What caused this colorful rainbow?

Discussion in 'US Coins Forum' started by Insider, Dec 8, 2021.


What caused this characteristic?

Poll closed Dec 9, 2021.
  1. Chemical Cleaning

    9 vote(s)
  2. Ink

    0 vote(s)
  3. Album Toning

    2 vote(s)
  4. Lacquer

    8 vote(s)
  5. Photographic artifact

    2 vote(s)
  1. Insider

    Insider Talent on loan from...

    This is a colorful cent. Why does it look as this?

    potty dollar 1878 likes this.
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  3. Randy Abercrombie

    Randy Abercrombie Supporter! Supporter

    That looks like oil on water. I would have a hard time reconciling that to presenting itself on the surface of the coin so I am voting photographic artifact.
    Insider likes this.
  4. Treashunt

    Treashunt The Other Frank

    Insider likes this.
  5. potty dollar 1878

    potty dollar 1878 Well-Known Member

    I'm going with Lacquer.
    Insider and C-B-D like this.

    GDJMSP Numismatist Moderator

    I wanna see the whole picture ! :rolleyes:

    devil made me say it, I swear he did :D Sorry Mike, just couldn't help myself :D

    And Frank, you be careful be now. At your age you gotta watch what you're doin :p
  7. Evan Saltis

    Evan Saltis College Dorm Collector Supporter

    I went for 'chemical cleaning'.
    The fact it appears distanced from the lettering leads me to think whatever it was dipped or soaked in dried on it, or it's glue/lacquer.
    Last edited: Dec 8, 2021
    spirityoda and Insider like this.
  8. potty dollar 1878

    potty dollar 1878 Well-Known Member

    I'm going to take a crack at the coin,its a australia penny king george the V version from 1911 to 1936.
  9. C-B-D

    C-B-D Well-Known Member

    That’s lacquer residue. “Wavy rainbows”…. I’ve seen that before.
    Insider likes this.
  10. paddyman98

    paddyman98 I'm a professional expert in specializing! Supporter

    Someone eating Skittles touched it? :wacky:

  11. potty dollar 1878

    potty dollar 1878 Well-Known Member

    paddyman98 likes this.
  12. C-B-D

    C-B-D Well-Known Member

    @Insider , acetone it and it’ll come right off… but will the owner be pissed off when his rainbow toned cent comes back ICG MS63 RD without the toning?
  13. Insider

    Insider Talent on loan from...

    I did that at the ANA's Certification Service in DC. We received a 1936 satin Proof Lincoln sold in auction as having beautiful sea green toning. The coin was actually in a wet flip covered with liquid green PVC! I removed it without calling the customer and when :bucktooth: he got the coin back he accused us of a coin switch. :facepalm: He insisted his coin had blue-green toning and was not bright red. :( We had to buy the coin .
    imrich likes this.
  14. C-B-D

    C-B-D Well-Known Member

    Well dang. Communication is key I guess.
  15. -jeffB

    -jeffB Greshams LEO Supporter

    One of the clearest shots of interference fringes I've seen here. That's from a thin transparent layer of something with varying thickness. Assuming there isn't actual plastic wrap over the coin, yeah, I'll go with lacquer, or maybe even some sort of polymer glue.

    If I had it in hand, I'd be tempted to haul out a couple of polarizers, put one on a light and one in front of my lens, and see what happens as I turn them. Wouldn't contribute any knowledge of what's on the coin or what to do about it, but it might be a pretty trippy visual.
    ksparrow, Kentucky and Evan Saltis like this.
  16. Kentucky

    Kentucky Supporter! Supporter

    Wouldn't that count as "chemical cleaning"? :)
    paddyman98 likes this.
  17. Kentucky

    Kentucky Supporter! Supporter

    I voted for chemical cleaning with the thought that the cleaning agent had left a thin film...lacquer is possible too.
    Evan Saltis likes this.
  18. ksparrow

    ksparrow Coin Hoarder Supporter

    I'm guessing lacquer due to a thin film interference pattern
  19. Treashunt

    Treashunt The Other Frank

    Clearly not a "cent".
  20. GDJMSP

    GDJMSP Numismatist Moderator

    OK, but ..... the colorful effects we see from toning is also due to thin film interference. So, if this is caused by lacquer, what would distinguish this from toning ? Put another way, how could we tell one from the other ? If we could indeed tell the difference at all ?
    Kentucky likes this.
  21. messydesk

    messydesk Well-Known Member

    Short answer: The "shape" of the toning. Lacquer pools up in crevices of the coin differently from how toning does.

    Longer attempt at an answer: The thin film interference that causes toning is from layers of light-absorbing material building up on the coin in the 200-1000 nm thickness range. Once it's too thick, the coin is black.

    For lacquer, the root cause of the thin film interference is not likely the thickness of the lacquer, as the variation of the thickness across the surface of the lacquer on the order of micrometers, not nanometers. What is causing the rainbows in the lacquer is a change in the refractive characteristics of a layer of the lacquer nanometers deep. This is influenced both by the thickness of the lacquer and the exposure to air, which is causing the change. Put another way, the lacquer itself tones as it dries and cures over time. I imagine that with another 50 years' exposure to warm, dry air, the rainbows on the OP coin would change, and eventually disappear altogether as the lacquer layer became completely homogeneous.
    imrich, ksparrow, Dynoking and 2 others like this.
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