Listen to part of a lecture in an art history class.

Authenticating art can be tough, even harder when the artist in question is famous. Because so many people copy master works for personal gain or simply to learn. What's more, some of these copies are neither holy authentic nor forgeries.

Take Rembrandt, the 17th century Dutch painter. When he was just 20, Rembrandt began teaching his style to aspiring artists. Rembrandt even collaborated with some of his pupils. So a Rembrandt may have a corner or a face that was actually painted by somebody else.

Moreover, Rembrandt not only tolerated imitators, he invited students to reproduce his images. These and other factors make authenticating Rembrandt quite an ordeal.

Now, forgers know that fake Rembrandt are "easy to pass off as real" easy since there are so many student copies in existence. So many of these copies appeared to blend right in with the Rembrandt body of work. As a result, by 1860, 2 centuries after his death, as many as 15000 collectors believed they owned an original Rembrandt.

And we're not just talking inexperienced or untrained private enthusiasts here. These owners also included some of the most highly respected museums of the 1860s, who relied on their reputations as connoisseurs of fine art.

S: But even as prolific as a Rembrandt was, he couldn't possibly have made 15000 paintings.

P: You're right. After rounds of critical examination from scientists and art historians, the number of verified originals got paired down to a mere 419. I mention these numbers just to give you a sense of how common Rembrandt copies are, and how that might make some people nervous. More and more owners, particularly respected institutions like museums, wanted to ensure their Rembrandts were real.

S: But don't experts often disagree over a painting's origins?

P: That's largely why an organization the Rembrandt Research Project, was formed back in 1968. This Rembrandt Research Project, RRP, for short, consisted of six Dutch art historians who specialized in authenticating Rembrandts, or reauthenticating them actually.

Yet, in spite of their expertise, some of their judgments sparked a lot of contentiousness. Take, for instance, a painting called The Good Samaritan, which was dismissed by the RRP at first. The RRP thought the execution of The Good Samaritan was "too weak", that the architectural details didn't seem to be painted with much assuredness. This hesitation, this insecurity in the artists style, led them to suspect it wasn't real.

See, forgeries take lots of time to replicate, and this slow, methodical process often results in an appearance of uncertainty, as opposed to the confidence conveyed when one paints fluidly from one's own imagination or memory, as Rembrandt did.

However, after this painting was cleaned and some discolored varnish removed, a fluent, smooth style emerged, plus the colors were consistent with Rembrandts color choices at that time. The RRP also discovered a very typical Rembrandt signature the initials RHL seems what the RRP initially saw as lack of confidence, stemmed from how old the painting was.

S: Must've been pretty embarrassing. P: Yes, in fact, the dispute over The Good Samaritan grew so heated the RRP had to disband.

S: Just for reversing their opinion about one painting? P: Not exactly, although they were well versed in identifying true Rembrandts. Other art historians disagreed with many of their conclusions, but I think they came under pressure, not because their judgments were wrong, which I'd have sympathy with. Rather many owners got angry that suddenly some of their Rembrandts were called fakes, and thus relatively worthless, since there were hundreds of owners of supposed Rembrandts, and only six are RRP members, the owner's opinions and their collective influence just overwhelmed the RRP.

But it's noteworthy that the RRP didn't just go around denouncing alleged Rembrandts. They also discovered new Rembrandts that were previously overlooked. A good example is a painting called La main chaude. Historically, La main chaude wasn't considered an authentic Rembrandt due to its subject matter, people playing a popular, rather childish game. This common activity doesn't fit with the sorts of scenes Rembrandt usually painted.

But with more recent technology at their disposal, the RRP were able to conclusively date the oak panel that La main chaude was painted on. They also took advantage of new research efforts that, in addition to providing a wealth of new information about Rembrandts' painting technique, also shed new light on other aspects of the painting. For instance, they were able to identify the clothing worn by the figures in the picture as representing a short-lived fad between 1628 and 29.