ELECTRO-METALLURGY.
The electrotype, electro-gilding and plating, and the other applications of the deposition of metals from their solutions, by the agency of voltaic electricity, had their origin in the chance observation of peculiarities in frequently repeated experiments. In this, as in most other inventions, there are contending claimants for priority; but there is little merit due to any of the first discoverers of the process, who seem to have been guided altogether by accident. It seems strange, now, on observing the extensive use that is made of the deposition of metals, that it should have remained so long unapplied after the principle had been known.
The "revivification," as it was called, of metals from their solutions by voltaic electricity, was known at the beginning of the present century; for, in 1805, Brugnatelli, an Italian chemist, gilded a silver medal by connecting it with the negative pole of a voltaic battery, when immersed in a solution of ammoniuret of gold. It did not occur to him, however, that any use could be made of that mode of gilding, and the experiment had no result.
Nothing further was done, even experimentally, towards advancing the art of electrotyping, until Mr. Spencer, of Liverpool, when experimenting with a Daniell's battery, in 1837, accidentally coated a penny piece with copper; and when the thin sheet of metal was removed, he found on it an exact counterpart of the head and letters of the coin. Even this did not suggest any useful application; nor was it until, by a further accident, a drop of varnish fell on the copper of the negative pole, and showed that no deposition took place on the part so covered, that the idea occurred to him of turning the deposition of the copper to account. The method he adopted of doing so was to cover a copper plate with varnish or wax, and to etch a design through the covering. By then exposing the plate to the action of a solution of sulphate of copper, when in connection with the negative pole of a voltaic battery, the metal was deposited in the lines drawn through the varnish, and a design in relief was fixed to the copper. This slight advance in the art was not made known until it was announced, in 1839, that Professor Jacobi, of St. Petersburg, had made application of the same process. Mr. Spencer, indeed, was forestalled, even in this country, by Mr. Jordan, a printer, who published an account in the Mechanics' Magazine for May, 1839, of a method of making copper casts by the deposition of copper from its solution. In the autumn of the same year, however, Mr. Spencer exhibited to the British Association several more perfect specimens of electrotyping, that showed the process might be rendered valuable; and from that time rapid progress was made in bringing it into practical operation in a variety of ways.
The deposition of copper from its solution, when under the action of voltaic electricity, is not produced by the decomposition of the sulphate of copper, as might be supposed, but by the decomposition of the water that acts as the solvent of the metallic salt. Thus, when two platinum wires from the poles of a voltaic battery are introduced into acidulated water, hydrogen gas is disengaged at the wire connected with the negative pole, and oxygen at the other; but when a solution of sulphate of copper is substituted for water, the hydrogen that is disengaged combines with the oxygen that held the copper in solution, and the metal is liberated. The copper thus liberated from its combination with the oxygen is deposited, in a pure metallic state, on the surface connected with the negative pole of the battery.
The simplest illustration of electro-metallic deposition is obtained by immersing a silver spoon and a strip of zinc into a solution of sulphate of copper. So long as the two metals are kept apart, no change takes place on the silver, but on bringing them into contact, voltaic action immediately commences, and a coating of copper is deposited upon the spoon, and adheres firmly to the metal. If the action be continued, and the supply of copper be maintained by the addition of fresh crystals of the sulphate, the coat of copper may be increased in thickness to almost any extent.
The first applications of the discovery were directed to the copying of medals and coins. An impression of the metal was obtained in fusible metal, which is an alloy composed of tin, lead, and bismuth, melted together in the proportions of two of the latter to one each of the former. This alloy expands on cooling, and thus affords a very sharp impression of the medals; and as it melts at a low temperature, it may be easily removed after the copper coating has been deposited upon it.
An electrotype mould, obtained directly from the medal, is, however, more sharp in its definition than an impression, and is therefore preferable, when circumstances admit of its being so taken. For that purpose, the surface whereon the deposition is to be made is smeared over with sweet oil, or with black lead. It is then carefully wiped with cotton wool, but a minute quantity of the oil will still remain, sufficient to prevent the metal from adhering.
A simple form of apparatus for the electrotype process is shown in the accompanying diagram.
An earthenware jar, a, serves to hold the solution of copper, which should be maintained in a saturated state by the addition of crystals of the salt. A porous tube, b, holds a rod of amalgamated zinc, to the top of which a binding-screw is soldered. The copper mould or medal, c, is suspended in the solution by a wire, which is held tight by the binding-screw, d. The porous jar is then filled to the same height as the copper solution in the jar with diluted sulphuric acid, in the proportions of one of acid to twenty of water.
