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.
I wanna see the whole picture ! devil made me say it, I swear he did Sorry Mike, just couldn't help myself And Frank, you be careful be now. At your age you gotta watch what you're doin
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.
I'm going to take a crack at the coin,its a australia penny king george the V version from 1911 to 1936.
@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?
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 he got the coin back he accused us of a coin switch. He insisted his coin had blue-green toning and was not bright red. We had to buy the coin .
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.
I voted for chemical cleaning with the thought that the cleaning agent had left a thin film...lacquer is possible too.
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 ?
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.