How can I test my $20 gold coin for purity without harming it?

Tom D. writes: Dear Doc, I recently purchased a 1879 $20 gold Liberty Head coin. The coin was sold thru a popular auction to me as an AU grade coin. When I got the coin I noticed that the coin had very good eye appeal and that the coin had minimal wear. However upon more intricate review I noticed that the coin has been cleaned. The cleaning was truly expertly done. The whiz marks were barely noticeable. Does the coin have an intrinsic value left if it has been professionally cleaned. The coin is a real pretty coin yet you can notice under magnification the areas that were cleaned.

I purchased the coin prior to reading the 10 rules of coin purchasing never buy anything over $300 that hasn’t been properly graded. Is there any easy test I can do to prove the gold content besides the size, weight, eye appeal without injuring the coin? I am aware of chemical tests but I think they will devalue the coin even if done on the reeded edge. I am also aware of the specific gravity test but does exposing the coin to water represent any danger to the coin? I appreciate your expertise and any help you can afford.

Your coin may have been cleaned but if you are not expert at this it is easy to mistake die scratches (which are raised lines) for cleaning (incuse in the coin). “whizzing” with a wire brush leaves circular scratches in the surface that are immediately apparent to an expert as this treatment leaves an unnatural luster on the surface of the coin. This type of deceptive technique is hardly ever done to just one part of the coin. The whole coin is whizzed.

Most cleaning techniques, including whizzing, simply moves metal. Therefore the original standards for the coin and its gold content are usually intact. U.S. gold $20 coins contain .9675 of a troy ounce of pure gold (at $1200 gold the gold content would equal about $1161).

A specific gravity test in water does not harm gold coins (that is why gold is a precious metal, it is virtually inert). Maybe the saying “you get what you pay for” is appropriate. I suggest you send the coin to a third party grading service such as NGC or PCGS to get your coin evaluated.

Maybe it is a score and you just don’t know it. If it comes back as cleaned or whizzed you can always complain to the auction house if it was a mail bid since you relied on their expertise and description. Floor bid sales assume you’ve looked at the thing before the sale and will refuse to refund your money. Even if it is cleaned you should easily get 5% or 10% above melt for it.