Most Silver alloys are based on the Silver-Copper binary phase diagram, which shows a limited extent of strengthening via heat treatment. Copper is a much smaller atom than silver, so it is an effective solid-solution hardener, as it is with karat gold. Since the main silver hallmarks have a high fineness, we often operate in the single-phase silver-rich region of the phase diagram.
Compared to karat gold hallmarks, silver hallmarks allow a lower extent of possible alloying:
Earrings & Pendants
Very Soft, Not Durable
Most in the UK
Widely Used in Europe
Conventional sterling silver (Ag with 7.5 wt.% Cu) is very malleable with reasonable properties, although it suffers from firestain and a tendency to tarnish. Zinc is often added to improve ductility (for chain-making etc.), deoxidize the melt, improve fluidity in investment casting and make the alloy whiter.
Melting Range / ˚C
Hardness / HV
Elongation / %
22 (annealed), 100 (cold-worked)
50 (annealed), 5 (cold-worked)
66–76 (annealed), 116–130 (cold-worked)
40 (annealed), 2 (cold-worked)
50–70 (annealed), 136–148 (cold-worked), 100–120 (aged)
50 (annealed), 2 (cold-worked)
Non-sterling silver alloys
Britannia silver is an alloy of 95.8% silver and copper and is consequently not as hard as sterling silver. It finds little use today.
Lower finenesses of silver, such as 800 (80% silver-20% copper plus zinc) or 830 grades, find application in some countries. These have higher hardness and strength but lower ductility (higher proportion of eutectic phases). The higher copper content also imparts a more yellow color, and such alloys are often electroplated with silver to improve the color.
Improving Tarnish Resistance
Tarnishing is the discoloration of the surface due to the reaction of the metal alloy with chemicals and oxygen in the surrounding environment. The tarnishing of silver is a well-known phenomenon.
There are several so-called ‘tarnish-resistant’ silvers on the market, although none are tarnish-proof. These elements form a transparent oxide on the surface that hinders the formation of silver-copper sulfides, which are the cause of the black tarnish.
Typically, the alloying additions are germanium, indium, or even silicon. Hardening by heat treatment (i.e., age hardening) is possible in some of these.
Firestain is caused by internal oxidation of the copper in sterling silver. Many tarnish-resistant silvers also claim to reduce or eliminate firestain, but experiments show this is not necessarily true.
Other alloying metals, such as zinc, can form sub-surface oxides. Even though the surface may not appear darkened, subsequent polishing will reveal its sub-surface.
It would be desirable to eliminate copper, but additions such as Zinc and Tin do not sufficiently harden silver alloys. Some alloys based on silver-zinc-tin-indium can have a hardness below 50 HV.
Heat-treating Silver Alloys
Silver alloys show a limited capability to be strengthened by heat treatment. Heat treatment can be achieved by annealing in the single-phase field (750˚C) and then quenching in water. Aging occurs at around 300˚C for one hour. The resulting microstructure is a silver-rich phase with fine copper-rich precipitate particles.
Heat treatments and Soldering – The big problem
Soldering is the process of joining two metals by melting a metal alloy with a lower melting point and solidifying it between them. It involves heat.
The soldering temperature is below the solution-treatment temperature. Hence, soldering cannot be done before heat treatment as the solder would melt.
If we solder after heat treatment, the region around the solder will be hot enough to cause coarsening of the precipitates. This will cause overaging and a loss of age hardening.