Saturday, November 5, 2016

Silver Plating

Below is an article from Scientific America, 1866 on silver plating.

Silver Plating.
A correspondent asks for information about the above subject; we have, of course, no room for a full treatise on this matter, as it would fill a book; the following remarks will, however, place him and others on the road to the successful practice of this interesting art.
Formerly a copper plate was covered with a much thinner silver plate, and then rolled out together; in this way a very thin coating of silver covered tho copper entirely, sometimes on both sides ; of such silver-covered copper plates, different objects were manufactured, as teapots, pitchers, goblets, etc. This is still practiced ; however, to a very limited extent since the invention of the electro-plating process. The daguerreotypo plates are chiefly manufactured in this way.

Objects made of copper or brass may be coated in a simple way by a process described by Berzelius in his chemistry, by rubbing them with a chemical mixture consisting of chloride of silver, 1 part ; well dried potash, 3 parts; Paris white (very fine chalk), 1 part; common salt, a little more than 1 part. The brass surface, is well cleaned, moistened with a little salt water, and then the surface is rubbed with the above mixture till it is silvered. This is the customary way that the thermometer and barometer scales, clock dials, etc., are silvered, and it is well to cover them afterward with e colorless varnish, the process being so very economical, and the silver coating consequently so thin, that when dirty it can stand so much cleaning and polishing afterward without being removed. A iater invented method to accomplish the same purpose, and which many operators prefer, is to take 1 part of nitrate of silver and 3 parts of cyanide of potassium, rub these together and add a little water to make them into a paste ; rub this with a piece of flannel on the object to be silvered, which, however, before must have been carefully cleaned. This process is peculiarly adapted to copper and brass name-plates attached to apparatus, etc. The film of silver obtained in this way is also very thin, and it is also advisable to cover it with a colorless varnish. When calculating the price of this process after silverin g a great number of objects, it is always found to amount to only from 1 to 2 cents per square foot.

This method requires no friction whatever, and is accomplished thus: Make a saturated solution of common salt, dissolve cyanide of silver in it, and filter. A piece of clean copper or brass placed in this solution is soon covered with a silver coating which adheres very strongly.
If the object to be silvered is iron, it must first be coated with copper, as silver will not very well equally deposit on iron. This is simply accomplished by placing the iron object for a sufficient time in a diluted solution of blue vitriol (sulphate of copper) acidulated with sulphuric acid ; afterward wash and clean the object, and when it is well covered with copper, place it in the above described silver bath.
Care must bo taken not to touch the cleaned object with the fingers, as the silver will never deposit equally on the thus touched spots.

The most important way of silvering is, however, the electro-plating or galvanic process. This is founded on the fact that when an easily oxidable metal, like zinc, is placed in a properly prepared silvering liquid, it will combine with, the acids which hold the silver in solution, and the silver will be precipitated in its metallic state on any object in contact with the zinc, provided the surface of this object is a conductor of electricity. The simplest i xplanation of this curious fact, is that the action of the acids on the zinc generates electricity which repels the metallic silver from the zinc and carries it to any other metal plunged in the liquid, provided this metal is not acted on by the acid, and is in contact with the ziric, so that it may carry back the electric current to the latter metal. The metal receiving the silver deposit acts as it were like a sieve ■which lets the ethereal electric current pass through its mass, but retains on its surface the material particles of silver carried on with that current. We do not say that this is the latest and most approved explanation of the phenomenon, which, in reality, is more complex, but it is the simplest reasoning by which beginners, in this branch of study, may satisfactorily account to themselves for the curious results they observe.
In a continuation of this article, we will describe the various processes of electro-plating ; first, the simple process without battery, and, second, that with the help of galvanic batteries.

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