Professional Bench/Defining Quality
Prong Setting Round Brilliants, Part 2: Avoiding Prong Failure
Knowing how to accomplish this task will help keep gemstones safely in their settings
Prong failure when prongs fail or break during manufacture or after delivery to your customer allows the gemstone to loosen and fall out. Its caused most often by residual stress, says Fred Klotz, vice president of findings at Hoover & Strong, Richmond, VA. Residual stress is an internal condition within the metal that, if not relieved, leads to cracking and eventual prong failure.
This article will focus on settings made of white gold alloy containing nickel. While residual stress is a condition that is not exclusive to these settings, it most often occurs with them.
||This photograph shows a minute stress crack (A). Stewart Grice of Hoover & Strong took this photo at 10X using a metallurgical microscope.
||This prong failed (B) while being worn. Fortunately, the gem is still in place.
Characteristics of Nickel White Gold
Nickel, while acting as a whitening agent for gold alloys, can make the metal brittle due to nickel being insoluble with gold, says Klotz. As its melted, it mixes with other base metals to form nickel-rich boundaries which become brittle when stressed.
Potential Causes of Residual Stress
Causes of stress-related problems are subtle, but most are within control of the jeweler, says Klotz. The table on the facing page illustrates how residual stress can build during common assembly and settings procedures and the likely results of errors in workmanship:
To avoid residual stress, we offer the following table of basic procedures for the assembly and setting process with techniques for the reduction or elimination of residual stress-building problems:
Stress Corrosion Cracking
Residual stress often results in minute cracks. Undetected, these cracks act as a conduit for common corrosive elements, says Stewart Grice, metallurgist at Hoover & Strong. This condition is referred to as stress corrosion cracking.
Corrosive elements include chlorine, bromine and human sweat. The higher the concentration, the longer the exposure and the higher the temperature, the faster the settings will deteriorate. This condition severely hastens prong failure. Hoover & Strong conducted and rated the chemical effect on white gold settings. The results are posted on page 295 of Hoover & Strongs 2002/2004 catalog. Failure was not associated with settings made of palladium white gold and platinum in the worst test conditions.
Perform regular inspections and analysis of your customers jewelry, says Grice. Review the prongs for wear and minute cracks, and clean their jewelry on a regular basis. He advises thorough cleaning of customers jewelry on a regular basis because the build-up of foreign matter can lead to problems in key areas.
||Grease and skin cells (C) adhere to the inside of settings and serve as an ideal medium for entrapping corrosive liquids or residual salts, ensuring long-term exposure to corrosion for the inside of the setting, says Stewart Grice of Hoover & Strong, who took this photo at 10X using a metallurgical microscope.
To avoid potential prong failure when using nickel white gold, consider the following:
1. Rhodium-plate white gold nickel settings. The plating may contribute to a short-term fix.
2. Buy preassembled solitaires. They usually are soldered in a furnace with a controlled atmosphere where heating and cooling are maintained. The process yields a piece thats properly annealed and workable. This will help with residual stress related to heating, but the setting remains vulnerable to other forms of stress in the setting process.
Grice and Klotz advise using palladium white gold settings because, like platinum, theyre not vulnerable to stress corrosion and significantly lower potential problems related to residual stress.
Hoover & Strong offers a full line of findings, including palladium white gold and Tru-Seat® Solitaires in all alloys. The cost for palladium white gold is slightly higher than a 14k nickel white gold counterpart. The cost must be offset and considered against potential future failure, says Grice.
The Tru-Seat Solitaires, aside from the manufacturing advantages, offer reduction of residual stress-related problems. An overview of Tru-Seat Solitaires will be included in Augusts Professional Bench section.
Information in this article is based on:
1. A study conducted by Hoover & Strong and reported by Fred Klotz in an article titled Stress, Cracking and Prong Failure, published in Hoover & Strongs 2002/ 2004 catalog.
2. A presentation titled Failures in 14kt Nickel-White Gold Tiffany Settings, prepared and presented by Stewart Grice at the 2002 Santa Fe Symposium. The in-depth metallurgical paper in its entirety is included in the 2002 Santa Fe Symposium proceedings book available from Rio Grande (product number 550-476, price $49.95). Call Rio Grande at (800) 545-6566.
by Mark B. Mann
Technical Contributions by Fred Klotz, vice president of findings, and Stewart Grice, metallurgist and director of mill products, Hoover & Strong, Richmond, VA
Photographs by Stewart Grice
© 2004 Visual Connunications, Inc.
Sponsored in part by Jewelers of America