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Pearl 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. There are no fixed boundaries
between what is necessary and tolerable (for
instance cleaning and polishing the pearls
after harvesting) and what is deception. Also,
not all methods used can afterwards be
recognized.
Pearl Treatments and
Artificial Coloration
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. |
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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 example
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.
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All This
and More... Can Be Found At
Howard's
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Artificial Black Colors
Dyeing with Silver Salt
Japanese Akoya
Cultured Pearls
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
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
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
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 undrilled 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.
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