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.
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.
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.
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.