Opinions differ considerably when discussing the ease or difficulty of polishing platinum, but most agree that when a piece is well polished it tends to stay that way. Herein I will be covering the main points regarding platinum finishing, and I may possibly be able to offer some obscure shortcuts.
The polishing process begins after casting. The sequential steps to be followed are determined by the surface of the casting. The most common mistake in polishing platinum is misjudging the degree of surface defect and selecting an inappropriate corrective procedure. Surface porosity is a condition that needs to be identified and distinguished separately from the cast surface.
Whenever porosity is present, the casting should be burnished prior to filing. If you are unsure of the surface condition (by this I mean what is the depth of the present defect), use a #4 cut file and make a single pass over the surface. If the majority of the defects are removed, continue using the #4 cut file. If more than 50% of the cast surface defects remain, use a coarser cut file. The key is in finding the finest cut necessary to remove the defects in an efficient manner. If porosity is revealed during the filing process, it is best to stop and burnish that area before continuing. When filing it is important to remember that you should only allow the file to contact the piece on the cutting stroke. Back-dragging the file across the piece can be counterproductive to the filing action.
Files are manufactured in different ways. One of the most common methods of manufacture is stamping. Although quality files can be produced by stamping, the overall quality from lot to lot varies considerably depending on tool wear. Stamped files will often have ragged or tolled cutting edges, or in some cases of extreme tool wear the cutting edge may not be completely defined. The best quality files available will have machined cutting surfaces. Machining generally produces a file with a more uniform tooth set as well as a smoother surface. This is a benefit in reducing clogging during the filing process. Many people prefer to use the precision-made watchmaker’s files. One thing that can be done to help reduce the rate of clogging is to lubricate the files prior to and during the filing process.
The platinum polishing process requires that the surface be repeatedly abraded with finer and finer grits of abrasives until the subsequent scratches are so fine that they appear non-existent. Abrasives can be found in many forms, shapes and hardnesses. Some abrasives possess a quality referred to as friability, which means that they continually to break apart, constantly exposing fresh cutting surfaces. Some of the most common abrasives include emery, silicon oxide, tripoli, diamond, rouge, and many others. All of these abrasives have advantages and disadvantages in use. The most important aspect is the particle size. In natural abrasives, particle size can vary considerably. This problem is not as apparent with synthetic abrasives.
Synthetic particles tend to have a more uniform hardness and shape which allows them to be sieved more accurately. It is important to note that sieve sizes between different measuring standards vary. So if you are using an abrasive with a 400 FEPA sieve, it may not be the same as a 400 ASTM sieve size. It is best to stick with one standard when selecting your abrasives. One of the newer products on the market is the white “compo” bar. These bars are available in a variety of coarsenesses and are very uniform in particle size .
Just as there is a variety of abrasives on the market, there is an equal variety of support mediums. The most common include paper, plastic mesh, wheel, disc, and others. The effectiveness of these products vary depending on the bond between the particle and the medium. The bonding technique, as well as the medium, will also affect the effective cutting depth of the exposed particle. The deeper the particle is imbedded in the medium, the shallower the cut required. Some of the newer support mediums on the market include resins, rubber, waxes, soaps, and plastics such as polyvinyl acetate wheels. These particular sheets are available in a carefully chosen range of abrasives and rouges. This particular product is used extensively in Japan. The effectiveness of these wheels can be further enhanced with the addition of rouge.
You can improve the effectiveness of the polishing operation by operating in the optimum working speed range for the grit being used. This is measured in distance over time. The optimum working speed for coarse (400 mesh) burring is around 1,600 meters per minute and around 2,000 meters per minute for the fine grits (1,000 mesh).
One step that should never be overlooked in polishing platinum is the necessity of cleaning the coarser compound from the piece prior to moving to a finer one. Cross-contamination is one of the biggest problems encountered during the polishing process. If the binder of an abrasive is allowed to overheat it can plasticize and smear over the piece. If this material is not removed prior to moving on to the next finer step, the heat and action produced by the subsequent operation can liberate the coarser abrasive and intermix it with the finer one.
Though cross-contamination is less of a problem with the coarser abrasives, it is extremely problematic with the buffing abrasives. Here the particle is generally so small that the size difference is not visually apparent. If the white “campo” compounds are being used, be sure that the proper grit has been selected. Using a finer grit than is required will generally result in premature flogging of the buff. If the buff becomes clogged it can be cleaned with a dressing stone or a piece of carbide. It is a good practice to label or color code your buffs to help reduce the likelihood of cross-contamination.
Buffing wheels come in a variety of styles and sizes. With the standard polishing, motor, 8″ to 10″ buffs should be used to obtain the optimum working speed. Choices should be confined to the stitched buffs, either treated or non-treated. The unstitched buffs generally lack the stiffness to generate sufficient friction at the interface to allow the surface to flow and fill in the micro-scratches necessary to yield a high luster.
Any of the standard matte finishes used elsewhere may be applied to platinum. The only difference is that it is best to finish the piece to the point where the surface is one grade finer than the desired finish. This will allow the finish to be applied, usually in a single pass, thereby reducing the chance of clogging. In addition, the finish will generally appear more uniform. Variety can be added to the finish with areas of bright finishes contrasting with matte surfaces. Small areas of bright finish can easily be applied by burnishing the area with a burnisher made from tungsten carbide polished to a mirrored surface. Another point to consider is polishing the platinum pieces prior to assembly. This is possible due to platinum’s resistance to tarnishing.
If you intend to use a platinum sheet for die-struck product, it is better to polish the sheet prior to striking. Unlike gold, platinum does not tend to polish through ‘impact. As long as the face of the die is polished, the struck piece will retain its polish and the need for additional polishing is either reduced or eliminated altogether.
Platinum can be tumble-finished using processes similar to those of gold, but it is best to be very selective in what is chosen to be tumbled. Generally, pieces with smooth, rounded shapes are best. It is best to avoid tumbling pieces with intricate details or sharp edges as these may be rounded off. Pieces that are gold and platinum combinations should never be tumbled. Abrasives that break down during tumbling are preferred for platinum. You should follow the same general guidelines when using the cut down media as in polishing. Start with a coarser grit and progress to a finer one.
Stainless steel shot can be used for the burnishing process in the tumbling operation. Here it is important to consider the amount of impingement that is being applied to the piece. The amplitude of the vibratory tumblers should be reduced to minimize impingement. The shot shape will also affect the degree of impingement imparted on a piece. Pins tend to produce a better burnishing action than most other shapes. If a barrel tumbler is being used, reduce the rotational speed to cut down on the degree of impingement. Adding a polishing powder to the burnishing process will help improve the overall finish.
The time cycles in polishing platinum are going to be a matter of trial and error in order to produce the best results. But once you have established what works best in your machines, the process should be highly repeatable. As a general rule, the cycle times for platinum will be longer than those for gold and silver.