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 Post subject: Heat Treatment of Pearls
PostPosted: Thu Aug 10, 2006 3:55 am 
How do you go about detecting, and diagnostically identifying heat treatment in pearls?


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PostPosted: Thu Aug 10, 2006 10:19 am 
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I am not aware of heat treated pearls. What is your sourse for this information?

After harvest both freshwater and Akoya pearls are routinely bleached.

With pearls of lower quality, the following treatments can be a salvation:

Dyeing - The use of silver nitrate to darken the nacre of the pearl. Akoya pearls are often "pinked" to increase their rose blush.
Irradiation - The use of gamma rays to darken the nucleus of the pearl in akoya pearls and the nacre layers in freshwater.
Luster treatments - A coating treatment is placed on the surface of the pearl to artificially enhance the luster.


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PostPosted: Thu Aug 10, 2006 11:03 am 
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That was a new one on me as well.

I guess heat treatment is going on now to some extent.

Some reading here, down the page a bit.

http://www.tucsonshowguide.com/stories/ ... earlid.cfm


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PostPosted: Fri Aug 11, 2006 3:48 pm 
This heat treatment thing on has in fact gone on for some time. The chinese have pioneered this method, and are responsible for producing some beautiful golden coloured pearls.

The last time I encountered them was some months back. One beautiful strand of golden pearls, but there isn't much documentation about the treatment. I read something about fluorescence somewhere but I can't put my finger on it.

This pearl heat treatment thing just hit me in my recent trip to Vietnam(they have lots of pearls and jade there!) so I decided to find out more about this treatment.

Does anyone have lab experience with them?


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PostPosted: Tue Aug 15, 2006 5:59 am 
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Golden pearls are usually a result of natural color in pearls or by dyeing.
Natural coloured golden pearls are usually found in South Sea pearls (pinctada maxima) and sometimes Akoya pearls (pinctada fucata martensii) can have a medium yellow color.

Heat treatment is done by placing the pearls for a few days in heating cabinets at a temperature of up to 80°C.

Akoya pearls without treatment are often light yellow in color. This is not a popular color thus they are bleached or dyed golden yellow.

Heat treatment is done to improve the luster of pearls which is usually done after cold treatment/bleaching (cold treatment means soaking the pearls in bottles of hydrogen peroxide solution under intense light of about 33°C-40°C).

Pearls that do not undergo cold treatment before bleaching are usually soaked in distilled water to avoid drying out the pearls during heat treatment.

Heat treatment in Akoya pearls can make the nacre brittle and thus prone to peeling.

In freshwater pearls (usually hyriopsis cumingii), heat treatment is done first before cold treatment due heat treatment retains more conchiolin thus turning the pearls brown.


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PostPosted: Wed Aug 16, 2006 7:13 pm 
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Don’t they irradiate freshwater cultured pearls a lot as well too change them into many different colors as well as adding a sheen to them? And its very hard to detect as well from what I understand.
I don’t think there is much luck irradiating saltwater cultured pearls to change color if I remember correctly.

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PostPosted: Wed Aug 16, 2006 9:54 pm 
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Irradiating pearls is an expensive treatment than dyeing and Chinese factories cannot afford them. Thus, irradiation treatment is usually done in more expensive pearls. In Japan, Akoya pearls and some South Seas are irradiated.

Irradiation turns the pearls black. The many different colors in black pearls that you see maybe one of the following:
1) Tahitian pearls may have natural peacock colors even when not treated.
2) The pearls may have undergone metal vapor deposition. Niobium gives the pearls a rainbow metallic sheen.
3) Some freshwater pearls may retain their orient after being dyed inside out.


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