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Author Topic: Iridescent gold?  (Read 931 times)

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Offline Obryzum

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Iridescent gold?
« on: November 04, 2021, 09:13:15 pm »


Here is a tetarteron of Theodora with iridescence on the surface.  The coin is underweight at 3.49g.  At first I thought this might be a fouree, but there are a couple of deep scratches, and bronze does not scratch like that.  During Theodora's reign the tetartera were about 75% gold.  Can the other 25% account for the iridescence?  I have never seen such a feature before except on the highly debased tetartera of Alexius I, which were only about 25% gold (and later which went to 0%).


Offline Pawel K

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Re: Iridescent gold?
« Reply #1 on: November 05, 2021, 05:03:18 am »
Looks a bit like a "Boscoreale efect".

Offline Callimachus

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Re: Iridescent gold?
« Reply #2 on: November 05, 2021, 07:21:47 pm »
"Looks a bit like a 'Boscoreale efect'."

I've never heard of that term before, and GOOGLE was not much help.
Could you explain what it is?
Thank you.

Offline Pekka K

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Re: Iridescent gold?
« Reply #3 on: November 06, 2021, 02:49:15 am »

Take a look here: https://www.acsearch.info/search.html?id=6670047

nb. Boscoreale Hoard 1895.

Pekka K

Offline Simon

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Re: Iridescent gold?
« Reply #4 on: November 06, 2021, 10:39:02 am »
 This thread is more than interesting.

Pekka Thanks for that description.  I copy and pasted  from the link.  I want a Volcano coin now.

On April 13, 1895, excavators working on a Roman villa near Pompeii unearthed a vaulted box containing a treasure trove of silver vessels and the remains of a leather bag containing more than 1,000 gold aurei. The intense heat from the volcanic ash, lava and pyroclastic flows from the eruption of Vesuvius left nearly all of the gold coins with a distinctive reddish discoloration which has come to be known as "Boscoreale toning." Most of the silver pieces were later purchased by the Baron Edmond de Rothchild, who donated them to the Louvre in Paris, where they are still exhibited. The coins, however, were dispersed to local collectors before any formal records could be compiled. It is known the hoard consisted of aurei from the late Roman Republic and early Empire up to and including AD 79. Although it is usually impossible to tell for certain whether any particular coin was from the Boscoreale Hoard, the presence of deep reddish toning on an aureus dating to before the eruption is regarded as highly suggestive that the coin was from this hoard, or elsewhere in the region buried by Vesuvius.
https://www.forumancientcoins.com/gallery/thumbnails.php?album=5633 My main collection of Tetartera. Post reform coinage.

Offline Pawel K

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Re: Iridescent gold?
« Reply #5 on: November 06, 2021, 05:22:19 pm »
Everything about has been told before in the thread.

But in general Boscoreale efect is based on the effect of high temperature on gold, which results a specific patina on gold coins. Search for "boscoreale aurei" to see more of this.

Offline Mark Fox

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Re: Iridescent gold?
« Reply #6 on: November 07, 2021, 02:36:50 pm »
Dear Board,

I think the iridescent areas of coloration on the tetarteron Obryzum brought to our attention stem from the alloy and not the effects of intense heat.  (I observed the latter process [on a much less dramatic scale!] on US copper-nickel clad coinage when someone left some pocket change on the stovetop of an antique wood-burning stove for an extended period of time.)  On the Byzantine coin, notice that the areas affected are present either on the high points of the coin or in the fields where contact with an object (such as someone's finger) is easy.  This implies to me that the gold content is probably rather high in an alloying agent such as copper (rather than silver).  See, for example:

https://encyclopedia-of-money.blogspot.com/2010/01/byzantine-debasement.html

Gold, as most of us know, is an extremely noble element resistant to corrosion.  At high temperatures, however, strange and rare compounds can of course form, but in the case of the Boscoreale hoard coins, I suspect what happened was the little bit of copper (and/or possibly silver?) in the alloy (rather than directly the gold itself) became oxidized from the searing heat at the coin's surface. 


Best regards,

Mark Fox
Michigan 
   

Offline Obryzum

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Re: Iridescent gold?
« Reply #7 on: November 07, 2021, 10:54:07 pm »
Thanks for all the answers. Very interesting. I also agree that this is very likely a result of some type of reaction with impurities in the gold or on its surface. 

I also noticed that it the effect on the tetarteron is more pronounced on the raised surfaces, whereas for the aureus there is a strong effect in the field

 

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