Firestain is a common problem in silver manufacture, associated with corrosion. It commonly becomes a problem during polishing since it is difficult to remove.
What is Firestain?
Most sterling silver is based on silver-copper alloys. When heated in the air (or an oxidizing atmosphere), the copper can oxides. This oxidation occurs on the surface and penetrates deep into the material, forming a dark black oxide known as Firestain.
In silver, oxygen diffuses into the surface regions faster than copper can diffuse out to the
Hence, it penetrates deep into the surface, forming a copper oxide scale.
While pickling in a hot acid bath can remove the surface oxide layer, it cannot remove the deeper copper oxide. The result is a soft, nearer-pure silver surface with a silver plus copper oxide layer underneath. If heated further, e.g., during soldering, the oxygen can quickly diffuse through the silver layer, causing further oxidation beneath. Polishing reveals dark, disfiguring stains. Deep firestain can also cause cracking during working and during soldering.
Preventing Firestain
Fire stain was traditionally prevented by:
Protect the metal surface throughout heating by using flux
This is time-consuming and only practical for larger work
Working in a protective non-oxidizing atmosphere
This is expensive and only feasible for small pieces of work
Heavy polishing or electrochemical stripping in a cyanide solution
This is wasteful and dangerous.
Fire-stain resistant alloys
The development of Silver-Copper-Germanium alloys, known as Argentium, has been a major development in silversmithing. Many other competing alloys, with comparable alloys have also been developed.
Germanium has a greater affinity for oxygen than copper or silver and fulfills a protective function in the molten state and the wrought/cast alloy. The alloys are two-phase, with a copper-rich and a silver-rich phase. The germanium forms a protective and transparent oxide layer on the surface, which prevents firestain during soldering and annealing.
In addition to being a powerful deoxidizer, germanium also helps to improve the fluidity of the melt, which is desirable for investment casting. These alloys can be age hardened, although some properties differ from usual silver alloys.
Firestain caused by Zinc
Other alloying additions to silver, such as zinc, can also oxidize quickly, forming an internal oxide scale. For zinc, this scale is often whitish in color and so is less obvious to observe.