Great Facts / A Popular History and Description of the Most Remarkable Inventions During the Present Century

by Frederick C. Bakewell

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.