21. January 2008, 17:19 Uhr Uncategorized admin
While Japan was at war, and for some years afterwards, production of cultured pearls was almost at a standstill.
In 1939, the industry was in full swing, and then cultivation was being carried on in at least eight localities in Japan, as well as in the island of
Palao.
The farms employed about 2,500 workers over a period of six months, May to October. Round beads were drilled and strung on the small island of Toba, where 250 workers were em?ployed. Of the
200 necklaces strung daily, only some 20 were exported in strung bunches of approximately the same qualities, the number of pearls and the weight in grains often being noted on each necklace.
The
principal selling centers were Paris, London, Bombay, New York, Chicago, Los Angeles, Tokyo, and Kobe. But disturbances caused by the outbreak of war in 1939 gradually brought the industry to a standstill.
Further, tidal waves and other natural disasters added to the destruction of the oysters. But new equipment and materials have been brought to the breeding grounds, and the industry has been revived. Exports of pearls are, however, controlled, and most of the pearls are being sold to the U.S.A.
It was in March 1935 that the first experiment by which 3,000 white Banda shells, native oysters of New Guinea, were transplanted successfully to Palao.(does he mean Palau??)
Further shipments were then made, and the transplanted oysters were cultivated profitably. This has led to similar experiments being made from other areas.
The detection of cultured pearls is not always obvious, especially in the better qualities . . . (
cultured pearls sometimes have small convex marks and small blackish spots on their surface).
The large nucleus, generally a mother-of-pearl bead, makes all the difference, and it is on this that scientific tests for detection are based.
An examination of the drill hole with a lens often reveals the dividing line between the skin and the bead nucleus. Although the cheap qualities possess nothing more than a thin film of nacre, which often splits and falls away from the mother-of-pearl like an egg shell, the
better qualities have a nacre thickness of about one half to two and a half millimeters. These latter take about five years to produce, but the thickest skin does not necessarily produce the best pearl in appearance, as is often believed.The luster of
cultured pearls is often somewhat greasy
compared with the natural pearl.
Their
(cultured pearls') specific gravity is generally slightly higher: 2.735 or 2.80,
compared with that of natural pearls: about 2.715,
as the mother-of-pearl nucleus is denser than the outer covering. A rough test, therefore, would be to place specimens in a
solution of bromoform, the
density of which should be lowered to
2.73 (No problemo here . . . I always do that to my bromoform).
If the pearls sink, they are probably cultured.
Although specific gravity tests help to distinguish between the different varieties of pearls, these tests cannot be conclusive since both natural and cultured pearls vary. To be more exact, the heavy liquids
used in the laboratory should have densities varying from 2.70 to 2.76, and these may be prepared by a
dding mono-bromonaphthaline (density 1.5) to bromoform (density 2.9).
The pearls should not be kept in this mixture longer than necessary
, and they should be rinsed in benzine or toluene after testing to avoid injury. This liquid should be handled with care as it is toxic.
Most
natural pearls from the Persian Gulf have a specific gravity which falls between 2.68 and 2.74;
Australian pearls should give a figure of about 2.74, and
freshwater pearls a lower figure still.
On the other hand, some natural pearls give a reading of only 2.30.
The
mother-of-pearl bead, which forms about sixty per cent by weight of the cultured pearl, has a specific gravity of 2.80 to 2.85, and the covering nacre
2.63 to 2.70. This nacre is, of course, rarely more than one millimeter in thickness.
The
specific gravity of the cultured pearl will therefore fall generally
between 2.74 and 2.78, appreciably higher than that of the natural pearl. But
the fact that the specimen sinks or floats when tested with this liquid is not an infallible test of its origin.
The drill holes in genuine pearls are often no smaller than those in cultured pearls, nor are they always perfectly straight in either variety.
If any doubt exists regarding given specimens, an instrument known as the
endoscope should be used.
This is an improvement on the nacrescope, which was formerly in use, and a description of this instrument and the effects it shows will be found in a later chapter.
