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What is Electroplating?

Electroplating is the process of using electric current to coat or “plate” a layer of metal onto the surface of another material. This technique may be used to add metal that has a desirable quality, such as corrosion protection or abrasion resistance, onto a surface that lacks that property. Electroplating can also be used to make an object appear more valuable, as is often the case with gold and silver plating, or simply to make a surface more attractive.

How does the process of electroplating actually work? First, a metal salt of the metal that will be transferred to the surface (substrate) of another object is dissolved in water to produce a plating solution. In the illustration shown below it is a metal salt of silver, silver nitrate (AgNO3), that has been dissolved to create a solution of silver ions (Ag+) and nitrate ions (NO3-). Next, a solid piece of the metal that will be transferred (the same metal as in the plating solution) is placed in the solution and becomes the anode by connecting it with a wire to the positive pole of a battery. It is this solid piece of metal, along with the metal salt in the plating solution, that provide the metal ions that coat (plate) the object. That object (here a spoon) is also immersed in the plating solution and becomes the cathode by connecting it to the negative pole of the same battery. The electric circuit that is created provides the current (flow of electrons) needed to reduce the metal ions (Ag+) in the solution.

 

Punch Bowl
Punch Bowl, 1900-1915
Possibly made by Joseph Heinrichs, American (active New York and Paris)
Copper and silver with stone arrowhead decoration; interior electroplated with gold
13 1/2 x 23 3/4 x 18 inches (34.3 x 60.3 x 45.7 cm)
Purchased with the Richardson Fund and the Thomas Skelton Harrison Fund, 1993
1993-65-1
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Electroplating process:
Ag+ (in the solution) --> Ag0 (on the spoon) when an electric current is introduced to provide a flow of electrons.
 
It is at this point, when the metal ions are reduced (becoming Ag0), that the object begins to be plated with metal.

Demonstration of electroplating: copper onto silver. View related burnishing demonstration >>
Electroplating is a widely used technique because it can plate a variety of metals onto a variety of substrates to produce smooth coatings. The thickness of a coating generally can be increased as long as the electric current is flowing and there is a source of metal to be reduced (the metals commonly electroplated onto other surfaces are outlined on the periodic table). The interior of the punch bowl shown above exemplifies the luminous gold surface that can be achieved with this process.

On the frame of the late-nineteenth-century shaving mirror pictured below, the manufacturer has proudly advertised the quality of the electroplating. Banners that roll down the sides and across the top proclaim “HARD White Metal WITH QUADRUPLE PLATE OF SILVER” and “Silver Plated Ware of Highest Grade and most artistic design, Manufactured by James W. Tufts, Boston MASS.” The “white metal” refers to the substrate metal onto which the silver was plated, usually an alloy of tin, antimony, and copper. The “quadruple plate” suggests that the plating was four times thicker than “normal” silver plate.
 

Shaving Mirror (detail) 1994-10-1,
silver electroplated on white metal.
Shaving Mirror
Shaving Mirror, 1880-1890
Tufts and Company, Boston, American
White metal electroplated with silver
Height: 14 inches (35.6 cm)
Purchased with funds contributed by anonymous donors and with funds contributed in memory of William Bradley, 1994
1994-10-1
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Why would the manufacturer choose to quadruple plate an object? Because natural wear will lead to some loss of the plated metal, and in the case of silver, typical polishes remove untarnished silver along with the tarnish (surface discoloration caused by oxidation). Over time, electroplated silver coatings can be completely worn or polished away; the thicker the coating, the longer it will last. In the activities section you will find a laboratory procedure for converting tarnish back to untarnished silver without losing any of the silver plate.

Historical Use of Electroplating

Electroplating was an important technological advance in metal finishing techniques. In 1805 the Italian chemist Luigi Brugnatelli conducted the first scientific experiment that successfully plated gold onto silver. The process that Brugnatelli devised was made possible by one of his peers, Allesandro Volta, who in 1800 invented the first battery able to produce electricity from a chemical reaction. The use of electroplating as a decorative tool did not become widespread, however, until British scientists George and Henry Elkington patented a much improved procedure in 1840.

Now that we have seen that electroplating can successfully coat a metal object, we can move on to electroforming, a process that can actually form a metal object.

 

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