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ON THE APPLICATION OF SCIENCE TO ELECTRO
THE success of an important enterprise has frequently de
1 pended upon an apparently trivial circumstance, and it was even so with the important process of silver electro-plating in the early period of its development. In the year 1838, Messrs. G. R. & H. Elkington were engaged, commercially, in coating military and other metal ornaments with gold and silver, by immersing them in various solutions of those metals, some of which were composed of ferrocyanide of potassium, and also carbonate of potash, with the oxides of gold and silver dissolved in them.
By this process of simple immersion, although the action was electrical, the articles received only an extremely thin film or coating of the precious metals, and it was highly desirable that the thickness should be considerably increased, on account of the greater degree of durability required.
About this period, Professor Jacobi, of St. Petersburg (October, 1838), and Mr. Spencer, of Liverpool (May, 1839), published their processes of electrotype for copying engraved plates in copper by separate currents of electricity, by means of which a firm coating of copper, of considerable thickness, could be readily obtained. From the moment that this method of obtaining thick deposits of firm copper by means of a separate electric current was made known, Mr. John Wright, a surgeon of Birmingham, and Mr. Alexander Parkes, a modeller and experimentalist, in the employ of Messrs. Elkington, were constantly engaged in experiments, to obtain thick deposits of silver and gold by similar means. A great variety of liquids were tried for the purpose, but all the solutions of gold and silver operated upon gave only a thin deposit of firm metal, which, as it increased in thickness, became loose in aggregation, and assumed the character of a dark-coloured or black metallic powder, completely useless for the purpose required.
At this particular juncture, Mr. Wright met with the following passage in Scheele's “Chemical Essay" (pages 405 and 406). Speaking of the solubility of the oxides and cyanides of gold, silver, and copper, it says, that “if after these calces” (i.c., the cyanides or ferrocyanides of gold and silver) “have been precipitated, a sufficient quantity of precipitating liquor be added, in order to redissolve them, the solution remains clear in the open air, and in this state the aerial acid” (i. e., the carbonic acid of the atmosphere) “does not precipitate the metallic calx.”
This liquid, “the precipitating liquor," is obtained by burning dried blood with potash in an iron vessel, cooling the melted product, and dissolving out its soluble portion by water; the resulting solution contains cyanide and ferrocyanide of potassium, the latter of which is commonly known as " yellow prussiate of potash.”
Mr. Wright had been working during several months, expressly to obtain a thick deposit of silver, with the aid of a separate electric current, before he met with the above passage. He then took a solution composed of chloride of silver, dissolved in an aqueous solution of ferrocyanide of potassium, and succeeded in obtaining what had never been obtained before,a thick deposit or coating of firm silver by electrolytic action,
The first article that received the successful coating was a small vase, which was coated at Mr. Wright's residence, and the next was a figure of a small kid. The mode of proceeding adopted by him in these cases, was to place the article in the silver solution contained in a porous vessel (a common garden-pot), immerse the porous cell in an outer vessel containing dilute sulphuric acid, place a cylinder of sheet-zinc around the porous vessel in the dilute acid, and connect it by means of a copper wire with the article to be coated; the electrical action then commenced, and the article gradually acquired a thick coating of silver : this method is commonly known as the “single cell” process.* It was about a month after this when a solution of actual cyanide of silver and potassium, the substance now universally used for electroplating, was first employed by Mr. Wright for the purpose, although cyanides, in several forms, had been used both for coppering and silvering by simple immersion, without the aid of a battery or zinc, about sixteen months previously.
The above-mentioned successful results were immediately submitted to Messrs. Elkington, who made arrangements with Mr. Wright for the security of the discovery and invention, and patented it on March 25th, 1840; this patent was the basis of all successful electro-silver-plating and gilding. A handsome remuneration was made to Mr. Wright by this firm for his invention, and an annuity paid to his widow (afterwards
and the ar
after this the “singling of silver
* See Plate V. fig. 1, and explanation at end of article.
patention was ways the sub earliest perioder. Wright of nick
the wife of Mr. Cammell, of Sheffield), after his death. A separate battery, to generate the electric current, was not employed until January, 1841, nearly a year after the date of the patent; and, in the early stages of the process, the silver solution was employed hot; now it is always used cold. Nickel silver was always the substance employed as a base for receiving the deposit from the earliest period of electro-silver-plating, or at least soon after the date of Mr. Wright's patent. The edges of articles are made of the best quality of nickel silver, which is very white and very hard, suitable for resisting friction, whilst the body part, or mass of the article, is made of a commoner quality, not only because it is less expensive, but also because it is worked more easily.
Electro-deposition of the precious metals was theoretically accomplished long before cyanide of potassium was known; as early as the year 1805, Brugnatelli "gilt, in a complete manner, two large silver medals, by bringing them into communication, by means of a steel wire, with the negative pole of a voltaic pile, and keeping them, one after the other, immersed in ammoniuret of gold, newly made and well saturated.” Wo should therefore not condemn theoretical science, because we are not able, even with fair and persevering trial, to apply it to any useful purpose, but wait patiently until circumstances ripen for its application. Many discoveries and inventions which are inapplicable in one state of knowledge, become applicable by the progress of science and art; the idea of electro-gilding, first attempted in 1805, by means of a solution of ammoniuret of gold, had to await the discovery of cyanide of potassium, in 1815, before it could be practically applied ; and the idea of an electric telegraph, first attempted by the aid of frictional electricity, had to abide the development of the voltaic battery, and the discovery of electro-magnetism, before it could be successfully carried out.
