Tuesday, April 14, 2009

Silver Coating

Silver was one of the first metals to be electroplated from such solutions, and the electroplating industry was thus founded in the 1840s. A great incentive to its development was the ability thereby to provide an attractive appearance and the hygienic handling of foodstuffs without the expense of making a whole article out of the costly metal. For some decades prior to the introduction of silver plating this difficulty had been overcome by the use of Shefield plate, a composite sheet metal made by rolling silver and copper together. Tableware and decorative articles were then made from the sandwhich in such a way that only the silver was visible. Electroplating was able quickly to take over this market and establish itself.


Silver plating has changed little since it was introduced. The solution is traditionally used at room temperature or only slightly warm, although this limits the permissible current density to between 5 and 10 A/ft2. As however, the silver atom is heavy, this corresponds to a rate of deposition of 0.0008 to 0.0016 in/h. The silver electroplate obtained from the simple double cyanide solution in milky white and matt, but it is very smooth and soft. It can readily be color buffed or burnished to very high luster. Minor addition to the simple silver plating bath have long been known which limit the grain size of the deposit and produce a lustrous or bright coating. These are either organic sulfur compounds, typified by carbon disulfide, or small addition of other heavy metals such as lead or antimony. The increase in lustre lessen the labor of polishing, but the main advantage is that the bright deposit is harder and more wear resistant.


For house hold and table articles, silver is usually applied to one of the copper alloys and predominantly to nickel silver. Nickel silver is an alloy of copper, zinc and nickel, and is thus essentially a brass, bleached to a yellowish white color by the addition of nickel. A minimum 9% nickel is necessary to eliminate the brassy color, and 12 to 15% gives a white alloy.