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Treatments
and Artificial Coloration
It is a known fact that cultured pearls are
bleached, polished, artificially stained and dyed, irradiated and coated in
order to improve their appearance. The fact develops into a problem if no one
talks about the treatments.
It
is assumed that all white Akoya and Freshwater pearls are bleached in the
standard cleaning and polishing the pearls.
Imperials grading reports will
disclose any treatments.
There
are no fixed boundaries between what is necessary and tolerable (for instance
cleaning and pol¬ishing the pearls after harvesting) and what is decep¬tion.
Also, not all methods used can afterwards be recognized.
In
an interview he gave to the American “National Jeweler” magazine in early 1998,
the American pearl dealer Salvador Assael accused the pearl trade of often
failing to declare treated and artificially colored pearls to the consumer.
Assael, who also spoke openly about artificially dyed golden South Sea pearls
in this interview, provoked a storm of disapproval from his colleagues.
However, the trade can only secure
consumer confidence if precise definitions of processing, improvement,
embellishing, treatment, artificial coloration and falsification are declared.
In the United Stated, the rules of the Federal Trade Commission (FTC) fulfill
this role, and the CIBJO Pearl Book assumes it in Europe and a number of other
countries. The Pearl Book allows bleaching and polishing and also allows pink
vegetable dyes which cannot be detected. However, a declaration of artificial
coloration, irradiation and a number of other treatments is insisted on.
Methods of Treatment
Until
today, Japan has remained the master of the so-called “pearl processing
technology”, which uses technically perfect and subtle methods in order to
improve the appearance of cultured pearls. Other countries, for ex¬ample China,
use their own methods which are not yet so perfect.
Bleaching
Only a general description can be given
here. The Japanese harvest is certainly always bleached and the same is true of
the Chinese Akoya production. Bleaching is an age-old method which was also
applied to natural pearls. George Frederick Kunz wrote in 1908 that dealers in
Bombay immersed their pearls in water-filled bottles and then placed them in
the sun on their roofs.
The
Japanese factories use a weak solution of hydrogen peroxide. In addition, the
pearls are slightly heated over a prolonged period of time or are placed under
an intensive light source which produces white, fluorescent light. Sometimes
only a light source is used and sometimes this is replaced with ultraviolet
light. The pearls are placed in wooden boxes which are painted white on the inside
or are lined with an aluminum foil.
The bleaching process changes the color
pigments contained in the organic substance. The time required is between seven
to sixty days, and it is applied until the pearls have reached a uniform white
color. A period of more than thirty days may prove damaging to the pearls, as
they may begin to show cracks due to the progressive desiccation of the
conchioline substance.
Pink Coloration
 About
95 per cent of all pearls in Japan are treated with a coloring agent after the
bleaching process, producing a light even hue and a more or less pink overtone.
The coloration is hardly perceptible and can be compared to a cosmetic
treatment. The method was already applied before WWII when eosin a vegetable
dye was used. The actual process may take up to sixty days and the pearls are
usually heated slightly as well. Other coloring agents, as for instance cobalt
salts, are probably used in place of eosin today, but information is kept
strictly secret. Mikimoto himself already tried to develop a method in which
the pink overtone was initiated during the pearl growth. According to his
biographer, he even asked his staff on the last day of his life how far the
experiments had progressed.
Both the bleaching and the dyeing
processes require experience and knowledge and there is never a guarantee of
achieving the required result. Different pearls react differently to the same
methods of treatment and there is always a certain risk, although the techniques
have been perfected over the last few decades and most factories can achieve
the results they want.The pink coloration is more or less accepted in the trade as long as it is
permanent, does not look artificial and cannot be recognized at the drill hole
or the surface of the pearl. The CIBJO rules do not demand that this treatment
be declared, and it is usually not mentioned when the pearls are sold.
Artificial Black Colors Dyeing with
Silver Salt
Japanese
Akoya Cultured Pearls
 All
black Akoya Pearls are Dyed
Treatment
with silver nitrate solutions (AgNO3) is applied since the nineteen thirties to
Japanese Akoya cultured pearls. The method was used at the end of the 19th
century in the mother-of-pearl industry and natural pearls were also dyed this
way.
The pearls are immersed for a period of
several days or weeks to several months in diluted silver nitrate solution (the
dilution ratio is 3 to 4 per cent) and are kept in a dark room. After this,
they are exposed to a strong light source (either sun light or artificial
light) or they are treated with hydrogen sulphate. Both methods lead to the
deposition of metallic silver in an extremely fine distribution in the pearl,
causing the dark color. The method enables the production of lighter or darker
hues, depending on the quantity of silver produced. An immersion period of
several months is required in order to produce truly black colors while shorter
periods lead to more brownish or grayish hues.
The silver nitrate solution intrudes
into the ultra-thin conchioline layers and circulates there. The black color
will be caused by the deposition of silver oxide within the layers, but
possibly also by the deposition of non-crystalline carbon as a reduction
product of the organic matrix. In the case of natural pearls with prismatic
layers, the solution will possibly become concentrated and drain away between
the calcite prisms, and a comparably small amount will be left to intrude
between the aragonite platelets of the upper layers. In such cases, they will
not assume a dark color, although the pearls will appear dark to the naked eye.
