ELECTROPLATING AND ELECTROTYPING
In our chapter on the accumulator or storage cell we saw that a current
of electricity has the power of decomposing certain liquids; that
is to say, it is able to split them up into their component parts.
This power has given rise to the important art of electroplating and
electrotyping. Electroplating is the process of depositing a coating
of a rarer metal, such as gold, silver, or nickel, upon the surface of
baser or commoner metals; and electrotyping is the copying of casts,
medals, types, and other similar objects. The fact that metals could
be deposited by the decomposition of a solution by a current was known
in the early days of the voltaic cell, but no one seems to have paid
much attention to it. An Italian chemist published in 1805 an account
of his success in coating two silver medals with gold, and some thirty
years later Bessemer transformed lead castings into fairly presentable
ornaments by coating them with copper, but commercial electroplating
may be said to have begun about 1840, when an Englishman named
Elkington took out a patent for the process. Since then the processes
of electroplating and electrotyping have rapidly come more and more
into use, until to-day they are practised on a vast scale, giving
employment to thousands.
Electroplating on a small scale is a very simple affair. A solution
of the metal which it is desired to deposit is placed in a suitable
vessel. Two metal rods are placed across the top of this vessel, and
from one of these is suspended a plate of the same metal as that in
the solution, and from the other is hung the article to receive the
coating. The positive terminal of a voltaic battery is connected to
the rod supporting the plate, and the negative terminal to the rod
carrying the article to be plated. As the current passes through the
solution from the plate to the article the solution is decomposed, and
the article receives a coating of metal. The solution through which the
current passes, and which is decomposed, is called the _electrolyte_,
and the terminal points at which the current enters and leaves the
solution are called _electrodes_. The electrode by which the current
enters the electrolyte is called the _anode_, and the one by which it
leaves is called the _cathode_.
If we wish to deposit a coating of copper on, say, an old spoon which
has been dismissed from household service, a solution of sulphate
of copper must be made up and placed in a glass or stoneware jar.
Two little rods of brass, copper, or any other good conductor are
placed across the jar, one at each side, and by means of hooks of
wire a plate of copper is hung from one rod and the spoon from the
other. The positive terminal of a battery of Daniell cells is then
connected to the anode rod which supports the copper plate, and the
negative terminal to the cathode rod carrying the spoon. The current
now commences its task of splitting up the copper-sulphate solution
into pure copper and sulphuric acid, and depositing this copper upon
the spoon. The latter is very quickly covered with a sort of “blush”
copper, and the coating grows thicker and thicker as long as the
current is kept at work. If there were no copper plate forming the
anode the process would soon come to a standstill, on account of the
copper in the electrolyte becoming used up; but as it is the sulphuric
acid separated out of the electrolyte takes copper from the plate and
combines with it to form a further supply of copper sulphate. In this
way the strength of the solution is kept up, and the copper anode
becomes smaller and smaller as the coating on the spoon increases
in thickness. It is not necessary that the anode should consist of
absolutely pure copper, because any impurities will be precipitated to
the bottom or mixed with the solution, nothing but quite pure copper
being deposited on the spoon. At the same time if the copper anode
is very impure the electrolyte quickly becomes foul, and has to be
purified or replaced by new solution.
[Illustration:
_By permission of_] [_W. Canning & Co._
FIG. 35.--Small Electroplating Outfit.]
To nickel-plate the spoon we should require a nickel plate for the
anode and a nickel solution; to silver-plate it, a silver anode and
solution, and so on. Fig. 35 shows at simple but effective arrangement
for amateur electroplating in a small way.
