The same criteria used to judge the quality of the finish on gold and silver jewelry apply to platinum jewelry, however. A polished surface should be mirror-like in appearance and free of blemishes such as scratches, irregular bumps or waves and there should be no visible porosity. A finish that is applied to platinum should be consistent and defined, whether it is a matte, hammered or grooved (Florentine) finish.
It should be noted that finishing implies a surface treatment that alters the outward appearance of an as-cast jewelry item, such as matter or brush finish, Florentine or hammered, as opposed to polishing, which implies a mirror-like luster on the surface of a jewelry item. In our discussion, finishing will refer to the process of preparing a jewelry object for the final polishing or the actual mechanical finish.
As in all processes, a methodical and systematic approach will increase efficiency and quality of the desired result. The use of the proper materials (abrasives, compounds and polishing wheels) will also increase the success of the finishing and polishing operation.
The Right Stuff
Having the right materials is important for getting the best possible finish on platinum. While the polishing compounds used are up to the individual, compounds should be considered for desired speed in cutting or polishing. The suggested compounds listed below are available from most jewelry supply companies.
*Silicon carbide or aluminum oxide abrasive papers. Grits usually required are #220,#280, and #320.
*Coarse, medium and fine rubber wheels containing silicon carbide or aluminum oxides.
*Unitized or de-burring wheels. The most frequently used grits are #400, #500, and #600.
*White cutting compound (800 to 8000 grit) and orange polishing compounds are recommended to be used in conjunction with stitched muslin buffs and rock-hard laps.
Step by Step
The following instructions are for hand-finishing as-cast platinum jewelry. Please note: it is extremely important that abrasive papers and polishing wheels be used on platinum only. This will eliminate the possible cross-contamination from other metals such as gold and silver, which may be present from previous finishing operations.
Contamination generally shows up at a later stage of the manufacturing process, during operations such as brazing and welding, when the platinum piece will show darkening and oxidation. Since platinum does not normally oxidize, such discoloration is a sure sign of earlier contamination.
It is also good practice to change abrasive papers and dress buffing wheels often, re-charging polishing buffs after each dressing. This is done because platinum particles will become imbedded in the papers and buffs, causing scratches and intensifying the finishing operation.
The first step in finishing a platinum casting is to sand off the sprues using #220 grit abrasive paper. Sand in a diagonal direction, then repeat the sanding in the opposite diagonal direction.
The diagonal direction used in this process will reduce the reappearance of scratches produced by the original direction of the sanding operation. This diagonal direction applies to all steps of the finishing and polishing procedures. Failure to use alternating diagonal directions during the finishing process will produce deep grooves that will show up during the polishing process.
Rotary-file the inside of rings very lightly. Do not remove too much metal in this procedure. Sand the inside of the rings using #280 grit abrasive paper. Repeat the procedure using #320 grit abrasive paper. Due to the high reflectivity of platinum, it is only necessary to polish the inside of a ring with a white cutting compound of the finest grit (8000 grit). You may also choose to leave the inside of the ring with a satin (matte) finish.
Rubber-wheel the outside surfaces of the castings. Start with a coarse-grit wheel, then a medium-grit and finish with a fine-grit wheel. Move the wheel in alternating diagonal directions using overlapping strokes.
Next, lightly burnish or hammer the surface to minimize the appearance of small pits, known as micro or gas-porosity.
Micro-porosity is very prevalent in platinum castings, due to platinum’s propensity to absorb gases such as hydrogen and elements like carbon the volatize at the high temperatures required to melt platinum. It is best to treat this potential condition before the porosity becomes visible in the final stages of finishing. There is nothing more disappointing to a manufacturer than investing hours laboriously finishing a platinum product and producing a high polish, only to find that the end result is the appearance of minuscule pits marring the freshly polished surface.
Burnishing can be achieved by using a reciprocating hammer or burnisher. The hammer and burnisher should have surfaces that are highly polished and, if possible, the burnisher should be made of tungsten. Burnishing will also produce a hardened surface which in turn will be easier to bring to a highly polished finish.
Buff-stick all flat surfaces using #280 grit abrasive paper, then #320 grit paper. Again, stroke in alternating diagonal directions and overlap the strokes.
Using a unitized wheel, reduce the surface scratches left by the abrasive papers. Start with a #400- then a #500-, and finish with a #600-grit wheel. Remember to use overlapping and alternating diagonal strokes.
Polish the inside of rings with white cutting compound or leave a satin finish. One note of caution about this step: A rough surface or hollow section within a ring will promote the capture of household chemicals, dirt, moisture, body oils, and acids that may lead to skin irritation. This skin irritation, known as dermatitis, may lead the wearer to believe they are allergic to the precious metal used in the ring.
It is important when polishing to keep the polishing buffs, brushes, wheels and laps clean and freshly charged with the cutting/finishing compounds. These compounds are more abrasive when fresh, allowing for quicker cutting and polishing.
The outside surfaces should be buffed with white cutting compounds. Buff the outside surface with the white cutting compound in a diagonal direction, then reverse the direction of the buffing to cross over the original direction.
Flat-lap all flat surfaces using a white cutting compound. Clean the surface and repeat the lapping with an orange polishing compound (aluminum oxide). Again, clean the surface when the operation is complete.
Using a muslin buff charged with orange polishing compound, lightly buff the outside surfaces. This should give a very high luster and a reflective surface.
If platinum is being joined to karat gold, it is important to finish the platinum segment completely prior to assembly because karat gold polishes faster than platinum. Failure to finish or polish the platinum prior to joining it to karat gold will result in over-polishing the karat gold component.
Platinum will not oxidize during the brazing/welding process, so the finish will not be altered by brazing or welding. However, the use of an anti-oxidizing glaze, such a boric acid and alcohol solution, should be used to protect the polished surface of the gold from oxidation.
In the event that you are re-polishing an already polished jewelry item, care should be taken not to finish or polish out the manufacturers hallmark. This hallmark is the guarantee that the product is platinum. If it is removed, it should be replaced before the product is released to the end user.
A matte or brush finish can be applied to the surface of a platinum ring using a separating disc, rubber wheel, emery papers, unitized wheels, etc.
The depth of the cut can be determined and specified by the individual manufacturer. A matte finish is especially functional on combination platinum and gold jewelry items. The matte finish allows for a better contrast of the two colors, since platinum is highly reflective and often reflects the yellow color of the gold, making the platinum seem to disappear.
Standard gravers such as Florentine gravers can be used for mechanical finishes on platinum. Although platinum can dull cutting tools during normal applications, the use of lubricants such as cutting oils or powdered Teflon in conjunction with positive tool angles can minimize this dulling effect.
In summary, with care and practice, one should be able to produce consistent finishes and brilliant polished surfaces on platinum jewelry. The art of finishing platinum lies in first-hand exposure to the processes of platinum jewelry.
Christopher Cart is the manager of technical education at Platinum Guild International USA Jewelry. Manufacturers in need of assistance can contact Cart on the Platinum Technical Hot Line (714) 760-8882.
Platinum Guild International USA
620 Newport Center Drive, Ste. 800
Newport Beach, CA 92660
Telephone: (949) 760-8279
Fax: (714) 760-8780