those we have described. We have succeeded by first dissolving it in sulphuric ether (or hydrocyanic acid with the aid of heat), and find the effect upon the solution to be the same." The proportion either of silver or of free cyanide in the silver bath to which the brightening solution is added does not appear to noticeably affect the result. The brightening solution acts more slowly than the usual plating solution and requires a stronger current. The brightening exhibits itself first at the lowest portions of the articles and spreads from these gradually over the whole surface, requiring usually about a quarter of an hour to completely brighten. The brightening solution is intended to be used simply to impart a superficial finish upon plated articles, and is unfit for ordinary use in plating. A bright" vat should, therefore, be kept for finishing only, and is found especi ally serviceable in plating surfaces that are difficult to scratch-brush. In adding brightening solution to the plating bath, the caution should be carefully observed to use as little of it as possible; the addition of too much is apt to cause the articles to become spotted and streaked with a bad, brownish discoloration. Urquhart (Electro-Plating, 1880, page 136) conveys the following hints on the "bright" plating process: "This is a process employed in the plating factory by which a brilliant appearance or finish is given to the goods after they have been plated in the ordinary way. It saves a great deal of trouble, because the surface is finished as if it had been burnished. It is questionable whether the solution is of use to the amateur, as it does not appear to work well in very small volumes. "To make this solution, place a quart of ordinary plating solution in a special bottle, add to it four ounces of liquor ammonia, two of ether, and four of bisulphide of carbon. Shake well, and stir in an ounce or two of this mixture to every twenty gallons of plating liquid it is required to brighten about twice a day. This is done as the bath is working. All the bisulphide of carbon mixture must not be added at once, this would spoil the whole solution. As the plating goes on, a little should be stirred in after the day's work, as mentioned. It is of the greatest importance to use as little as possible of this destructive bisulphide of carbon, as it is very apt to ruin the solution altogether; and no solution should be thus 'brightened' that is required to plate ordinarily with again, as it will be unfit for the purpose." Should the spotting or discoloration of the articles indicate that too much "brightening" liquid has been used, the same author recommends as the best remedial plan, to work upon a silver cathode until the excess is worked off. Plante's method of bright silvering consists in the addition of sulphide of silver to the plating bath. Of other methods suggested for the same object, Gore (ElectroMetallurgy, 168) makes mention of sulphur and collodion; a solution of iodine and gutta percha in chloroform, which is affirmed to be more permanent in its effects than the bisulphide of carbon; also one and a half ounces each of the carbonate and the acid carbonate of potassium, added once in nine or ten days to a plating liquid containing twelve ounces of cyanide of potassium, and three and a half of silver per gallon. This authority remarks concerning these suggested substitutes, that they do not equal the bisulphide. Respecting this operation, it may be said that it is the generally expressed opinion of experienced platers that it is difficult to secure uniform results. The experience of the William Rogers Manufacturing Co., of Hartford, as expressed to the author, is to the effect that if the articles are disturbed or moved in the "bright" vat, they will plate a dirty-white color. If kept perfectly still, and all conditions are favorable, they will plate bright, but to obtain uniformly good results is difficult and uncertain, and if too much brightening solution has been added, the bath is sure to work unevenly, and to plate roughly and badly. On the other hand, the Meriden Britannia Company, in response to inquiries, state that they employ the " bright" bath regularly, and find no serious objection to its use. Gore affirms, in explanation of the peculiar behavior of the brightening solution, that the silver deposited is not the pure metal, and that he has found sulphur in it "by dissolving it in pure dilute nitric acid, determining the amount of silver, and testing for sulphuric acid in two separate portions.' Finally, it may be added that it is important to rinse the "bright" plated articles in boiling water immediately after their removal from the bright vat, and that the rinsing should be so thorough as to completely remove all traces of the brightening liquid, as otherwise the articles will quickly blacken. From all that has preceded, our readers will be able to draw the conclusion that while the use of brightening solution is attended with some difficulties and uncertainties, they may be overcome by the exercise of proper care and good judgment. |