A great deal of misunderstanding exists as to the effects of steel ball burnishing on the surface quality of “as cast“ gold and silver jewelry. This presentation will explore, in detail, the use of four types of finishing equipment (vibratory, disc finishers, magnetic pin and roll burnishers) and focus on the resulting smoothness and hardness, as well as visual perception of finish resulting from the use of these systems.
There will be an in-depth review of how the surface finish is affected when “as cast“ pieces are prepared, employing the use of vibratory, disc finishing, magnetic pin or roll burnishing equipment for the ball burnishing process.
Finally, the paper will touch on the benefits and drawbacks of utilizing a burnishing procedure prior to wet-cutting operations or final hand buffing.
Burnishing Definition Steel media has been used for brightening metal surfaces since early in the manufacturing industry. Jewelry manufacturers have used steel media to produce a shiny, bright finish to their earrings, pendants, chain, etc., to enhance their visual appeal to the consumer.
Generally speaking, a wrought or cast piece of jewelry has a dull appearance when first formed. The act of rubbing, rolling or hammering a jewelry piece with a highly polished, hardened ball will reduce the average height of the microscopic peaks of the metal’s surface. By doing so the surface becomes more reflective of light and hence brighter to the eye. Brightness, however, cannot be confused with smoothness. A smooth (level) and bright surface is far more appealing than brightness alone. Brightness achieved by ball burnishing usually has an overall “orange peel“ surface profile. Through the history of jewelry manufacturing, ball burnishing has had decreased use for high-end jewelry finishing, being replaced by hand polishing or wet and dry mass finishing.
This research is to explore a side benefit of steel ball burnishing that has been used on the industrial side of surface finishing. Steel media weighing about 300 pounds per cubic foot (about three times heavier than any other media) impinges on the surface of the work piece, imparting a compressive stress that work hardens the surface.
This compression and hardening of the surface may be beneficial in mechanically reducing surface porosity and imperfections by compressing the grain structure of the surface of the metal. Subsequent mass finishing and/or hand polishing with the proper abrasives may produce a better finish with less metal removal.
The subject castings will be processed by means of several different types of burnishing equipment, after which the following observations and measurements will be taken:
- Visual inspection
- Microscopic surface inspection
- Surface profile measuremen
- Surface hardness measurement
The subjects* were cast by means of the lost wax method. The star-shaped design was chosen for its slightly convexed surface as well as its rather large cross-section. The gold is a 14kt alloy and the silver is 925. After casting, the only operations that were done were de-spruing and grinding the gate. No other surface imperfections were removed so as not to disturb the “as cast“ grain structure.
Four types of equipment were used to accomplish the burnishing operations. Test samples marked with #2 were processed** in a vibratory finisher. Samples marked #3 were processed in a roll burnisher. A centrifugal disc finisher was used for samples #4 and finally, #5 samples were processed with a magnetic pin finisher.
All the products were burnished using the same liquid compound and the same size stainless steel media with the exception of those processed in the magnetic pin finisher. Because of the difference in material hardness, the silver samples were processed for one hour while the harder, 14kt gold samples were processed for two hours.
The results of both the silver and gold samples are relatively parallel. The measurements and observations are presented as follows:
Visual of Surface:
Here we see the effects of burnishing as is evident from the photographs showing the raw casting as compared to the four samples that have been burnished. The visual appearance of jewelry, as you know, is the essence of its appeal. In photographs Au1 and Ag1, we see the “as cast“ pieces appear very dull. This finish is an example of how a surface with minute hills and valleys diffuses the light, giving the effect of a matte finish. Photographs Au2 and Ag2 represent a vibratory ball-burnished finish. We see that there is more reflectivity and the surface appears brighter; however it seems that the length of time or the machine amplitude was not enough to reduce the differential of the hills and valleys to what we had hoped. Photographs of pieces processed in a disc and roll burnisher (Au3, Au4, Ag3 and Ag4) are by far the brightest and most appealing finishes, while pin-finished pieces (Au5 and Ag5) are diffusing the light again and appear matte.