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
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.
Possible causes include:
- Perspiration (everyone's body chemistry is different, hence this
is why some are more susceptible than others); for women, the time of
the month can influence their body chemistry.
- Perfume, hair or deodorant sprays,
- Tarnishing during storage (storage boxes may contain organic sulphur
- Leaching of acid/ cleaning solutions from surface microporosity from
cast jewellery; this causes corrosion locally (such porosity may even
trap perspiration during wear, causing local corrosion)
- Preparation of vegetables such as onions and spices (many foodstuffs
contain sulphur compounds and others are also acidic).
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.
Possible solutions to the problem include:
- Store jewellery in a pouch or bag in a dry atmosphere (unpolluted,
e.g. by exhaust fumes, solvent vapours, where possible).
- If stored in a box or pouch or plastic bag, ensure it is free of sulphur-containing
compounds (from solvents, fabric treatments, adhesives, etc.).
- Polish jewellery regularly with a soft cloth to remove any early tarnish
films, perspiration and other contaminants (sprays, etc). Clean after
wear and do not put on perfume, deodorants, sprays, etc whilst wearing
- Where microporosity is a cause, thoroughly clean in an ultrasonic
cleaning bath to remove any trapped tarnishing agents.
- Electroplate the item with pure, 24 ct gold to provide a tarnish resistant
surface. This will eventually wear through during use. An anti-tarnish
(silver) lacquer may help.
Blackening of High Carat Gold Jewellery in India
and the Middle East
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 Skin Smudging from Carat Gold Jewellery
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:-
- The blackening is due to a tarnish/corrosion product from the jewellery,
as described above. This rubs off the jewellery during wear.
- It is harmless, although unsightly. It occurs most frequently on low
carat jewellery but also on 18 ct and higher.