Why are art forgeries so hard to spot, and why do they matter?

Forgeries are hard to spot because a good forger builds the fake to pass the tests, and they matter because a painting's price and meaning depend on who made it. Art forgery is the creation and sale of works that are intentionally and falsely credited to other, usually more famous, artists.

The key word is intent. Copies, studio replicas and reproductions are not forgeries, though they can become forgeries through knowing and willful misrepresentation. Look at Edward Hopper. His first signed oil painting, Rowboat in Rocky Cove, was copied from a reproduction in The Art Interchange, a journal for amateur artists, and his other earliest oils have been identified as copies of paintings by Bruce Crane and Edward Moran. Most of them carry his signature, yet Louis Shadwick, the researcher who traced them, believes Hopper had no intention of passing them off as originals. A young painter learning his craft is doing something very different from a dealer who sells a copy as someone else's original.

Money is what turns copying into crime. Once art became a commercial commodity, an artwork's monetary value came to depend on the identity of the artist. A forger therefore needs two things: a convincing object and a false provenance, a made-up origin that raises its value at the buyer's expense. And forgers are often proficient in the current authentication methods, so they reverse-engineer their work to cover mistakes that could get them caught.

Even the experts can be wrong in both directions. Vermeer's Young Woman Seated at the Virginals was regarded as a forgery from 1947 until March 2004, when it was declared genuine, though some experts still disagree.