Working with silver (annealing & tempering)
Sterling Silver is an alloy of 925 parts silver and 25 parts copper. The copper is added to the silver to increase its strength and durability. The hardness or malleability of this metal can be manipulated by deliberately tempering or annealing with heat, unintentional ‘work hardening’ also occurs during the shaping process.
Whilst work is being performed on sterling silver it becomes ‘work hardened’. This is a result of hammering, folding, bending, sawing, rolling and other physical actions on the metal. A piece that has been work hardened becomes more difficult to work as it becomes less flexible and more brittle. Continued bending and folding after this occurs will eventually cause the metal to crack and break.
To counteract the hardening that occurs through working the metal a
process called annealing is used to return the metal to a more pliable
state. This may be carried out several times during the course of
shaping the silver object being made.
Annealing is the process of changing the crystal structure of the metal.
We are aiming for a smaller crystal structure to increase the malleability of the metal. This can be done by heating the silver to around 650 degrees Celsius for a minute or two and then rapidly quenching it in cold water. Using a controlled atmosphere oven or coating the metal in flux reduces oxidisation of the metal. This technique produces a Vickers hardness of around 56.
Tempering is the opposite of annealing and is done to harden the metal. This is achieved by heating the metal to around 750 degrees Celsius for around 30 minutes and then rapidly quenching in cold water. The metal is then heated to around 300 degrees Celsius for one hour and then allowed to air cool. Sterling silver tempered in this manner will have a Vickers hardness or approximately 110 to 120.
The Vickers Hardness (HV) Test was formerly called the diamond pyramid hardness (DPH) test and is used to measure the hardness of materials, and is the standard test for metals. The tip of the tool is a square based diamond pyramid used to create an indent in the metal being tested. The result is calculated from the force applied and the area of the indentation produced during a 10 to 15 second duration. This number is frequently converted to Pascals (but should not be confused with pressure, which also uses this unit)
The hardness ratings of some common metals in their pure form are given here: Copper 369 MPa, Silver 251 MPa, Gold 216 MPa, Nickel 638 MPa, Palladium 461 MPa, Platinum 549 MPa and Rhodium 1264 MPa. It becomes very obvious from these figures why Rhodium plating is frequently used over white gold and silver, not only due to its hardness but also due to its high resistance to corrosion.
9ct gold has a rating of 120HV, 18ct gold is slightly higher at 125HV. This surprises many people who always had believed that 9ct gold was harder than 18ct gold. The actual difference in everyday life is small that they are effectively the same hardness.
Burnishing is the process of using friction to bring out the shine on the surface of a metal. A very smooth metal burnishing tool is rubbed in all directions over the surface to be burnished. It can be used dry or dipped in a mild detergent solution frequently. This process hardens the surface of the metal being treated. It is common to use a tumbler for burnishing as it saves lots of time and many pieces can be done concurrently.