The fascination with Platinum has endured down through the ages. Civilizations have coveted the dense, durable white metal and used it for religious
adornment and to demonstrate power and wealth. Today the fascination with Platinum still exists; it is no longer reserved for royalty and is much more readily available to the general public. Platinum is used more commonly for bridal
jewelry, as 25% of the fine jewelry manufactured today for weddings is made from Platinum alloys.
Since the 1950-1960s, mass media finishing of precious metal jewelry has grown in popularity in proportion to the demand from the
public for more fine jewelry. Critical manufacturing time and labor choices now must be made regarding whether to mass media finish or to hand polish the jewelry item. Improved surface quality of Platinum castings has expanded the
possibilities for mass media techniques to now be used on Platinum. It has been said that there is no one-size-fits-all approach to mass media finishing and that each pattern style must be evaluated individually before the final finishing
technique is chosen.
Project Objective
In the conception of this project we asked some questions about what metal characteristics make particular Platinum alloys more popular than others for jewelry manufacturing. It was
speculated that one factor might be the color and look of the finished jewelry's Platinum surface. With the assistance of Jurgen Maerz, Platinum Guild International USA, one of the first tasks was to choose the most popular Platinum alloys
on the market today and to obtain samples of these alloys for test purposes. The three Platinum alloys chosen were 90/10 Iridium Platinum, 95/5 Ruthenium Platinum, and 95/5 Cobalt Platinum.
The key to identifying the particular
characteristics that appeared to be desirable was to see if we could measure the surface brightness of a Platinum alloy and compare the surface qualities of several popular Platinum alloys to each other.
We chose to use a mass
media finishing model developed by Manfried Dreher for his 1991 Santa Fe Symposium paper on the Comparative Study of Metal Finishing Techniques on Standard Samples of Cast Gold. The model was developed by Mr. Dreher to represent the
all-different surface configurations found in jewelry designs and especially all of the surface challenges present in jewelry patterns that are typically mass media finished.
The mold and control patterns were sent to Dr. Helmut
Frye of TechForm Advanced Casting in Seattle, Washington to be cast.
Dr. Frye cast three control patterns in each of the different Platinum alloys. The raw castings were bead-blasted to remove the casting skin surfaces and to give the casting all the same surface finishes to start with.
We went to
Minolta and asked to use their best test equipment for measuring light reflectivity. Minolta loaned us a Minolta Luminance meter, model #LS-110. A lightproof box was then designed in which to conduct the experiment.
Inside of the box, a matte black magnetic tool-holding fixture was added to hold an optic light source securely in place. The beam of the illuminator was positioned so that it aimed a beam that reflected directly off the test pattern and back up to a target in the dead center of the lens of the Luminance meter.
Once the experimental equipment was deemed ready and all initial light readings were documented on the bead blasted Platinum test patterns, then it was time to progress with the finishing and continual measuring of the reflectivity
of the pattern surfaces.
Equipment Selection
We elected to use a vibratory machine for the cut down and dry polish steps and a rotary barrel machine for the wet burnishing step. The goal of our research was to provide
useful data to the largest population as possible, and a very limited sample uses high-energy equipment like the centrifugal disc. The steel burnishing processes were completed in a rotary barrel machine.
Compounds
Compound selection was based on products that were readily available and had minimal or no chemical reaction with the workpiece. The compound used in all cut-down processes is pH balanced and the compound used in the burnishing processes
is slightly acidic.
Media
Media selection was limited to those media that have been tested and proven effective in other nonferrous applications. The cut-down media was selected to emulate a course, medium, and fine
cut. The selected dry polish material was Green Buff. This material uses chrome oxide and alumina as polishing agents carried by walnut shell and corncob.
Processes
The term process was used in this experiment to represent
the sequence of finishing steps selected to achieve surface refinement. The selected processes were chosen from previously successful runs of nonferrous applications. Measurements were taken at predetermined times throughout each process.
Measurements
The measurements for this experiment include the reflective reading from the front of the sample (representing the outside dimensions of the workpiece), the reflective reading from the back of the sample
(representing the inside dimensions of the workpiece), the current, and the weight.
Data
The Platinum alloy samples were given an alphanumeric label identifying the sample number and the process type performed on the
sample.
TEST 1
VALIDATION TEST
The first test was conducted for validation of experimental technique. All variables were held constant including source, alloy, and process.
TEST 2
SAMPLE COMPARATIVE
This test was conducted to compare the front of the sample representing the outer dimensions of the workpiece to the back of the sample representing the inside of the same piece. After review of this data we
elected to concentrate our remaining testing on the front reflective value since the back value did not appear to be as sensitive to the experiment. This may be caused by our methodology or for the simple fact that mass media finishing is
not as effective on inside surfaces.
TEST 3
SOURCE COMPARATIVE
This test was conducted to compare metal suppliers and their impact on the ability to finish out the samples. The alloy and process were held
constant with the only variable being metal source.
TEST 4
ALLOY COMPARATIVE
This test was conducted to compare metal alloy. The PtIr (Iridium) alloy compared to the PtRu (Ruthenium) alloy. The metal source
and process were held constant with the only variable being metal alloy.
