An elaborate and very lucid article on the Electrotype and Daguerreotype,
being a review of “An Account of Experiments in Electricity made by
Thomas Spencer--Annals of Electricity, January 1840,” and of the account
of M. Daguerre’s discovery of Photogenic Drawing as published by
himself, has appeared in that excellent work “The Westminster Review”
for September. Our space not allowing us to enter so fully into details
as our admirable contemporary, we present our readers with as concise an
article as the nature of the subject will permit, confining ourselves for
the present to the Electrotype, as being less generally known, though not
less curious.
The electrotype is another instance of the application of invisible
elements to the uses of man, by which powers and influences, of whose
nature he is as yet wholly ignorant, are made subservient to his
purposes, and obedient to his rule.
To define accurately what electricity is, would be, as yet at least,
impossible. Many conjectures have been, are, and will be hazarded, but
the knowledge of its production, power, and effects, is only in its
infancy, and so full of promise of a gigantic growth, that time will be
better spent in its cultivation than in debating upon what it is.
The truth of this proposition is fully borne out by the subject of our
present paper; for whilst many scientific men have been exhausting their
energies in the production of plausible theories upon the nature of the
electric fluid, other more matter-of-fact philosophers have addressed
themselves to its application; and whilst some of these devote themselves
to the developement of its motive powers, in the well-founded hope of
its superseding steam, others press its services to far different uses.
Amongst the last, Mr Spencer holds a foremost place.
Before entering into the description of the electrotype, we must say
a few words on the subject of electricity to the less informed of our
readers. The electric fluid, as it is called, may be produced in various
ways: the most ordinary is by the friction of glass against silk, as
exemplified in the electrical machine, which is familiar to almost every
one. But galvanic and voltaic electricity is differently produced. In all
cases its production is the consequence of combination, but particularly
in the galvanic battery and voltaic circle. The latter, being Mr
Spencer’s apparatus, we shall briefly describe.
An ordinary voltaic circle is formed by a plate of zinc and another of
copper being placed upright in a vessel containing acid or a saline
solution. Zinc is more oxidisable than copper, that is, it has a greater
affinity to, or inclination to unite itself with, the gas called
oxygen, the combination of which with the particles of metal produces
that appearance which is called “rust.” Whilst the zinc and copper are
separate, the oxygen of the fluid operates upon both; but if they are
united by means of a wire connected with each, the oxygen forsakes the
copper altogether, and proceeds with increased force to unite with the
zinc, and a current of electricity is immediately formed, which proceeds
from the zinc plate through the fluid medium to the copper, thence along
the connecting wire to the zinc, and thence round again in a constant
circulating stream, until the zinc has been entirely decomposed, or
oxidised.
Electricity being thus produced by combination, its progress and effects
are marked by a wonderful power of separation or decomposition, which it
exerts upon substances brought within the circle; and this is the power
which Mr Spencer has turned to his use, the great object which he has at
present in view being the multiplication of engraved plates of copper for
the purpose of printing from.
Every person who has seen metal of any description in a state of fusion,
must have remarked that it never forms a thin fluid such as water,
capable of insinuating itself into the smallest interstices, but is what
would be called _thick_ even at the fiercest heat, consequently incapable
of entering into such fine scratches as are necessary to be accurately
and clearly defined upon an engraved plate. Again, the contraction and
expansion of all metals by the application of heat and cold, would offer
an almost insuperable bar to the utility of casting, even if the fusion
could be rendered perfect. But the application of electricity removes all
the inconveniences, and opens a new field of science.
Mr Spencer’s apparatus consists of an earthenware vessel, in which is
suspended another, much smaller, of earthenware or wood, with a bottom
formed of plaster-of-Paris. Into the larger vessel is poured a saturated
solution of copper (the copper being dissolved in sulphuric acid)
sufficient to rise up along the sides of the lesser one, which is filled
with the acid or saline solution intended to operate upon the zinc. The
plaster-of-Paris being very porous, allows the two liquids to meet in
its cells, but prevents them from mixing; by permitting them to meet,
however, the current of electricity is enabled to circulate through all.
In the larger vessel, and beneath the bottom of the smaller one, is
placed the copper plate from which the cast is to be taken, or upon which
the pattern is to be raised. It is suspended by the wire, which is to
connect it with the zinc, being fixed on the edge of the inner vessel,
in which is the zinc plate, suspended by its connecting wire. The two
wires are then brought into contact, fixed together by a screw, and the
voltaic circle is complete. The acid in the upper vessel attacks the
zinc, the electric current descends through the plaster bottom, thence
through the solution of copper, where its separating or decomposing power
is brought into operation, causing the infinitely minute particles of
copper suspended in the solution to separate from the sulphuric acid, and
descend upon the plate, through which itself proceeds to the wire, and so
round again.
Now, here is probably the most wonderful part of the process. It is only
on the copper plate that the particles of copper, disengaged from the
solution, will descend and settle. If the copper be varnished, or covered
with a coat of wax, they will not deposit themselves or go together at
all; but where they find the clean surface of the metal, they at once
not only settle, but fix and adjust themselves in their proper forms,
building up as it were a metal structure, not eccentric or uneven, but
forming a correct plate of new metal, so pure, so hard, and so free
from defect or extraneous matter, that engravers prefer copper plates
thus formed to any other for working upon. But the perfection of this
operation consists in the wonderful accuracy with which the finest lines
of the most beautiful engravings are copied: the particles which float
in the solution are so indefinitely small, that they can enter into the
finest cuts, the slightest scratches; and as they undergo no process of
heating or cooling, their form is in nowise altered.
