Author Topic: Acetate problem  (Read 1050 times)

0 Members and 1 Guest are viewing this topic.

Offline Figleaf

  • Administrator
  • Honorary Member
  • *****
  • Posts: 31 166
Acetate problem
« on: November 26, 2019, 10:30:49 AM »
Had this coin for some time, trying to figure out how to get rid of the red. I met a professional metal restorer of a French provincial museum last week and asked her about it. Her verdict: the coin is from an archeological environment. Its metal was replaced partly or wholly by acetats, the red. If you succeed in removing them, you will at best be left with a blank piece of metal.

BTW, she also gave me what looks like a great, but costly reference: Metals and Corrosion, by Lyndsie Selwyn. Does anyone have this book?

I will return the coin to its rightful owner, of course, but not before showing it with the above warning. That said, can you point me in the right direction to determine what it is? Edit: my own guess is 8th century Khwarezmia, but I don't find that crown with the half moon on top.

Peter
« Last Edit: November 26, 2019, 11:23:44 AM by Figleaf »
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline Manzikert

  • Honorary Member
  • *****
  • Posts: 1 464
Re: Acetate problem
« Reply #1 on: November 26, 2019, 12:20:56 PM »
Yes, Khwarezm, I believe the ruler is Kanik, date c.725 AD, my specimen below, 3.60 gm, 27.75 mm.

The references I have are: cf Zeno 219314, 119512, 108172; Vainberg ΓIV/6-7 (See Vainb. legend 1015, Tabel V.); Kh 53

Offline Figleaf

  • Administrator
  • Honorary Member
  • *****
  • Posts: 31 166
Re: Acetate problem
« Reply #2 on: November 26, 2019, 02:49:53 PM »
Very helpful indeed, Manzikert! Thank you very much. The best of the coins on zeno shows much more legend.

For the record:
Obverse: MR'Y MLK' k'nyk (... malik Kanik)
Reverse possibly: šs'thwt'w

Note that the characters on my coin have their tops turned the other way.

It also confirms that the rider is holding an object in his right hand,  something the other pictures did not show.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline THCoins

  • Honorary Member
  • *****
  • Posts: 5 853
Re: Acetate problem
« Reply #3 on: November 30, 2019, 02:21:06 PM »
Hi Peter !

"..metal was replaced partly or wholly by acetats, the red."
Copper acetate is blue green, not red. I'd rather think the red is cuprous-oxide Cu2O. This typically forms on copper surfaces which were silvered. With a thin porous silver layer in a moist environment an electrochemical reaction will occurr between the silver and copper. Parallel to "bronze disease" this is known as the "red plague".

Offline Figleaf

  • Administrator
  • Honorary Member
  • *****
  • Posts: 31 166
Re: Acetate problem
« Reply #4 on: December 01, 2019, 09:06:30 AM »
Are you saying that there is in fact a way to get rid of the red without destroying the coin? The expert I spoke to did not see the coin, but I did give a detailed description that included "dull red Bordeaux" more than once.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline THCoins

  • Honorary Member
  • *****
  • Posts: 5 853
Re: Acetate problem
« Reply #5 on: December 01, 2019, 10:21:34 AM »
Cuprous oxide dissolves in ammonia. I used that several times on silvered copper coins which had small patches of red. I suspect your coin is to far gone though  :-[

Offline gpimper

  • Meritorious Member
  • ****
  • Posts: 754
Re: Acetate problem
« Reply #6 on: December 02, 2019, 07:39:46 AM »
I've killed a number of coins trying multiple cleaning methods...it's sad but I've learned.  Soft soap and warm water for most seems to work but when you've got a crusty I've found electrolysis with a low v battery and distilled water works well.  Still learning :-) 
The Chief...aka Greg

Offline bruce61813

  • Moderator
  • Meritorious Member
  • *****
  • Posts: 694
    • Gringgotts Coins
Re: Acetate problem
« Reply #7 on: January 12, 2020, 06:36:21 PM »
The deep red is pure copper, from a bronze coin. The tin was destroyed, the copper let[ it might still be an alloy, not sure about it.] I have seen brass items develop the same red color [very old rifle brass, that has been subjected to long exposure to water] and do not know of a safe cleaning method. Since the coin is legible and not bad looking, give it a wax coat and leave it alone.

Bruce

Offline Figleaf

  • Administrator
  • Honorary Member
  • *****
  • Posts: 31 166
Re: Acetate problem
« Reply #8 on: January 12, 2020, 07:33:08 PM »
Since the coin is legible and not bad looking, give it a wax coat and leave it alone.

Thank you, Bruce. That's what I have done. I regard the red stuff as part of the coin's history now, no longer as crud.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline bruce61813

  • Moderator
  • Meritorious Member
  • *****
  • Posts: 694
    • Gringgotts Coins
Re: Acetate problem
« Reply #9 on: January 13, 2020, 02:32:45 AM »
I will see if I can find some samples to show.

Bruce