In 2000, people around the
world celebrated the millennium. In 2003, we in the jewelry industry have
a cause for celebration: This year marks the 200th anniversary of the
discovery of rhodium-a brilliant, highly reflective, silvery white metal
that is completely resistant to corrosion and tarnish.
William Hyde Wollaston, an English chemist and physicist, discovered
both palladium and rhodium in 1803 while studying platinum. However, it
wasn't until the 1930s that rhodium was first electroplated to produce
a hard, bright-white finish on white gold jewelry.
Since then, plenty of information has been published about how rhodium
plating solutions should behave under ideal conditions-but little data
and real-world information exists for manufacturers and bench jewelers
to apply to their own plating operations. Consequently, we have found
that too many of them do not plate properly: they immerse pieces in the
plating solution for an insufficient amount of time, producing a very
thin deposit that will eventually wear off.
That's where our tribute to the great white metal comes into the picture:
What better way to pay homage to Wollaston's monumental discovery than
with some original research into rhodium plating techniques? Over the
past several months, the team at Red Sky Plating Corp. in Albuquerque,
New Mexico, conducted a series of plating experiments, the data from which
appears on the following pages. Our findings should be helpful to anyone
involved in rhodium plating.
The Plating ProcessBefore sharing the results of our research, we'd like to offer an overview
of the plating process in general. Plating solutions for decorative rhodium
are typically solutions of sulfuric acid, phosphoric acid, or a combination
of both, and they contain about 8 grams of rhodium per gallon. One of
the more popular formulas used in the jewelry industry is a combination
bath with 50 ml of sulfuric acid and 100 ml of phosphoric acid per gallon.
(This is the formulation that was used for this study.) The rhodium solution
should be maintained at an operating temperature of between 900F/320C
and 1150F/460C. Operating voltage should be between
1 volt and 5 volts, and current density should be between 0.15 amps and
0.4 amps per square inch (ASI).
Preparation of the jewelry items is the first step toward good results.
As in all plating processes, the appearance of the rhodium finish will
depend largely on the quality of the initial surface. The items should
be polished to a high luster, with all scratches and surface imperfections
removed. Very small scratches are much more visible after plating because
of the highly reflective nature of rhodium.
After polishing, it's essential to remove all polishing compounds and
other soil from the jewelry items. The importance of this step cannot
be over-emphasized. Even a small amount of contamination from dirt or
polishing compounds can ruin the rhodium solution. A good way to remove
the polishing compound is by using an ultrasonic cleaner with a cleaning
solution designed for this purpose.
Once the jewelry is clean, it must be activated. Activation requires
two process baths, plus rinsing. The first process involves immersion
in electrocleaner, a heated bath usually operated at 1500F/650C.
Either a steel anode or a steel container that also acts as the anode
is attached to the positive pole on the rectifier (DC power source). The
item to be plated is then attached to the negative pole and immersed into
the electrocleaner solution for one minute at 5 volts. Hydrogen gas bubbles
form rapidly to help clean and activate the surface. (This solution is
not designed to remove polishing compound or other heavy soil.) The item
must be rinsed thoroughly after electrocleaning.
The second activation step is an acid dip. There are some good proprietary
solutions on the market, but you can also use a solution of 10 percent
sulfuric acid to 90 percent water. Remember to always pour acid into water
while stirring; never pour water into acid.
The item is immersed in this bath for one minute. This step should not
be skipped. The acid not only helps to further activate the surface, but
also removes any remaining electrocleaner, which is otherwise very hard
to rinse off completely. Small amounts of electrocleaner introduced into
the rhodium bath will raise the bath's pH and possibly cause the rhodium
metal to precipitate from the solution. The pH must remain below 2 in
a rhodium plating bath.
After one minute of immersion in the acid bath, the item should be rinsed
with distilled or deionized water. Tap water should not be used, as it
will contaminate the rhodium.
It is a good practice to check for "water break" as the piece
of jewelry is removed from the rinse water following the acid dip. This
simple test can be done by observing the piece to see if water is wetting
the entire surface evenly and not breaking away in areas. Water breaks
indicate the surface is not clean and active. If water breaks are detected,
the process must be started over from the electrocleaning step. If the
water looks even, the item is ready for plating
The rhodium plating solution should be placed in a Pyrex-type glass container
or, for larger baths, a well-leached polypropylene container (a plastic
container treated to remove contaminants). The container is then heated
to 900F/320C. A platinum- or platinum-coated titanium
anode is attached to the positive pole on the rectifier, and the power
is set to 2.5 volts. The jewelry item is then attached to the negative
pole and immersed in the solution for one minute to get a thickness of
5 microinches of rhodium. After rinsing and drying, the piece is successfully
rhodium plated. (Note: Deionized or distilled rinse water used exclusively
to rinse rhodium plated items can be poured back into the rhodium plating
bath to replace water lost through evaporation and drag out.)