In the case of Japanese Akoya cultured pearls, the silver solution only rarely
intrudes into the mother-of-pearl nucleus. This means that the nucleus remains
white, while the outer pearly layer takes on the dark color. The deposition of
silver seems to concentrate in the conchioline layer which is often present
between the nucleus and the pearly layer. The distribution of silver gets less
towards the surface of the pearl. In the case of undrilled pearls diffusion of
silver oxide starts however from the surface while in the case of drilled
pearls the silver solution will more directly find its way to the border area
between nucleus and outer pearly layer.
Tahitian
Cultured Pearls
Most Tahitian Cultured pearls are not treated the
notable exception are “Chocolate” colored pearls
The
method is also used for Tahitian cultured pearls, where light colored pearls of
a low color range are dyed black (originally, white South Sea cultured pearls
from other countries may have been used for the same purpose). Artificially
dyed pearls are found in a variety of sizes and shapes, and they are above all
seen in large, so-called circle shapes.
Chinese Freshwater Cultured Pearls
Freshwater Pearls come in white and pastel colors
from pink to purple all other colors should be considered dyed
In
1997, artificially dyed black Chinese freshwater cultured pearls in round
shapes and sizes of up to 11 mm were offered on the market in Hong Kong. Since
then, large quantities have reached the international market, where they can be
purchased at reasonable to low prices. They have an astonishing resemblance to
Tahitian cultured pearls, as they almost perfectly replicate the green to
purple overtones and are already called “peacock pearls” in the Asian pearl
trade.
Several companies in Hong Kong offer the treatment and are
often speaking of “laser treatment”. However, this is not true and neither is
irradiation used. Although the exact method has not yet been made known, it is
probably correct to assume that silver salts and other coloring agents are
used.
South Sea Cultured Pearls
South Sea Cultured pearls are not treated the
notable exception are “Golden” colored pearls Golden pearls are a natural South
Sea color but some are dyed
The
artificial coloration of South Sea cultured pearls is a more problematic issue,
as it imitates yellow and golden hues which also occur naturally the coloration
represents an improvement or even a deception and it cannot always be easily
distinguished from natural colors.
Since 1994, a wholesaler from New York
has offered artificially colored South Sea cultured pearls on the market. He
uses un drilled pearls of an originally light green or light yellow body color
and otherwise good quality factors and bleaches and subsequently treats them with
a chemical dye. Another company from New Jersey applies a heat treatment.
Prices
for artificially produced strong golden hues are in the range of only 10 to 30
per cent of the prices for pearls with a natural color. The price difference is
less pronounced for pearls with an artificial yellow color which cost only
about 20 to 30 per cent less than pearls with a natural yellow color. The
reason for this is that yellow hues do not have the same high ranking value as
the golden hues. The same is true for more bronze-colored yellow hues which
have lately been offered on the market and which seem to be artificially
treated Tahitian cultured pearls.
The
American companies declare the artificial coloration of their pearls. They
assume that dyed pearls will make the sale of golden pearls possible to people
who cannot afford to buy natural colors. The last five years have seen a rising
demand for dyed pearls, above all in Asia. In addition to Japan and Korea, this
applies to Thailand, the Philippines, Indonesia and Malaysia.
Artificial
yellow and golden colors now come mainly from Japan, where different methods
are apparently used. Some companies are supposed to use organic dyes, which
have the disadvantage, however, that they fade under strong light and tend to develop
an irregular distribution of color. Other companies are reported to neither
apply bleaching nor dyeing but to use a method which has remained undisclosed
so far, but produces a permanent color change.
Insiders believe today that 80 per cent
of the yellow and golden South Sea cultured pearls are dyed artificially. When
the Board of the South Sea Pearl Consortium met in Japan in November 1997, its
members (who came from the best-known companies in the pearl industry) had to
admit that they were no longer able to distinguish treated from untreated
pearls by visual inspection alone.
The problem is that there still is no
method to prove in all cases the presence of an artificial golden color with
certainty. Traditional methods only help in certain cases. The Gemological
Institute of America, which has received generous donations from the South Sea
Pearl Consortium and other organizations over the last few years, is currently
working on the development of a reliable testing method.
The
sale of artificially dyed pearls is only a problem if the treatment is not
declared. The artificial golden colors are therefore not really a danger to the
market, but they are not good for the image of the South Sea cultured pearl, as
they create uncertainty. The majority of treated pearls are sold without
disclosure. The lower prices do, however, reflect the facts and buyers should
be on the alert. It cannot be denied that artificially dyed pearls of strong
golden colors are occasionally offered on the market for the same high prices
as natural golden pearls. This means that the consumer can only rely on the
integrity of the jeweler and the jeweler is well advised to buy only from
reliable importers and dealers.
Other Coloration
Chinese freshwater cultured pearls in
striking artificial colors have been seen on the market since the nineteen
eighties. In addition to yellow, there are red, pink, or¬ange brown blue green
purple and lilac although rice crispies, baroques and flats were used mainly in
the beginning; there have been more rounded shapes also over recent years.
Intensely blue, green, yellow, pink, red and purple nearly round pearls, no
doubt dyed with chemical agents, have been seen at international trade shows
since the late nineteen nineties. No detailed in¬formation is available as to
the methods used. The colors seem to be permanent and — needless to say — fall
into the lowest price range. They can be identified
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