Precious Metal Alloys Part 3

The hardening of gold and silver is now easier than ever before thanks to increased experience in the heat treatment of certain alloys. This hardening process has long been available, yet has been met with great skepticism from goldsmiths in particular, despite the obvious advantages. The jewelry industry can produce particularly manageable results using a protective gas atmosphere in annealing furnaces.

Goldsmiths can also upgrade their enameling ovens (with temperature and time controls) to include a protective gas chamber (nitrogen gas) or use a method which is somewhat less controllable but also effective by annealing the pieces of jewelry using an open gas flame followed by quenching and then placing the items in the kitchen oven for several hours. It's thanks to metallurgists like Mark Grimwade and Dr. Christopher Corti from England or Dr. Jorg Fischer- Buhner from Germany that expertise in this field is finally being applied. Their research and in particular the lectures they have presented at international symposiums have made these metalworking processes comprehensible at last. Precious metal separating plants now also provide information materials on hardening and tables of values for the various alloys of precious metal.

Wedding rings in particular are subject to a high level of wear and tear. Scratching the ring on a rough wall is enough to leave behind a deep groove. A goldsmith can, of course, work on the ring but this means losing some of the material. So why not simply increase the hardness of the ring using a heat treatment process, with very little additional effort required? This increased level of hardness and wear resistance can be emphasized as an additional quality feature when selling and advertising the item. A harder material would be particularly beneficial for mechanisms, bangles, tie pins, money clips and other such items. This will also increase the elastic force of the item and make it easier to polish, while filigree items of jewelry will bend less easily.

The increased hardness of the material created using cold forming processes, such as forging or milling, is often lost through the high temperatures of soldering unless other techniques such as riveting or lasering are applied to the items of jewelry. The process of increasing the hardness through cold forming can also be applied to pure precious metals whereas hardening through heat treatment usually requires a sufficient level of copper to be present with precious metal alloys. Gary Dawson runs a goldsmithing studio in Eugene/Oregon, USA and has worked intensively on the thermal hardening of precious metal alloys at his studio: "l truly believe that heat treatment may be one of the most under-utilized techniques in jewelry manufacturing. The fairly simple hardening procedure can add years of life to the products without adding much time or expense to the current processes".

Structural composition of a gold/silver/copper alloy (FEM)

Microstructure of 925Ag after rolling (-66%) and homogenization at 730°C and 800°C/1h/water quench (FEM)

Abrasion of jewelry surefaces, yellow gold and sterling silver. FEM, Research Institute Precious Metals & Metals Chemistry

Increasing hardness through homogenization, quenching and tempering

With so-called thermal hardening, certain time periods need to be respected. The first step in thermal hardening is homogenization either in the oven at high temperatures for approx. ¼ - 1 hour for an even structure or annealing with an open gas flame. The second step is quenching in water without the item of jewelry having time to cool down beforehand. The third step of the thermal hardening is tempering in the oven followed by cooling in the open air. During tempering, the item of jewelry is heated up to 280-400°C for 30 mins to an hour in the case of gold alloys. This often doubles the hardness of the item of jewelry. The times and temperatures vary greatly depending on the alloy. Even the size of the item plays a role. Precious metal separation plants and the FEM (German Research Institute of Precious Metals in Schwabisch Gmund, Germany) also provide additional information.

Internal processes within the metal during annealing

During annealing or homogenization, the atoms are re-ordered. The fact that silver and gold alloys have limited solubility in their solid states is responsible for this annealing ability. Increased cooling can, for example, lead to the atomic structure of silver absorbing less copper atoms; the solubility is said to decline or becomes limited and copper-rich crystallites are deposited between the silver-rich crystallites. According to the same principle, adding 1-2% cobalt leads to the hardening of 990 gold alloy. Certain alloys, such as a 750 red gold alloy, form a different structure when cooled below 400°C and harden as a result.

925 Ag, cast condition. Influence of homogenization temperature on hardening at 300°C

Hardening directly from cast condition

Picture series: Workshop Gary Dawson. Cold forming of gold using rollers, annealing/homogenization using an open flame, quenching in water

From the workshop

Gary Dawson: "Torch annealing at the bench requires slightly higher temperatures because times are much shorter. I have found that torch annealing can sometimes be much more effective than furnace or kiln annealing in my shop, depending on the mass of the material being annealed. Small mass seems to allow objects to cool significantly between the kiln and quench water to make the process less effective. When torch annealing, I can follow the material with the torch from the heating platform right into the quench water, and therefore maintain annealing temperature right into the quench… One relatively simple alternative (to a kiln) is a salt bath, wherein a composition of salts (covering the temperature range of 160°C to 1200°C) are contained in a heated iron pot. The additional benefit of salt bath usage is that it prevents oxidation or contamination of titanium in the high-gold-content 990 alloys. Safety hazards posed by this method require the use of protective clothing and facemasks. In addition, it's important to always quench in water, not pickle." When a flame is used for annealing in a goldsmith's workshop, a reduced flame (blue flame with low oxygen supply) is particularly important with silver. Silver also responds well to being covered with borax to prevent oxidation with the oxygen in the air. If the copper, present in the silver as an alloy component, is oxidized by the oxygen in the surroundings and pickling then takes place, the alloy cannot be hardened across the low copper surface. Tempering of silver also requires the use of temperatures which usually lie above the solidus point of normal silver. Soldered items of silver jewelry are therefore not suitable for thermal hardening. However, homogenization in the oven can be replaced by a soldering process using a gas flame. Gemstones such as rubies, sapphires and diamonds can be set before hardening and placed in the oven or under the flame with the metal. Care should be taken with fissured stones; oil treated rubies lose their oil and become unattractive.

Workshop Katrin Erben, tempering in an enameling oven

Solution annealing at high temperature. Uncontrolled annealing atmosphere, strong internal oxidation.

Solution annealing at high temperature. Controlled protective gas atmosphere (FEM)

The hardening possibilities of silver alloys

Dr. Jorg Fischer-Buhner, former head of the Division of Physical Metallurgy and Precious Research at the FEM, has conducted research into the fundamentals of casting, soldering and deformation in the hardening of silver as part of a project with the 'Industrielle Gemeinschaftsforschung' (Joint Industrial Research program). There are various methods of testing the hardness of metals, including the Vickers (HV) method. Fischer- Buhner: "The hardness of sterling silver alloys can be increased significantly from 60-70 HV up to approx. 140-160 HV using suitable heat treatments. All heat treatments are carried out in a protective nitrogen atmosphere." Furthermore, Fischer-Buhner says of new silver alloys: "Recently new sterling silver alloys have been developed with grain refinement, fire-stain resistance and improved investment casting properties. Compared to standard 925 sterling silver, the copper content is significantly reduced in modern commercial alloys. As a consequence, the as-cast hardness (or soft-annealed hardness) of these alloys is even lower than for standard sterling silver (down to 50 HV), so that hardening through subsequent heat treatment possibly will gain greater importance."

Dr. Christopher Corti, metallurgical consultant of the World Gold Council, will be giving presentations on 'The role of hardness in jewelry alloys' at The Jewellery Technology Forum at the Vicenza Oro 1 in January 2008 and at the Santa Fe Symposium on Jewelry Manufacturing Technology in May next year in Albuquerque, USA. These presentations will be about hardness as a measurement, its use in jewelry manufacturing and service performance.

www.santafesymposium.org