What exactly makes the Silver Surfer quarter illegal?

There’s been a bit of discussion in the last week or so over the problems the U.S. Mint has with the promotion 20th Century Fox and the Franklin Mint created for Fantastic Four: Rise of the Silver Surfer. To recap, the two parties took 40,000 quarters – legitimate U.S. currency – and put an image of the Surfer from the movie’s promotional materials on the “tails” side of the coin. People could then find the coins and use them to register for prizes related to the movie. The U.S. Mint recently expressed its displeasure with the promotion.

I was wrong in my initial feeling that it was the defacement of currency that had them upset. It’s not actually illegal to do so as long as they’re no effort to defraud people. But there is, according to the New York Times, a law forbidding the use of U.S. currency for advertising purposes. The Franklin Mint maintains the altered coins are “commemorative” and not meant as advertisements, though their use a current promotion certainly makes that argument questionable.

The Silver Surfer and other Marvel characters, the NYT says, will show up on a set of stamps coming later this year from the U.S. Post Office.