PRECIOUS METAL FINISHING

Jewellery Packaging - the most common tarnishing accelerants of jewellery articles
(Dippal Manchanda, MSc CSci CChem FRSC, Technical Director)

One of the most common uses of Precious Metals is in alloys to be used for jewellery manufacture and in this line of business satisfying the final consumer is hugely important. While corrosion is not of course a problem for the pure “noble” metals the phenomenon of tarnishing can be a big problem with precious metal alloys, particularly for sterling silver and for low carat golds.  The main reason for buying jewellery is its aesthetic value and this diminishes significantly when the metal succumbs to tarnishing, altering the colour, brightness and pleasing appearance.

Jewellers have been seeking advice on the subject from independent experts at The Birmingham Assay Office for decades.  The Laboratory has developed tests to determine susceptibility to tarnishing and its most recent research has produced interesting and helpful findings, 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. These elements, which are responsible for accelerating tarnishing are abundant in the environment. 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, such as sulphur dioxide, hydrogen sulphide gases and organic vapours, all contribute to the tarnishing process, which is also accelerated by other factors. Common causes of deterioration are: perspiration, which is essentially rich in sodium chloride (common salt), 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 naturally occurring accelerants, jewellery manufacturers themselves may inadvertently contribute to the tarnishing process. Leaching of acid or cleaning solutions from surface micro porosity of cast jewellery causes associated corrosion and may even trap perspiration during wear, causing local corrosion. 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; elements such as sulphur, moisture, oxygen and chlorides that accelerate tarnishing are ever present and in time, low carat gold and silver and silver plated jewellery will eventually tarnish.  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. However, the jewellery alloy or composition only plays a relatively small part; in that it determines the speed at which tarnishing occurs, but cannot prevent it unless all the oxidising metals are removed from the alloy. Similarly the onset of tarnishing can also be delayed by specialised processes such as e-coat, lacquer, ‘chromate conversion treatment’ and other treatments where by a coating is applied to metallic surfaces. This ensures that no further oxidation occurs and therefore no tarnishing can commence.

The majority of treatments that are applied to increase tarnish resistance can lead to other undesirable properties in jewellery items. Repairs, additional working   i.e. sizing etc. can become extremely difficult or even impossible when such alloy additions are made that make the alloy too hard or brittle etc. and all surface coatings will be removed by polishing, if the wearer requires scratches to be removed. Applying some of these protective coatings involves very complex multi stage processes and a jewellery repair shop may not be able to reapply the same coating. Also re-melting and recasting these alloys is becoming increasingly difficult as complete refining is usually required, which increases the costs associated with these low intrinsic value metals.   

Packaging – the most common tarnishing accelerants
Tarnishing is an inevitable long term reality for precious and costume jewellery items.  There are treatments as described above which can delay the onset of tarnishing, and elements which accelerate it. 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 is a common problem.

The main compound in packaging causing silver to tarnish or stain is usually reducible sulphur. The forms of sulphur believed to be reducible under test conditions are (1) sulphide (released by the acid, not actually reduced), (2) elemental sulphur, (3) thiosulphate, (4) other polythionates or polysulphides and (5) sulphite, sulphate are excluded.

It is well known that recycled paper and cardboard actually has a lower sulphur content than virgin paper; additionally re-processed paper has lower levels of other harmful tarnishing causing constituents. 

From experimental investigation, it has been shown that packaging material with less than 0.0008% (8 mg/kg or 8 ppm) reducible sulphur, may be assumed to be non-tarnishing, but if more than 0.0008% it may cause staining or tarnish silver,  the extent of which depends upon the sulphur concentration.

As far as paper/cardboard packaging material is concerned, it is not only sulphur that is involved. All wood products have a certain level of formaldehyde inherently present, which converts to formic acid over time and thereby increases the acidity of the paper product.  Unless manufacturers employ rigorous quality control practices to their input stock paper and board they could see large variations in the tarnish causing constituent. 

If the pH of paper is low e.g. 4.0 to 4.5 (cold extraction), as little as 0.0002% of reducible sulphur may cause tarnishing, whereas, if the pH is higher, even a much higher quantity of sulphur may not cause tarnishing and might be tolerated.

So called “acid free" paper from Third World Countries has been found to contain high concentrations of sulphur and other contaminants.  “Acid free” and “Acid neutral” are often used interchangeably, however they have very different meanings.  “Acid free” is a process to remove the acids from the board and often relies heavily on non-wood components (rag is a typical constituent). “Acid neutral” is simply a pH buffering material (which leaches out over time, leaving behind an acidic board or paper) that is added to the paper or board to bring the pH of the system to neutral. ‘Acid free’ paper/card board is more important from tarnishing point of view.

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 with sulphur releasing agents such as sulphurous adhesives etc., 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.

It has been observed that, depending on where the products are manufactured, there can be a huge difference in the adhesives employed. Economics drive many decisions and in under-developed and developing countries the first is price and second specifications.  Product quality depends upon the colour of the money. Without further experimentation utilising samples of good and bad packaging materials it is difficult to see where the problem originates.

Some synthetic foam, besides sulphur, contains high levels of Chlorides and Fluorides -- both of which will cause tarnishing of silver and other metals used in jewellery.

The accelerated tarnish test procedure utilised by The Laboratory of the Birmingham Assay Office for testing packaging materials involves the identification of the presence or absence of components in packaging responsible for tarnishing or staining silver products which come into contact with the packaging material. This procedure is primarily for testing packaging 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. Test conditions are such that if any volatile sulphides or any other compounds which could cause tarnishing are present, then they will cause the control sample of uncoated silver to tarnish.  

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