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.