We have already observed, that if the plate of metal be covered,
even with varnish, the particles will not descend or form upon it;
nevertheless, if some slight substance be not interposed, the depositing
particles adhere so firmly to it as to be inseparable, and it is upon
this property that one of the processes--that of engraving in relief on a
plate of copper--entirely depends for success. When a cast of an engraved
plate is required, the plate must be coated with bee’s-wax, mixed with
a little spirits of turpentine. It is laid on the plate in a lump and
melted, and when just cooling is wiped off, when, although apparently
clean, enough remains to interpose between the new and original plates,
and prevent a too strong cohesion. It is not necessary that the engraved
plate should be copper: it may be for instance lead or type metal, in
which case it need not be waxed, as the application of heat, expanding
the metals unequally, causes them at once to start asunder.
A piece of wire having been soldered to the back of the plate, its back
and edges should be covered with a double coat of thick varnish, or it
may be embedded in a box with plaster-of-Paris or Roman cement. This
precaution is necessary, to prevent the plate from being inclosed, and to
limit the deposition to a proper extent.
It may now be suspended in the apparatus, and the wires being placed in
contact, the operation begins. Particle by particle the new metal is
formed, until the plate is of sufficient thickness, when it is withdrawn,
and heat being applied, the two plates are separated, one being the exact
counterpart, in relief, of the other. Care must be taken in all cases to
change the solution of copper frequently, for by merely _adding_, the
separated particles of the sulphuric acid would accumulate to such extent
as to mar or injure the operation.
From the plate thus formed in relief, as many casts as may be required
can be obtained, by making it the mould.
To copy or multiply medals and coins the operation is very simple, for
a mould can be easily obtained by compressing the medal or coin between
two plates of milled sheet lead, and by varnishing the lead round the
impression, the deposit will be formed in the hollow only; and for
this purpose a very simple apparatus will suffice, and one that may be
very easily made. For the outer vessel an ordinary glass tumbler or
finger-bowl will answer; and for the inner, a cylindrical gas-glass,
having a bottom made of plaster-of-Paris. The solution of copper being
in the tumbler, and the acid with the zinc in the gas-glass, the mould
should be suspended by its conducting wire between the bottoms, the wire
of the zinc connected with it, and the operation will proceed. In all
cases it must be observed that the edge of the mould should be up, as, if
it be placed horizontally, extraneous substances, sinking by their own
weight, may be deposited upon it.
To produce a raised design upon a plate of copper, or as it is rather
erroneously styled, “Engraving in Relief,” the operation is thus
performed:--
The plate upon which the design is to be raised having had the conducting
wire soldered to it, is covered with a coat of wax about one-eighth of an
inch or less in thickness, and upon the surface of this coat the design
is drawn. With a graver, the end of which must be of the form of a thin
parallelogram, so as to make grooves in the wax equally broad at the
bottom as at the top, the lines of the drawing are to be carefully cut
down to the plate; care being taken that the plate is perfectly cleaned
throughout each line, and also that the grooves are not narrower at the
bottom than at the top. In order to lay the surface of the copper at
the bottom of the grooves perfectly bare, the plate must be immersed
in diluted nitric acid (three parts of water to one of acid), and the
particles of wax that may have escaped the graver are driven off by the
fumes of the acid. The plate is then placed in the apparatus, the circle
closed as before, and the operation commences. As the particles of copper
require a metallic base, they avoid the wax and seek the metal in the
grooves; they there attach themselves to it, and to each other, until the
hollows are quite filled up, when the plate is removed. If the surfaces
of the ridges thus built up be not perfectly smooth, a piece of pumice
stone or smooth flag, with water, being rubbed to them, will soon reduce
them, after which the wax can be melted and cleaned off with spirits of
turpentine; and so firm is this formation of metal thus raised, both in
the adherence of its particles to each other and to the original plate,
that it may be printed from at any ordinary printing-press.
One general remark applies to the production of electrotype copper,
and it is, that the strength and solidity of the formation depends
upon the slowness and deliberation of the process. The more slowly and
deliberately the particles separate from the solution and proceed to
their places, the more fitly they appear to take them up, and the more
firmly they adhere; whilst on the contrary, if the operation be hurried,
the metal is brittle, so much so as sometimes to powder under an ordinary
pressure. The thicker and finer the partition of plaster between the two
fluids, the more slightly are they connected, and consequently the slower
is the circulation of the electricity. The proper length of time to be
allowed for the process varies according to the nature of the work, and
the strength or solidity required. Forty-eight hours seems to be the
least time for forming a design in relief, and somewhat more than a week
for a plate with sunk lines.
The laws which govern matter are mysterious. The entire of this process
is so wonderful, that to descant upon it would be unnecessary; and, after
all, it is but another step taken upon the path of science, each advance
upon which, whilst disclosing new scenes and greater wonders, is only the
needful preliminary to another which will display yet more!