Gold Plating Primer
The usage of rhodium plating as a bright protective finish on decorative objects has become increasingly popular with our customers. At Metal Arts Specialties, we offer high quality Rhodium electroplated finishes for a wide array of decorative and industrial applications.
Here are some of the most frequently asked questions that we believe will help provide a basic understanding of rhodium plating and its many decorative advantages.
We also invite you to review the related technical papers on rhodium that can be found in the articles section on this site.
What is rhodium?
Rhodium is a member of the platinum metals group; the other members include platinum, palladium, ruthenium, iridium, and osmium.
When was rhodium discovered?
Rhodium was first isolated in 1803 by English chemist William Wollaston. Due to the distinctive red color of the dissolved rhodium compounds, Wollaston named the new metal Rhodium based on the Greek word rhodon meaning “rose”.
Where does rhodium occur in nature?
The primary sources for rhodium are in Russia, South Africa, and Canada. The industrial extraction of rhodium is complicated by the fact that it occurs naturally with numerous other native ores. Mixed with other metals such as platinum, silver and nickel, the separation and refinement of rhodium ores are quite expensive. Only about 9-10 metric tons are produced annually worldwide. As of this writing, its trading range on the world spot metals market are quite volatile. The costs per troy ounce of rhodium range between $1,100-$1,400.
From a decorative standpoint, what makes rhodium so desirable?
Among numerous other physical characteristics such as hardness, and high reflectivity, rhodium has an extraordinary resistance to most acids and corrosive substances. It is one of the few “white” metals that will remain bright and reflective under all atmospheric conditions at ordinary temperatures. As a result, electroplated surfaces that utilize rhodium plating remain scratch resistant, bright and attractive for years.
What were some of the early decorative usages for rhodium electroplating?
Rhodium was used as a novelty in the early 1920’s to demonstrate its chrome-like brightness to prospective customers by the plating trade. Commercially, rhodium’s presence grew quickly by the mid-1930’s. Rhodium electroplated finishes on Art Deco style cigarette lighters, writing instruments and other fashion accessories became very popular. Some excellent examples of rhodium plated products were produced by the Ronson lighter company (“Fine Line”style circa. 1934) and on fashion jewelry from companies such as Cartier. By the start of WW ll, rhodium as well as the other members of the platinum metals group were declared strategic metals and their use became restricted for military applications only. It wasn’t until the late 1950’s that rhodium plating began to re-emerge as a decorative finish.
Typically, what decorative metals are plated with rhodium?
Rhodium can be plated onto just about any metal surface. Primarily, our customers utilize rhodium to plate metals that are easily tarnished such as sterling silver, nickel, and copper alloys.
What are the typical plating thicknesses for Rhodium finishes?
The plating thickness for decorative rhodium platings used by jewelry manufacturer can range from .05 microns to 1.5 microns. For most decorative applications, a thickness from .10 microns to .50 microns are utilized for maintaining long term “brightness”. Industrial platings for specialized applications may utilize thicknesses up to 5.0 microns. However, rhodium finishes above 2.0 microns have a tendency to be brittle and will need special annealing processes to relieve the stress within the rhodium plated layer.
Will a thicker rhodium plating conceal scratches and surface blemishes?
No, as a rule the electroplating process does not hide surface imperfections. In fact, due to the diffusive reflection of incidental light rays, pre-existing cosmetic imperfections are even more noticeable on a brightly plated surface! For best results the item should be polished to the desired level of brightness prior to rhodium plating.
How scratch resistant are rhodium plated objects?
Among the “white” decorative platings, rhodium is considered “best in class” in terms of toughness and wear resistance. Most items plated with at least .5 microns of rhodium are usually more scratch resistant than prior to being plated! Electroplated rhodium has a hardness ranging from 400-550 vickers which makes it very resistant to incidental abrasions. Over sterling silver objects, rhodium plating can prevent the formation of fine scratches that occur from normal handling and cleaning.
How does the brightness of polished rhodium compare to silver?
One of the minor cosmetic trade-offs for plating rhodium onto silver is that the rhodium finish will not be as bright or reflective as the original silver finish. Rhodium reflects about 72%-80% of the light rays that strike it. Silver reflects up to 95% of all incidental light that strikes it. However, over the long term rhodium will stay “brighter” than silver because of its superior resistance to tarnishing and scratching.
Why do some rhodium plated jewelry items seem to tarnish despite being plated?
With the exception of nickel and silver, any object plated with less than .10 microns of rhodium will eventually discolor due to the corrosive oxides that are “rising” through the microscopic pores within the rhodium’s surface. This type of surface tarnish will eventually appear “yellowish’ bright to the observer and is commonly found on decorative items such as jewelry chains and bracelets. Constant contact with skin oils and chemicals help accelerate surface discoloring. Most objects properly plated(assuming “active”porosity is eliminated) with .10 to .25 microns in thickness, will remain bright well into the future.
What are the most common decorative objects that Metal Arts Specialties plates with rhodium?
Our most popular items plated with Rhodium are products made of sterling silver. Silver chains, bracelets, cuff links and holloware being the most frequently plated items. We are also plating stainless steel watch cases and men’s accessories with rhodium to replace their original chrome finishes.
Can rhodium be plated over chemically etched or sand blasted surfaces?
Yes, as long as the surface of the base metal (i.e.silver) is free of contaminants, rhodium can be successfully plated over just about any surface treatment.
What are the commercial trade designations for rhodium plated sterling silver objects?
Sterling silver items that are electroplated with a rhodium finish must still be designated as “Sterling silver” or one of its equivalent legal markings. For more detail on this topic visit the www.ftc.gov web site for specific guidelines.
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Metal Arts Specialties
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How might a piece of jewelry, that is 925 Sterling Silver, be marked if it has 5 µm of Rhodium plating?