In the "water break"
test, the piece is ready for plating if the water covers the surface
evenly (left), as opposed to beading up (right).
||This test piece is being plated in a 500
ml beaker, commonly used by bench jewelers for small-scale plating.
Original ResearchOur plating tests were conducted with solution temperatures of 1150F/460C
and 900F/320C. The test pieces we used were cut
from 14k gold sheet. Each piece is a one inch square with a small hole
in one corner, giving us two square inches of surface area. The advantages
to these pieces are:
Besides temperature, the other variables were plating time and current
density. Although the voltage values would be somewhat higher if you were
performing a larger operation, the current density data is valid regardless
of bath size.
- They are easy to make.
- They are easy to measure for surface area.
- They are of a material that is commonly rhodium plated.
- The chosen shape has both very high and very low current density areas.
Current density, which can be expressed in amps per square inch (ASI),
varies according to the geometry of an item being plated. Generally, the
higher the density, the more metal deposited. A flat square shape, such
as the one we tested, has very high current density at the corners, and
a little less along the very edge. The current density drops gradually
as you move toward the center. To put this in the context of a familiar
piece of jewelry, think of a ladies' solitaire diamond ring. It has a
very high current density at the top of each prong, a lower density on
the outside area of the shank, and an even lower density on the inside
of the ring. The rhodium plates thicker at high current densities, so
the top of the prongs will receive the most metal.
To obtain basic information about the rhodium solution we used, we plated
a 3 inch by 4 inch test panel in a standard 267ml Hull Cell at 3 amps
for 3 minutes. (A Hull Cell is a miniature plating unit designed to produce
a cathode deposit that records the character of electroplate at all current
densities within the operating range.) The resulting piece has a matte
haze beginning at the high current edge, which becomes less prominent
towards the right. The haze becomes gradually less apparent at about 0.4
ASI, but is still somewhat visible down to about 0.2 ASI. Thickness readings
were 15 microinches at 0.4 ASI, 12 microinches at 0.2 ASI, and 7 microinches
at 0.1 ASI.
Armed with this information, further tests were conducted using a 500
ml beaker, such as a jeweler would use at the bench. Initially, the test
pieces were plated at higher current densities, which were decreased in
subsequent tests, as shown in the charts on page 27. After 15 seconds,
the first piece looked bright to the naked eye, but a slight haze was
visible at the corners with a 10-power loupe. The second piece was plated
at the same current for 1 minute and exhibited haze visible without a
loupe all along the edge. This haze is the result of the current being
too high; it is referred to as "burning." The pieces plated
at 0.3 ASI remained bright to a respectable thickness.
Once a current was found that showed no burning, the time was increased
at both temperatures to find the limit of thickness while maintaining
a haze-free deposit. The haze on piece number five is the result of the
thickness and not from burning. In general, rhodium will stay bright to
about 10 microinches.
Two methods of thickness measurement were used: Direct readings were
made using X-ray fluorescence, and an average thickness was calculated
using a mathematical formula: weight of the rhodium deposit in milligrams
times five, divided by the area of the piece in square inches [(5 x weight)/area].
This average is designated on the charts as WtTh. (All thickness figures
are in microinches.)
Using the test results to plate a ringWe can conclude from these tests that 2.5 volts is the optimum power
setting, within a specific time period, for jewelry pieces with surface
areas similar to the test samples, at which settings, prongs, and other
protrusions are not likely to burn. The tests also reveal that rhodium
plates slower at 900F/320C than at a higher temperature,
but the deposit stays haze-free to a greater thickness.
Using this information, we plated a 14k gold ladies' solitaire ring,
size 7 1/2, set with a 6.5 mm CZ and having a surface area of 0.8 square
inch. We plated the ring for one minute at 900F/320C
and at 2.5 volts. The thickness results ranged from 5.8 to 26.8 microinches,
with the highest reading taken at the tip of one prong. The average thickness
was 8.1 microinches, based on the rhodium weight of 0.0013 grams, and
the ring looked bright and white. According to this data, we could plate
380 rings of the same size using one pint (500 ml) of rhodium solution
containing one gram of rhodium metal.
Many manufacturers and bench jewelers today are plating rhodium for 15
seconds, producing a very thin deposit that will eventually wear off.
The data collected in this article shows that a longer immersion period,
coupled with the proper voltage and temperature, can result in a more
durable, brighter, and whiter second skin.