Tarnishing is superficial corrosion of the carat gold surface and is evident by a usually dark discolouration – the tarnish film. Pure gold, of course, is not susceptible to tarnishing and this property is generally not greatly reduced by alloying to carat golds as long as the gold content is high enough, i.e. about 18 carat (75% gold) and above in the coloured golds (but there are circumstances in which higher carat golds tarnish – see later).
Thus tarnishing is generally seen only in the lower carat golds (8-10 ct), occasionally in14 and 18 ct and even higher caratages in some countries. It is the base metals, particularly copper, and the silver in the carat gold alloys that are attacked by the corrosive agent(s). Copper oxides are red – black in colour and silver sulphides* are black, although the tarnish films may be more complex in nature, such as hydrated oxide/sulphide mixtures.
The oxygen and sulphur compounds in the atmosphere (sulphur dioxide & hydrogen sulphide gases, organic vapours, etc) are possible sources of tarnishing. Moisture assists the tarnishing process. Perspiration, which is essentially rich in sodium chloride – common salt, can also cause tarnishing, as may other agents such as perfume or deodorant sprays. Some foodstuffs contain acid and/or sulphur compounds (fruit juices, pickles, onions, etc). Organic sulphur-containing compounds, present in the materials of storage boxes, are another known source that can cause tarnishing.
Another possible mechanism may be surface micro-porosity on the surface of investment (lost wax) cast items. This porosity may trap acids and other cleaning solutions, sprays, or perspiration and cause a local corrosion which ‘creeps’ over the surface of the item.
The tarnish films formed are generally harmless although unsightly and may lead to a black smudging of the skin (see below). Such films can be easily polished off by a jeweller to restore the bright gold colour.
Reports of blackening of 21 and 22 carat gold have been received from customers in countries such as India. This is very unusual in that such blackening is usually found only in low-medium carat golds and is attributable to tarnishing (see below). It is normally considered that high carat golds (such as 22 carat) do not tarnish in the conventional sense. It is a problem that is peculiar to India and other countries in the Middle East, apparently.
Recently, World Gold Council has had an opportunity to examine examples of blackened 22 carat gold returned to retailers in India. The analysis of the blackened layer has shown that it comprises silver (and copper) sulphide. This is a true tarnish layer. That such high carat golds should tarnish is unexpected and points to the jewellery being exposed to a particularly ‘corrosive’ environment at some stage. Perhaps, it is due to being worn during food preparation (some foods & spices are very high in sulphur compounds). Maybe, the jewellery is stored in aggressive sulphur-containing environments. Maybe, the jewellery surface is more susceptible to tarnishing for some reason. At present we cannot be certain as to the cause; we can only speculate. Certainly, the evidence suggests that lifestyle or the local conditions in countries such as India are different from other parts of the world as the problem is not reported elsewhere.
We can say that the jewellery examined was not undercarated or defective in any significant way. Therefore, the manufacturer does not appear to be responsible for the appearance of this blackening effect (tarnish).
If blackened jewellery is returned to the retailer, he should be able to clean off this black layer and re-polish it. Some advice on minimising its occurrence is given above.
Black smudges on the skin and even staining of clothes by jewellery is a well known phenomenon but the causes are not clear. Some people are more susceptible than others. Some salient points are:-
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