A great number of experiments had to be made, and many serious difficulties to be surmounted, before practical and remuncrative electro-plating was an accomplished fact. One of the chief difficulties consisted in making the silver adhere firmly to all parts of the underlying metal; this want of adhesion arose partly from the employment of too many cells in the battery, and partly from the use of too strong mercurial solution in preparing the surface, and was eventually overcome by lessening the number of cells, and dipping the previously cleaned articles in a very weak solution of mercury immediately before placing them in the plating liquid. If the deposit of silver is not firmly attached to the whole of the surface of the article, it is apt to rise in blisters and peel off when the articles are subjected to the after-process of burnishing. This non-adhesion of the silver was particularly apt to occur with articles made of Britannia metal, and with this particular alloy it was not overcome for several years; it was then effected by first coating the articles with copper, by electro process, in a solution composed of sesquicyanide of copper dissolved in an aqueous solution of cyanide of potassium, a liquid invented by Mr. Wright for the purpose of coppering articles of iron previous to silvering them, and patented in September, 1841. The method now in use for coating Britannia metal is different; it consists in first forming a thin deposit of silver upon the article, by a powerful battery, in a solution containing but little cyanide of silver and much cyanide of potassium, thus transferring the article to the ordinary silver-plating liquid, and completing the deposit therein in the ordinary manner: this process was first employed by Mr. Thomas Fearn, from whom it was purchased by Messrs. Elkington for a considerable sum of money.
Another difficulty arose in a tendency of the deposited silver to assume a granular or semi-crystalline state upon the surface of the articles, especially at their edges, during the process of deposition ; this was overcome by employing proper proportions of the ingredients in the plating solution, and carefully adapting the power of the battery to the size of the articles. In addition to these difficulties another was experienced; the articles after being plated and finished, in a few months became much discoloured: this was a consequence of too rapid deposition. And a still more serious difficulty, which required several years to surmont, arose from the opposition of the manufacturers of plated wares of Sheffield to the new method of plating; they objected to take licenses for the new process; but now the electric method is the only one they employ. A dispute also arose between Messrs. Elkington, of Birmingham, and Ruolz, of Paris, the latter having obtained a knowledge of the process and taken out a patent for France a short time before Messrs. Elkington, and a trial at law resulted, in which it was established that Messrs. Elkington were the original patentees of the cyanide of potassium solution, and it was finally settled by a compromise between Messrs. Ruolz and the patentees for the use of the process. In consequence of these and other difficulties, it was at least seven years before it became both practical and remunerative.
In electro-plating the deposited metal spreads as readily over the most elaborately engraved or figured surfaces as over the plainest forms, and in consequence of this property the new process caused a new and great extension of trade in plated articles to spring up, because articles of complicated forms, or with elaborate designs upon them, could not be made by the old method without very great expense. By the new method they were first cast of the requisite shape in German silver, and then coated with the precious metals, whereas by the old plan they required to be made from flat sheets of copper previously plated with silver by fusion; the different parts of a complex figure were first stamped into the requisite forms separately, and then the various pieces soldered together to make the entire figure; and thus a figure which could be made entire in a single piece by the new mode, required to be formed of several or many pieces by the old one, and after a moderate amount of wear the lines where the parts were soldered together became visible, and greatly disfigured the object, and those lines could not again be covered with silver. By the old plan, every portion of a figure which was “undercut,” i.e. in which the external parts overhung the internal ones, as the mouth or ear of an animal, for example, required to be made of several pieces, whilst by the new method such parts could be made entirely in one piece with the whole figure, and be coated with the precious metals all over, without any seam or joining. A great scope for the extension of beauty and taste in designs of metallic figures and vessels thus commenced, which has gradually extended itself to electro-plated articles of very moderate price, such as tea-pots, coffee-pots, cream-jugs, sugar-basins, &c., the base of which consists of Britannia metal; and the electro process has thus enabled persons of limited incomes to enjoy the use of articles of elegant design previously inaccessible even to the wealthy.
The next event of importance in the history of electro-plating consisted in the application of magneto-electricity instead of electricity from a voltaic-battery to depositing purposes. In August, 1842, J. S. Woolrich took out a patent for the use of a magneto-electric machine instead of a voltaic battery for electro-plating. This machine, which is in use at Mr. Fearn's electro-gilding works, Birmingham, and various other places, consists of a revolving wheel containing at its outer edge a number of short bars of soft iron, upon which are wound coils of insulated copper wires, giving to the bars the appearance of a series of reels; the wheel is surrounded by a set of powerful steel magnets, which are immovable, and fixed in a case, and have their ends, or poles, all pointing towards the wheel, so that as the wheel revolves, each of the reels of wire with its iron core, passes in succession between and very close to the poles of each magnet. As each of these coils approaches a magnet, a current of electricity is developed in one direction, and as it leaves the magnet a current is produced in the opposite direction, and similarly with the whole of the coils. All the corresponding ends of the coils are connected with the axle of the wheel, from which the positive electricity of all the