Avoiding Firestain

Firestain occurs when the copper contained in silver alloys is oxidised during annealing or soldering – it can be seen as a dark stain on the surface of the metal after polishing.

• Light firestaining can usually be polished out, or may respond to pickling in a bath of 10% sulphuric acid or another proprietary pickle.
• Silver plating may be the only answer to heavy firestain
• If a protective atmosphere is not available to carry out annealing operations, the following steps can be taken:
• Avoid prolonged heating wherever possible.
• Ensure that a large enough flame is used to get soldering and annealing jobs completed in the minimum amount of time.
• The entire workpiece can be coated with a flux which will form a glassy coat on the metal, protecting it from the atmosphere.
• Use a large, bushy flame when annealing, which allows the job to be completed quickly and provides a slightly reducing atmosphere.

Prevention or Removal of Surface Oxides

Sterling Silver is supplied fire free. Annealing under oxidising conditions, for example in air, will discolour the surface of the silver due to the oxidation of the base metal present, e.g. copper. Copper oxide, particularly, is hard and brittle and, if it is not removed from the surface, can be forced into the silver by subsequent working to create hard spots. Cracking may occur in these oxide rich arears during spinning operations.

Oxides are removed from the surface by pickling the annealed silver in hot dilute (10%) sulphuric acid. Their presence in the final workpiece produces unsightly fire stains that are extremely difficult and sometimes impossible to remove during finishing or polishing operations. The silver should be pickled after every anneal that is carried out in air. It is strongly recommended that protective atmospheres are used wherever possible to prevent the formation of surface oxides. Alternatively, Argo-tect may be used to minimise the formation of oxide on or near the surface of silver items being heated under oxidising conditions. The proper use of Argo-tect gives a continuous protective film over the piece. Very deep firestain may require mechanical action to remove.

Tarnishing in Silver Alloys

One characteristic of silver and its alloys is the tendency to develop a layer of black tarnish on the surface. This is a layer of silver sulphide and is caused by sulphurous compounds in the air and in human sweat. As is the case with gold, pure silver will tarnish less rapidly than sterling silver, which is alloyed with 7.5% copper to increase its hardness. Tarnish is usually easily removed by polishing.