Voltaic action immediately commences, and the copper will continue to be deposited from the solution as long as the supply of fresh crystals of sulphate of copper is continued. In about twenty-four hours the coating of copper will be as thick as a thin card, and it may be then removed. When detached from the medal, it will be found to be an exact counterpart, in the minutest details, of the original; those parts of the medal which are in relief being, of course, the reverse in the mould.
The extreme minuteness and delicacy of the electrotype process is strikingly exemplified in its application to the transference of engraved copper-plates. A highly finished engraved copper-plate has a film of metal deposited over its whole surface, which, when detached, exhibits all the lines that are cut into the copper-plate in relief. That electrotype cast then serves as the mould for further depositions, in which every line in the original engraving is so perfectly developed, that it is impossible to detect a difference in the impressions taken from the two plates. By this means any number of casts may be made and worked from, whilst the original is preserved uninjured. The objection to this application is that the metal deposited is not so hard as the hammered plates, and will not, therefore, bear the wear and tear of copper-plate printing so well as the plates made by hand.
It was at one time supposed that the depositing of metal on surfaces, by voltaic action, might be applied to the manufacture of numerous kinds of copper articles without manual labour. For this purpose, casts were made of plaster of Paris, which were covered with black lead, to give them the property of conducting electricity, and the metal was then deposited upon them. But, independently of the practical difficulties attending the operation, it was found that the metal was not sufficiently hard, and the cost of the requisite voltaic batteries rendered the economy of the process questionable.
One of the successful applications of Electro-Metallurgy is founded on the original application of it by Mr. Spencer. As already stated, he covered metal plates with wax, and after scratching through the coating, and exposing the metal, he submitted the plate to voltaic action in a solution of sulphate of copper, and thus obtained a representation, in relief, of the figures cut through the wax; but he does not seem to have thought of the application of this mode of deposition, since adopted, by which engravings in relief are obtained, and printed from with the ordinary letter-press, in the same manner as woodcuts. The name given to this new art is "Glyphography," and it is used with great advantage when the effect of copper-plate engraving is required; for cross lines, which are difficult to cut in wood, can be worked by this method with as great facility as in copper-plate etching.
Another application of Electro-Metallurgy that promises to be useful, is the coating of glass and earthenware vessels with copper, so as to enable them to be placed over the fire without being cracked. A glass sauce-pan might thus be made, which, protected by metal covering, would neither break nor crack when placed upon the fire, because the metal would diffuse the heat over the whole surface, and prevent the unequal expansion of the vessel, which is the cause of the cracking of glass and earthenware when placed upon the fire. A patent was granted last year for a mode of coating earthenware vessels with copper or iron by electro-chemical deposition. The earthenware is first covered either with copper leaf or with bronze powder, to obtain an electrical conducting surface on which the copper can be deposited, and the vessel is then placed in a solution of sulphate of copper, and put in connection with the negative pole of a voltaic battery.
The electrotype is frequently applied with advantage to the preservation and multiplication of objects of art and natural productions, for even the forms of flowers may be in this manner rendered durable; but the most important use that has been made of the process is in plating and gilding. To effect that object, it is necessary to employ a voltaic battery separated from the vessel in which the decomposition takes place. The annexed diagram shows an arrangement of this kind. A single cell of a Daniell's battery, a, is connected by wires from its positive and negative poles, with metal rods placed across the decomposition cell, b. The articles to be plated are suspended by wires from the metal rod, f, and a plate of silver is attached to the rod, e. Thus, when the vessel is filled with the silvering liquid, a voltaic current is established, and the deposition is effected on the articles connected with the negative pole.
The menstruum best adapted for electro-plating is a solution of silver in cyanide of potassium. During the process of deposition, the same quantity of metal that is deposited from the liquid on the objects connected with the negative pole of the battery is restored to it, by dissolving an equal quantity from the silver plate connected with the positive pole, and in this manner the solution is maintained at the same strength. Any thickness of silver may be deposited by continuing the process, but about one ounce and a half to a square foot of surface is considered a full quantity. Those parts on which no silver is required to be deposited are covered with varnish or wax, which protects them from the voltaic action.
Where the operation of electro-plating is carried on extensively, decomposing troughs, holding nearly 300 gallons, are employed, and the silver plates in a single trough expose a surface of thirty square feet to the dissolving action of the menstruum under the influence of the voltaic battery.
By the aid of electro-plating the most elaborate designs of the artist in metal can be covered with silver on every part; and a group, finely engraved in copper, may be made to resemble in every particular a work cut out of solid silver. The metal is usually deposited in a granulated state, resembling "frosted" silver, and the parts required to be bright are subsequently burnished; but by the addition of a few drops of the sulphuret of carbon to the solution, the silver may be precipitated perfectly bright.