Silver has the ability to contain up to ten times its own volume of oxygen when molten and although silver does not oxidise, the copper added to traditional sterling silver alloys to increase hardness does (see Blog post, Hardness – Why it Matters on 12th May). This property has a great influence on the manufacturing techniques used with silver alloys that are needed to cast, torch anneal and solder (braze) successfully.
Copper can form two different oxides. The first is cuprous oxide, also known as copper (I) oxide (Cu2O); this has a reddish colour and if viewed under polarised light using a microscope it appears a bright blood-red colour. If cuprous oxide is reacted with carbon or hydrogen it can be reduced back to copper; however this reaction leaves bubbles or blisters present in the silver alloy. If cuprous oxide is allowed to react with more oxygen it forms cupric oxide, also known as copper (II) oxide (CuO) which is black in colour. This copper oxide can easily be removed from the surface of traditional sterling silver alloys by pickling in a warm solution of 10% sulphuric acid.
In reality where copper oxides have been produced in silver alloys as a consequence of a manufacturing process by the silversmith they are usually a mixture of these two types of copper oxide.Where they are present just at the surface of the alloy they can be removed mechanically (by filing and polishing) or chemically (by pickling). If they have penetrated beneath the surface they are much harder to completely remove and this deeper penetration of copper oxide is what leads to the grey firestain effect; the surface has had the copper oxides removed but the deeper copper oxides remain at a sub-surface level beneath a silver-rich surface layer.
When sterling silver sheet or wire is commercially cast, oxygen is prevented from entering the molten metal by a gas and/or charcoal cover. In addition the molten sterling silver alloy can be deoxidised by plunging carbon (graphite) rods into the molten metal prior to casting or making an addition of an element such as phosphorous, which scavenges the oxygen present in the molten sterling silver alloy. However where phosphorous used it needs to be carefully controlled as it has a tendency to cause hot-shortness (a cracking effect at elevated temperatures) which can be displayed in some investment castings or when torch annealing or soldering (brazing) with the higher melting point solders (brazing alloys). When investment casting, the modern vacuum casting machines use nitrogen or argon gas as protection to prevent oxygen entering the molten metal.
When annealing on a large scale protective atmosphere furnaces (nitrogen or nitrogen with hydrogen) are typically used to protect the sterling silver alloy and prevent copper oxide formation. When torch annealing there are different proprietary coatings that can be used to reduce the oxidation of copper present in the silver alloy, but these require some chemical treatment to be completely removed or the surface copper oxides have to removed by either chemical or mechanical means as detailed earlier. Similarly when soldering (brazing) the flux that is used to remove surface oxides and allow the solder (brazing alloy) to flow also needs chemical treatment to remove it assisted in the most tenacious cases by mechanical action.
This is where the Argentium silver alloys show their advantages for the silversmith. Peter Johns recognised the ability of germanium to inhibit the formation of copper oxides and understood the benefits that this would offer to the silversmith. There are now a vast number of different silver alloys which attempt to mimic the properties available with the Argentium silver alloys and as they say, imitation is the sincerest form of flattery.
