Burnishing Platinum …an important step in finishing platinum
One of the most important steps in finishing a platinum casting is burnishing. Burnishing will work-harden the surface, compress it, close small cavities and make polishing easier. This step, when done properly, will reduce rejects from the final polish and save time.
When cast and no other step has been performed, the platinum casting has the hardness of the alloy in an annealed state. Burnishing will add extra hardness to the casting, preventing a score of problems that may show up later if this step is not taken. Castings may bend and take on the shape of the finger and the rings will scratch more easily when not burnished.
Therefore, as a general rule, all platinum castings should be burnished. The best way to burnish a platinum casting is using a tungsten burnisher. The highly polished tungsten burnisher is rubbed across the surface of the piece. This is done with considerable pressure. This action will move the surface, compress it and close small porosity holes in the process.
When platinum is being cast it is almost impossible to avoid some minor porosity from affecting the casting. This may be caused by the structure of the metal, the alloy combination, the temperature, the atmosphere; in short, there may be many reasons for it. As you know, porosity is defined as small cavities in the surface or the interior of the metal. When polished, micro porosity may appear as a haze, larger porosity may appear as small holes. A good caster can use several techniques to reduce this problem to a minimum and produce fine castings.
Since the burnisher is highly polished, it will also create a high gloss polish on the surface treated. The rubbing action hardens the surface and thus, final polishing becomes a much simpler task. It is important to remember that any scratch on the burnisher will be transferred to the piece. So the better the polish of the burnisher, the better the surface of the piece being burnished. It is advisable to use a small amount of oil to lubricate the burnisher. This prevents scratching and makes a smoother better surface possible.
Burnishing Platinum continued