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.
[Illustration]
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.
[Illustration]
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.