Reduce The Rejects
It’s no wonder why burnishing is one of the most important steps in finishing a platinum casting. It work-hardens the surface, compresses it, and closes small cavities to make polishing easier. When done properly, burnishing not only saves you time, but drastically cuts down on your casting rejects.
Platinum Casting Characteristics
When the platinum cast is first revealed, it has the hardness of the alloy in its annealed state. Burnishing the casting at this point adds extra hardness to the casting, preventing a score of problems that may show up later. When not burnished, castings may bend and take on the shape of the finger, and rings will scratch more easily.
It’s also important to note that when platinum is being cast, it’s almost impossible to avoid some minor porosity from affecting the casting. There are a variety of reasons for this: The structure of the metal, the alloy combination, the temperature, and/or the atmosphere. When polished, micro porosity may appear as a haze and larger porosity may appear as small holes. This is why a good caster knows that all platinum castings should be burnished.
Burnishers Of Choice
To achieve the best burnish manual, use a tungsten burnisher. By using considerable pressure and rubbing this highly polished burnisher across the surface of the piece, you will move the surface, compress it, and close small porosity holes in the process. This will also create a high gloss polish on the treated surface and harden the surface, simplifying the final polishing process. Don’t forget to use a small amount of oil to lubricate the burnisher and prevent scratching.
NOTE: Any scratch on the burnisher will be transferred to the piece. So, be sure to use a burnisher with a clean polish to result in a well- burnished surface.
If you opt for a motorized burnishing process, many rotary burnishers do the job quite well. If you love to make your own tool solutions, you can fabricate an excellent burnisher by grinding and off-center surface on the end of a tungsten rod and polishing it. Another way is to bend the end of a bur to an L-shape configuration and, again, giving it a high polish. Place either of these into a rotary hand piece, and you’ve got two very effective burnishers.
Last Minute Saves
There may be times, no matter your burnishing skills, that porosity happens. If you have a laser, you can use it to close large surface holes and cavities by filling them with additional platinum. The laser melts the added metal, thus closing the cavities.
In large production, some burnishing can be accomplished by placing the castings in a rotary or magnetic tumbler. Steel beads and/or steel needles create the burnishing action, yet you still may need to burnish the pieces by hand.
But, no matter which method or tool you use, it’s wise to include burnishing in your platinum casting process. It prepares the surface for the final polish and eliminates many minor flaws.
[Adapted from the November 2004 Stuller Standard]