A question was asked online about how to work-harden fine silver. Someone had embedded fine silver wire into metal clay earrings and they wouldn't hold their shape. There was quite a bit of discussion on work-hardening fine silver.
I researched the answer and found this information.
What is a metal's hardness?
A metal’s range of hardness differs depending on its particular mixture due to its arranged patterns of crystals and concentrations of mass specific to that metal. Each metal type has a range of hardness from soft to hard only relative to that metal. In other words, each metal type has its own range of soft to full hard ratings.
How is a metal's hardness measured?
There are several tests developed for measuring hardness, Mohs, Vickers, and Brinell. Through research, I found that Vickers is most often used for measuring a metal's hardness because it has one of the widest scales among hardness tests, known as the Vickers Pyramid Number (HV). The Vickers test observes the metal's ability to resist plastic deformation from a standard source.
When comparing relative hardness, Vickers lists fine silver as having the most softness compared with sterling silver and argentium silver. It rates fine silver as the softest metal when it is fully hardened when comparing it to argentium silver and sterling silver. Even at full hardness fine silver is still softer than soft sterling silver.
For earrings, the metal must be work-hardened so that it holds its original shape and can spring back into its original shape when slightly bent. Work-hardened fine silver still isn't able to become work-hardened enough. This is why copper is added to fine silver, making sterling silver, so that it can be work-hardened until it is spring-hard.
So, use sterling silver in your metal clay earrings in order to gain the needed spring-hard hardness. Make sure you don't fire the sterling silver higher than 1300˚F (704.44ºC) or the metal becomes brittle.
There are a few ways to work-harden sterling silver.
- Hammering the sterling silver with a rawhide or plastic mallet against an anvil
- Twist sterling silver wire (posts) with pliers or compress them with pliers
- Heat harden sterling silver in a kiln
According to Jörg Fischer-Bühner from Santa Fe Symposium® Proceedings, 2003. These are the steps for heat hardening sterling silver.
Step 3: Heat the sterling again, this time to 572°F (300°C), holding at that temperature for 30–60 minutes. After cooling, Vickers hardness will range between 120–140dph; if lower temperatures are used, the sterling will not achieve this level.