Electroplating on a commercial scale is of course a much more elaborate
process, but the principle remains exactly the same. Fig. 36 shows
the general arrangement of a plating shop. It is obviously extremely
important that the deposit on a plated article should be durable,
and to ensure that the coating will adhere firmly the article must be
cleaned thoroughly before being plated. Cleanliness in the ordinary
domestic sense is not sufficient, for the article must be chemically
clean. Some idea of the care required in this respect may be gained
from the fact that if the cleaned surface is touched with the hand
before being plated, the coating will strip off the parts that have
been touched. The surfaces are first cleaned mechanically, and then
chemically by immersion in solutions of acids or alkalies, the cleaning
process varying to some extent with different metals. There is also
a very interesting process of cleaning by electricity. The article
is placed in a vat fitted with anode and cathode rods, just as in an
ordinary plating vat, and containing a solution of hydrate of potash
and cyanide of potassium. The anode consists of a carbon plate, and
the article is hung from the cathode rod. Sufficient current is passed
through the solution to cause gas to be given off rapidly at the
cathode, and as this gas rises to the surface it carries with it the
grease and dirt from the article, in the form of a dirty scum. After
a short time the article becomes oxidized and discoloured, and the
current is then reversed, so that the article becomes the anode, and
the carbon plate the cathode. The current now removes the oxide from
the surface of the article, which is left quite bright and chemically
clean.
[Illustration:
_By Permission of_] [_W. Canning & Co._
FIG. 36.--General Arrangement of an Electroplating Shop.]
When thoroughly cleaned the articles are ready to be placed in the
plating vats. These vats are usually made of wood lined with chemically
pure lead, or of iron lined with enamel or cement. Anode and cathode
rods made of brass are placed across the vats, and from these the
anodes of the various metals and the articles to be plated are hung by
hooks of nickel or brass. Any number of rods may be used, according to
the size of the vat, so long as the articles have an anode on each
side. If three rods are used the articles are hung from the centre one,
and the anodes from the outside ones. If a number of small articles
are to be plated together they are often suspended in perforated metal
trays. Small articles are also plated by placing them in a perforated
barrel of wood, or wood and celluloid, which revolves in the solution.
While the articles are being plated the revolving of the barrel makes
them rub one against the other, so that they are brightly burnished.
Dog chains, cycle chain links, button-hooks, and harness fittings are
amongst the articles plated by means of the revolving barrel.
The strength of current required for different kinds of plating varies
considerably, and if the work is to be of the best quality it is very
important that the current should be exactly right for the particular
process in hand. In order to adjust it accurately variable resistances
of German silver wire are provided for each vat, the current having to
pass through the resistance before reaching the solution. The volume
and the pressure of the current are measured by amperemeters and
voltmeters attached to the resistance boards. If the intensity of the
current is too great the articles are liable to be “burnt,” when the
deposit is dark coloured and often useless.
When exceptionally irregular surfaces have to be plated it is sometimes
necessary to employ an anode of special shape, in order to keep as
uniform a distance as possible between the electrodes. If this is not
done, those parts of the surface nearest the anode get more than their
share of the current, and so they receive a thicker deposit than the
parts farther away.
An interesting process is that known as “parcel-plating,” by which
decorative coatings of different coloured metals can be deposited on
one article. For instance, if it is desired to have gold flowers on a
silver brooch, the parts which are not to be gilded are painted over
with a non-conducting varnish. When this varnish is quite dry the
brooch is placed in the gilding vat and the current sent through in the
usual way. The gold is then deposited only on the parts unprotected
by varnish, and after the gilding the varnish is easily removed by
softening it in turpentine and brushing with a bristle brush. More
elaborate combinations of different coloured metals can be made in the
same way.
Sugar basins, cream jugs, ornamental bowls, cigarette cases, and other
articles are often gilded only on the inside. The article is filled
with gold solution and connected to the cathode rod. A piece of gold
wrapped in calico is attached to the anode rod, suspended in the
solution inside the article, and moved about quickly until the deposit
is of the required thickness.
The time occupied in plating is greatly shortened by stirring or
agitating the solutions. This sets up a good circulation of the liquid,
and a continual supply of fresh solution is brought to the cathode.
At the same time the resistance to the current is decreased, and
more current may be used without fear of burning. Fig. 37 shows an
arrangement for this purpose. The solution is agitated by compressed
air, and at the same time the cathode rods are moved backwards and
forwards. Plating solutions are also frequently heated in order to
hasten the rate of deposition.
When the plating process is complete, the articles are removed from
the vat, thoroughly swilled in water, and dried. They are then ready
for finishing by polishing and burnishing, or they may be given a sort
of frosted surface. During the finishing processes the appearance of
the articles changes considerably, the rather dead-looking surface
produced by the plating giving place to the bright lustre of the
particular metal.
