Tarnishing can be a big problem for precious metal alloys, particularly sterling silver and low carat golds. Dippal Manchanda MSc CSci CChem FRSC, technical director of Assay Office Birmingham, explains the problem and offers some solutions.
Many jewellers have sought advice about tarnishing from Assay Office Birmingham – a problem which soon alters the colour, brightness and appearance of the jewellery, eventually rendering it unwearable. In response, the Laboratory team has carried out research which has subsequently been applied to develop tests to determine susceptibility to tarnishing, particularly in relation to packaging materials.
What causes tarnishing?
Tarnishing of silver, which is typically a blue/ purple/black colour, is triggered by its reaction with elements such as sulphur, moisture, oxygen and chlorides. Silver reacts with sulphur containing gases in the atmosphere to create silver sulphide, which appears as a visible black residue on the surface of the metal.
Oxygen and sulphur compounds in the atmosphere all contribute to the tarnishing process, which is also accelerated by perspiration, perfume and deodorant sprays.
Some foodstuffs such as fruit juice, pickles, onions and garlic release high levels of acid and/or sulphurous compounds. In addition to these natural accelerants, jewellery manufacturers may inadvertently contribute to the tarnishing process. The leaching of acid or residual cleaning solutions from surface micropores of cast jewellery can cause corrosion and may even trap perspiration during wear, exacerbating the problem further. Organic sulphur-containing compounds present in storage box materials, particularly adhesives, are another well-known source that can cause severe tarnishing.
How can tarnishing be prevented?
Tarnishing of silver plated articles cannot be prevented completely as elements that accelerate tarnishing are ever present. However, there are measures that can be taken to delay the inevitable.
To slow the onset of the tarnishing process there are some metallurgical interventions, for example the addition of de-oxidising metals such as germanium or silicon to the alloy mix. The composition of the alloy therefore influences the speed at which tarnishing occurs. There are now some tarnish resistant alloys available which significantly reduce susceptibility to tarnishing.
The onset of tarnishing can also be delayed by specialised processes such as e-coat, lacquer, ‘chromate conversion treatment’ and other coatings. This ensures that no further oxidation occurs and therefore no tarnishing can commence. However, the majority of measures that are taken to increase tarnish resistance can lead to other undesirable properties in jewellery items. All surface coatings will be removed by polishing and long-term wear. Repairs and additional working, such as sizing, can become extremely difficult or even impossible when the composition makes the alloy too hard or brittle.
Re-melting and recasting these alloys is also difficult as complete refining is usually required, which increases the costs associated with these low intrinsic value metals.
Packaging – the most common tarnishing accelerants In the UK, the average level of sulphur in the atmosphere is only a few parts per billion and this is not sufficient in itself to cause rapid tarnishing. However, when an item is packaged in a small, sealed environment the levels of sulphur can become concentrated, especially in warm humid conditions. Experimental evidence has shown that even one small adhesive label can have drastic effects.
Packaging has been identified as the most common tarnishing accelerant of jewellery items. High sulphur and other tarnishing causing compounds released from paper, cardboard, certain cloths, foams and adhesives are a common problem.
Research has shown that packaging material with less than 0.0008 per cent (8 mg/kg or 8 ppm) reducible sulphur, may be assumed to be non-tarnishing, but if more than 0.0008 per cent it may cause staining or tarnish silver, the extent of which depends
upon the sulphur concentration.
Some synthetic foam contains high levels of chlorides and fluorides as well as sulphur – all of which will cause tarnishing of silver and other metals used in jewellery.
Hazards at the point of sale Retailers have also approached the Assay Office to investigate why product is tarnishing while on display.
Findings from intensive site surveys have identified specific display pads and some glass cleaning products as major culprits. Well intentioned staff vigorously polishing finger prints off showcases may inadvertently make the tarnishing problem worse.
The accelerated tarnish test procedure utilised by the Analytical Laboratory involves the identification of components responsible for tarnishing or staining silver products. This procedure is primarily used for testing packaging or display material that is in direct contact with silver, low carat gold and base material items. Test results indicate the type of tarnishing or staining and the relative distribution of the materials causing such tarnishing.
Tarnish treatments are still relatively new and Assay Office Birmingham continues to engage in proactive research and development in an effort to fully understand, assess and restrict tarnishing.
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