TEST 5
ALLOY COMPARATIVE
This test was conducted to compare metal alloy. The PtIr (Iridium) alloy compared
to the PtRu (Ruthenium)
alloy. The metal source and process were held constant with the only variable being metal alloy. Although the process was held constant in Test 5, it was changed from Test 4 to study the effects of a different process on the same two alloys
TEST 6
ALLOY COMPARATIVE
This test was conducted to compare metal alloy. The PtCo (Cobalt) alloy was compared to the PtRu (Ruthenium) alloy.
The metal source and process were held constant with the only variable being metal alloy.
TEST 7
ALLOY COMPARATIVE
This test was conducted to compare metal alloy. The PtCo (Cobalt) alloy was compared to the
PtCoCu alloy and the PtCoS+ alloy. The metal source was also a variable in the test due to limited sample size. The process was held constant.
TEST 8
PROCESS COMPARATIVE
This test was conducted to measure the
effect of different finishing processes. The alloy (PtIr) was held constant with variable process. The cut down cycles for each process is the same changing only the final polish or burnish.
TEST 9
PROCESS COMPARATIVE
This test was conducted to measure the effect of different finishing processes. This time the tested alloy was PtRu and held constant with variable process. The cut down cycles for each process is the same
changing only the final polish or burnish.
TEST 10
PROCESS COMPARATIVE
This test was conducted to measure the effect of different finishing processes. The selected alloy was PtRu and held constant with the
variable process of work hardening and its effect on the finish.
TEST 11
PROCESS COMPARATIVE
This test was conducted to measure the effect of different finishing processes. The selected alloy was PtIr and
held constant with the variable process of eliminating the fine cut plastic (FCP) step and its effect on the finish.
TEST 12
ALLOY/ PROCESS COMPARATIVE
This test was conducted to compare alloys (PtIr and
PtRu) when processed through a complete finishing cycle. This cycle includes a dry polish after the steel burnish.
CONCLUSIONS
The initial validation exercise (test 1) and the sample comparative (test 2) let us believe our
methodology was sound for the front of the sample. The back of the sample representing the inside dimensions of the workpiece did not provide the sensitivity required to draw conclusions. This may have been caused by our selected
target area or the simple fact mass media processes are not as effective on inside surfaces. These results were not unique to Platinum. We found similar results with sterling silver samples.
Therefore we concentrated the remaining studies on the front of the sample representing the outside dimensions of the workpiece.
Test 3 provided data comparing the finish acquired when measuring samples from three different metal
sources. We were able to conclude metal source does have an impact on the quality of finish. For a given alloy and a given process, the finished results can vary up to 45%.
The next set of tests compared alloys and their impact on the ability to finish sample work- pieces. The primary alloys tested were Platinum/Iridium (PtIr), Platinum/ Ruthenium (PtRu), and Platinum/Cobalt (PtCo). Test 4 compared PtIr with PtRu utilizing a typical cut down process and finishing out with a steel burnish. The PtRu alloy achieves a much higher reflective value when a steel burnish is implemented. Test 5 compared the same alloys replacing the steel burnish with a dry polish and resulted in a much higher reflective value for the PtIr alloy. Since the PtRu sample performed the best with a steel burnish step, we chose to compare this alloy with the PtCo in a process initiating the steel burnish as a work hardening effort and then finishing the process with a secondary steel burnish. The PtCo alloy yielded a much higher reflective value. We carried this testing one step further and compared the original PtCo alloy with the newer PtCoCu and PtCoS+ utilizing the same process. We found the PtCo still provides the best reflective value. If the style of the workpiece lends itself to a dry polish type media, the PtIr would be the alloy of choice. If, however, the style had considerable detail requiring the steel burnish process, we would conclude the PtCo alloy.
The process comparatives reinforced those conclusions made above. Test 8 compared the final step of steel burnish versus dry polish with the PtIr alloy. As expected the dry polish process gave much better values. When a similar
comparison was made in Test 9 with the PtRu alloy, the steel burnish process was far superior, also supporting earlier conclusions.
Since the PtRu alloy performs better with steel processes, we chose this alloy to test the effects of work hardening. The processes were held constant with the exception of running one of the samples through a steel burnish step at the as-cast state, to work harden the surface. Test 10 shows it is advantageous to work harden this alloy prior to final finish.
There are times we have been successful eliminating certain process steps when working with silver and gold. A typical sequence for these cast alloys would be medium cut, fine cut, followed by a steel burnish or dry polish. The
elimination of the medium cut and/or fine cut can often times yield a similar end result. In Test 11 we eliminated the fine cut step and found Platinum is not as forgiving as gold or silver. We do not recommend the elimination of process
steps when working with Platinum alloys. However, we did find throughout this study that the extended times (up to eight hours) may not be beneficial. We were not able to define the optimum cycle times.
The highest reflective value
was achieved in Test 12 using a five-step process. The first three steps included a heavy cut, medium cut, and fine cut followed by a steel burnish and dry polish. Both the PtIr and the PtRu alloys gave comparable results.
The
maximum metal loss when looking at all of the processes was 1.51%. Comparing this to silver and gold loss rates of up to 4%, we can conclude the cut rate of Platinum is much less than silver or gold alloys.
V7N4
Comparatives in Mass Media Finishing of Platinum
Curt Hensen, Elizabeth Brehmer & Steve Smith
Rio Grande
This is an abbreviated version of the original work. For full technical details, please consult the original paper.