[Illustration:
_By Permission of_] [_W. Canning & Co._
FIG. 37.--Method of agitating solution in Plating Vat.]
It sometimes happens that an article which has been plated and polished
shows little defects here and there in the deposit. In such a case it
is not necessary to re-plate the whole article, for the defects can be
made good by a process of “doctoring.” A piece of the same metal as
that forming the deposit is placed between two pieces of wood, and a
wire fastened to one end of it. At the other end several thicknesses of
flannel are wrapped round and securely tied. This strip, which forms a
miniature anode, is connected to the anode rod of the plating vat, and
the article is connected to the cathode rod. The flannel is saturated
with the plating solution, and the strip is rubbed gently over the
defective places until the deposit formed is as thick as that on the
rest of the article. If the work is done carefully the “doctored”
portions cannot be distinguished from the rest of the surface.
Electroplating may be employed to give ships’ plates a coating of
copper to prevent barnacles from sticking to them. The work is done
in sections by building up to the side of the vessel a sort of vat of
which the plate to be coated forms one side. The plate is thus at the
same time the cathode and part of the vat.
So far we have spoken only of electroplating objects made of metal. If
we tried to copperplate a plaster cast by simply suspending it as we
did our spoon, we should get no result at all, because the plaster is
a non-conductor. But if we sprinkle plumbago over the cast so as to
give it a conducting surface, we can plate it quite well. Practically
all materials can be electroplated, but if they are non-conductors they
must be given a conducting surface in the way just described or by some
similar means. Even flowers and insects may be plated, and by giving
them first a coating of copper and then a coating of gold, delicately
beautiful results are obtained.
Electrotyping is practically the same as electroplating, except that
the coating is removed from the support on which it is deposited.
The process is largely used for copying engraved plates for printing
purposes. The plate is first rubbed over with a very weak solution
of beeswax in turpentine, to prevent the deposit from adhering to it,
and it is then placed in a copperplating vat and given a good thick
coating. The coating is then stripped off, and in this way a reversed
copy of the plate is obtained. This copy is then replaced in the vat,
and a coating of copper deposited upon it, and this coating, when
stripped off, forms an exact reproduction of the original, with every
detail faithfully preserved. An engraved plate may be copied also by
making from it a mould of plaster or composition. The surface of this
mould is then rendered conducting by sprinkling over it a quantity of
plumbago, which is well brushed into all the recesses, and a coating
of copper deposited on it. As the mould was a reversed copy of the
original, the coating formed upon it is of course an exact copy of the
plate. If the copy has to be made very quickly a preliminary deposit
of copper is chemically formed on the mould before it is placed in the
vat. This is done by pouring on to the mould a solution of sulphate of
copper, and sprinkling iron filings over the surface. The filings are
then brushed down on to the face of the mould with a fine brush, and a
chemical reaction takes place, resulting in the precipitation of copper
from the solution. After the filings have been washed away, the mould
is placed in the vat, and the deposition of copper takes place very
rapidly.
Engraved copperplates are often nickel or steel-plated to give their
surface greater hardness, so that the printer may obtain a larger
number of sharp impressions from the same plate. Stereotypes also are
coated with nickel for a similar reason.
Before the dynamo came into general use all electroplating and
electrotyping was done with current supplied by voltaic cells, and
though the dynamo is now used exclusively in large plating works,
voltaic cells are still employed for work on a very small scale. A
cell which quickly polarizes is quite useless for plating purposes,
and one giving a constant and ample supply of current is required. The
Daniell cell, which was described in Chapter IV., is used, and so also
is the Bunsen cell, which consists of a porous pot containing strong
nitric acid and a carbon rod, placed in an outer stoneware vessel
containing dilute sulphuric acid and a zinc plate. The drawback to
this cell is that it gives off very unpleasant fumes. The dynamos used
for plating work are specially constructed to give a large amount of
current at very low pressure. Continuous current only can be used, for
alternating current would undo the work as fast as it was done, making
the article alternately a cathode and an anode.