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]