What's this white "stuff"?

This was a good possibility. However, since you knew the coin was not a cast, the deposit on it would be because the coin was USED TO MAKE the mold for a cast.

 

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Thank you for reiterating what I said in the first paragraph of my post.

 

So, the counterfeiter obtains a genuine coin and makes a mold of it. Then, rather than holding onto the original to make tracing more difficult, he sells it on the well known auction site Gorny & Mosch. Not only that, but he also doesn't both wiping off the deposit from the cast, which as was earlier stated, is a lousy way to copy a coin. This makes it easier to trace the fakes back to him.

There are some really stupid people out there, so maybe it's possible. Of course, I would like to hope that, were that the case, either Gorny & Mosch or Roma would have noticed. Gorny, for their part, mentions a "weisse Auflage" in their listing. The coating is evident in the photos from both auction houses.

 

People make casts of coins for legitimate reasons, such as sharing the coins with others for die studies. In years past, plaster casts were photographed for auction sales.

 

Aren't plaster casts generally made by making an impression of the coin in clay or hard seal wax and then pouring the plaster into the mold? The original coin itself is never in contact with the plaster, only the substance used to make the mold.

 

Soak it in pure 100% acetone. Not nail polish remover.

 

Exactly! I see nothing here that suggests a cast was made from the coin. We see a smear from a toothpick that suggests a bit of waxing/paste cleaner. I go back to the theory that someone thought that, if a gram of wax would improve the coin, a ton would make it wonderful.

Since this thread was started, I found my long lost box of casts in the attic including trades with Roger Bickford-Smith and the coins I made like plaster casts but in epoxy resin that are indestructible. I wish I could find proper clay to make more but I have a lot to do sorting through all I found. I have a several boxes of the things. Years ago I attended what the Smithsonian billed as a travelling exhibit of ancient coins from a German museum. What they failed to mention was that there was not a single real coin in the exhibit. All were plaster casts that had been painted rather skillfully to look like coins. I have been known to paint plaster silver too make it easier to photograph but the coins in this exhibit had painted patinas and mottled toning. I felt cheated. It was made worse since the travelling exhibit was organized by a frat brother of my brother-in-law. Painted plaster can be pretty but they are not 'coins'. You can still get the book but it is not worth having. The only coin in the exhibit of Septimius Severus was misidentified.
https://www.amazon.com/Rome-Germans-seen-coinage-exhibition/dp/0961928107

 

TIF, posted: "Thank you for reiterating what I said in the first paragraph of my post."

I was not in agreement with you at all and tried to clarify what you wrote. However, I just figured out what you tried to get across! The WHITE DEPOSIT might be a plaster.

As for carelessly making fakes, it happened all the time. With the Internet today, it is much harder to get away with stuff. In the US coin series, a group of collectors has documented the sale of an expensive damaged coin, that is repaired and used as the model for the transfer die. Then the fakes and the genuine coin they were made from go on the market. Members of this forum have done the same type of detective work to spot counterfeits.