New Mineral Named After GIA’s John Koivula
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 Post subject: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 7:44 pm 
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Hi,

Hoping you can help. The title says it all, really...how do I identify a gem that has a foil backing versus one that doesn't? Foil back gems are always set with a closed back, so can only be viewed from the above.

Thanks,

Shelley


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 8:16 pm 
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Foil backs are pretty much always imitations, usually glass. I don't think I have personally encountered a genuine stone that has been foil backed.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 8:26 pm 
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Thank you.

Yes, paste is usually foil backed. However, Georgian era gems were almost always always mounted in a closed back setting, and they often used real gems (as well as paste) with a foil backing to enhance the colour e.g. foil backed garnets.

Assuming the stone is a gem (not paste), how do I identify a foil back?

Cheers,

Shelley


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 8:39 pm 
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I would think it would be apparent with microscopic observation.

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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 9:03 pm 
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Closed back mounting is very different than foil backed.

I would try to do the standard tests with the dichroscope, refractometer, microscope, uv lighting and filters as a start.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 9:15 pm 
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It is absolutely true that they are different, but Georgian jewelers often did both in their pieces. It was a wild time.

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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Thu Jan 03, 2019 11:58 pm 
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Foil back gems were very common in Georgian times, but descriptions of the methods of foil backing were first described in the treatises of Benvenuto Cellini on Goldsmithing and Sculpture written between 1500-1571.

Although often used with cabochons like garnet, sapphire or dark amethyst, the most beautiful foilbacks I've seen were the pastel green chrysoberyl and pinked topaz, originally mined in Brazil and brought to Europe for fashioning.
Here is an example of a Georgian Cannetille brooch with all gems foil backed.
Image

Foil back rose cut diamonds were also the vogue during this time. All the brilliance of the diamonds was the result of the reflection of the foil.
Image

Foil backs were (are) very fragile, even with the closed backs they always have. As lapidary styles improved into the Victorian era, we see fewer and fewer foil backed stones.

So, how do we go about identifying a foil backed stone.

The first factor to consider is the age of the piece.
Is it an antique? Georgian to early Victorian?
Are the stones backed with metal (gold, silver) cups?
Is the color of the stone concentrated at the base as you look through the side of the stone?
Can you detect a metallic luster coming through the table/crown of stone?
Do you notice any discoloration or crumpling of the foil behind the stone?

If you answer yes to these questions, you are probably looking at a foil back.

Also, as an aside, one could dramatically enhance or change the color of a gem by using colored foils. I remember seeing a lovely Georgian pendant with a pink diamond pear shaped dangle. It was impossible to tell by looking whether the diamond was a pale pink or backed with pinkish foil.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 04, 2019 12:50 am 
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Oh, I guess I misunderstood the question. I thought she wanted to identify the type of stone that was foil backed. Apparently she wanted to determine if the stone was foil backed. Sorry.

Barbra's answer is your best bet.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 04, 2019 7:01 pm 
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FYI, foil backs are still in use today in modern jewelry pieces using ancestral techniques.
For instance we still have in the south of France, around the city of Perpignan, a special way of setting rose cut garnets on 18 kt gold, using a foil which is called in french a "paillon".
All gold and garnet jewelry coming from there use foil backs.

https://www.legrenatdeperpignan.fr/
See on the left menu "Création et savoir-faire" to visualize the steps.
(Sorry, the link is in french)

I guess garnets used to be extracted in this geographic area, and now come from abroad, but the setting technique has remained the same.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 04, 2019 7:35 pm 
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Again Isi. You are correct.
Many of these are antique reproductions coming from India.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 04, 2019 7:46 pm 
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Check here, a nice picture of the red "paillon" (foil) being laid on the gold back.
http://jaimelafrance.tourisme.fr/grenat ... e-passion/


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 04, 2019 10:45 pm 
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Thank you for taking the time to reply! There are some good answers here that are helpful to me, particularly from Barbra and Isi. :)


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Tue Jan 08, 2019 7:31 am 
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Foil backing certainly goes back to Anglo-Saxon goldsmiths (800-1000 A.D.) and I would not be surprised if the Ancient Egyptians used the technique, but how to tell for sure (without taking the stone out of its setting), very difficult indeed.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 11, 2019 7:05 am 
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Very interesting post, Isi, thanks a lot.


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 Post subject: Re: How do I identify a foiled gem?
PostPosted: Fri Jan 11, 2019 11:03 pm 
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Alan F. wrote:
Foil backing certainly goes back to Anglo-Saxon goldsmiths (800-1000 A.D.) and I would not be surprised if the Ancient Egyptians used the technique, but how to tell for sure (without taking the stone out of its setting), very difficult indeed.


As usual Alan, brilliant.
I far from an expert on ancient Egyptian jewels but that seems very likely.


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