LUKE BURBANK, host:

When the Art Institute of Chicago acquired the sculpture, "The Faun" in 1997, they felt they were getting the work of the French master Paul Gauguin.

STEWART: Oops. A guy in England named Steve(ph) Greenhalgh forged the thing in his parent's shed.

BURBANk: What?

STEWART: It wasn't the first time. Last month, Greenhalgh was sentenced to four years in prison for fraud and for forging hundreds of antiques. His parents, who helped him sell the fakes, are also now facing jail time.

British authorities say the family had been defrauding art institutions since 1989. And apparently, he had quite a skill. Their forgeries range across many periods of history and artistic genres. The family is to have made over one a half million dollars off the sale of an estimated $20 million worth of forged arts and antiques.

Let's bring in Ronald Spencer, an art lawyer who specializes in authentication matters. He is also the author of "The Expert versus the Object: Judging Fakes and False Attributions in the Visual Arts." You sound like the right person to talk to, Ronald. How are you?

Mr. RONALD SPENCER (Lawyer; Author, "The Expert Versus the Object: Judging Fakes and False Attributions in the Visual Arts"): Good morning. Good morning. I sound like I might be. Yes.

STEWART: So, the family, you know, they're amazing. They faked works by Thomas Moran, Barbara Hepworth, Otto Dix. So in terms of being someone who makes forgeries, what can a family do right? We're not endorsing this, but I'm curious. What did they do right that they were able to get away with this for almost 20 years.

Mr. SPENCER: Well, apparently they did their homework, that is to say, presumably, with each of those artists they faked, they found out what pieces were out there, but not in the museums and not in public hands or - what pieces were lost so that they could - they wouldn't fake something that you could go around the corner and see the same thing in the British Museum.

So they did their homework to see what was out there. They would presumably do their homework to create a rational excellence-founding provenance - provenance being the historic trail from the artist's hand to the present owner; that they would do right. They'd also have to research how the artist worked, what materials he worked with, his style, his method of working. And they - so that they'd have to be quite well-up on the particular artist and with the particular medium that that artist worked in.

STEWART: I'm curious about the authentication process. How does it begin? How does it work? I mean, are there papers that people need to see or is it simply about eyeballing the piece of work and looking for certain signature marks or movements?

Mr. SPENCER: Eyeballing is a very - is the chore, all right?

STEWART: Yeah.

Mr. SPENCER: We call it connoisseurship. And I think…

STEWART: That's better than eyeballing, I guess.

(Soundbite of laughter)

Mr. SPENCER: Okay. What is that?

The three major tools that an expert has to determine authenticity of a work are your eyeballing/connoisseurship, one; two, a study of the provenance, which we talked about a second ago; and three, and lastly, perhaps, if it's required, scientific testing - paint, ceramic, cork samples - that kind of thing.

But the first and foremost tool that the experts hand is his experience having looked and hard at many works by that artist, knowing how the artist worked, knowing the material the artist used, but chiefly, having seen over many years - looking the work of the artist.

We all know how we can glance at anything we see from day to day, and we look at it and we hardly see it, and sometimes, then we stop and we look very hard at it; that's the difference between just glancing at it and the expert having looked at it long and hard over many years.

STEWART: It's like when I'm downtown, I see a fake Channel bag, when I notice the C is a little bit off.

Mr. SPENCER: Well, I was going to bring that up. That's right, that's the perfect example. If you look long and hard enough at just about anything, if you're into Channel bags, all right? At first, you don't see much, then you keep on looking and you see how it's stitched together and you'd see its proportions. You begin to see things here. You're educating your eye, all right?

STEWART: Now, I know you weren't all that shocked when you first read this story.

Mr. SPENCER: No, I wasn't because - well, I mean, museums are filled with excellent - in most cases, excellent experts - Jim Cuno at the Chicago Art Museum is one of the best directors in the country. But people make mistakes; they get fooled. But they don't get fooled if you're a real expert and you spend time on it. You don't get fooled that often.

So if a museum has 5,000 pieces in its collection, there'll be a couple fakes in there, but they won't be - but it won't rise to more than a miniscule amount of the whole collection.

STEWART: We've only got a brief time left, but I don't know if I've seen "The Thomas Crown Affair" or watched too much "CSI," but isn't there a science and technology and x-ray vision that can tell…

Mr. SPENCER: Yeah.

STEWART: …you if a painting is for real?

Mr. SPENCER: Well, most people think that scientific testing is the ultimate key. The problem, for example, is with - let's say, take Rembrandt. Most of the problems of Rembrandt attribution date right from the time that he was painting. So you - if you check the wood on the frame, you check the canvas he was painting on, you would find that whoever did that was using the same materials from Rembrandt's period - ditto for the paints, all right?

STEWART: Yeah.

Mr. SPENCER: There would be no pigments of unique binding media in there that were anachronistic, as we say, that is to say, came from a paint that weren't available in Rembrandt's time. So scientific testing has its place, but there is very - not many examples where scientific testing is going to be - is going to solve the problem.

STEWART: That's going to be the end-all.

Ronald Spencer, an art lawyer who specializes in authentication matters. The author of "The Expert Versus the Object: Judging Fakes and False Attribution in the Visual Arts." Thanks for explaining it to us, Ronald.

Mr. SPENCER: Thank you. Transcript provided by NPR, Copyright NPR.