The art forger Eric Hebborn, who was murdered in Rome in 1996, is a rather interesting figure whom the art world has tried to forget. From the article "The Art of the Master Forger" in Quadrant by philosopher David S. Oderberg:
His introduction to the forger's art began around this time, at the premises of a mysterious Mr Aczel in London (though he is possibly a composite character). George Aczel, Picture Restorer to the Trade, had a thriving business "touching up" paintings belonging to the well heeled, and trained RA students in his workshop. At first the training was fairly standard -- filling in damaged bits of the canvas with suitably similar colours, repairing cracks, and so on. The objective was "to make the mend invisible". The students progressed to mixing all kinds of complex combinations of paint and handling more "delicate" repairs. Aczel noticed Eric's proficiency at "painting-in large areas of missing colour in the style of the original artist", and so passed more "large-scale" work on to him: "Thus it was that under Mr Aczel's guidance I began, little by little, to develop my abilities and improve my knowledge of the materials and methods of the Old Masters until I would one day be able to `restore' a whole painting -- from nothing at all."
At this point Eric comments:
The borderline between what is restoration and what is simply repainting is not always clear. Nor should it be thought that old pictures are necessarily spoiled by modern alteration. This attitude arises from a scientific approach devoid of any aesthetic judgment. Would we return the Sistine Chapel to what it was before Michelangelo exchanged the Perugino frescos for his own? Well, no, but it would be nice to have the Peruginos as well. Would we remove the retouching with which Rubens was in the habit of improving his collection of mediocre Old Masters? No. The truth is that age in itself is obviously no guarantee of quality, and many old pictures are bad old pictures, some so bad it would be difficult to make them worse.
Hebborn soon discovered what Mr Aczel already knew, that:[p]ictures that are unsaleable are bad business; and by some warped kind of logic become bad art. Nobody wants bad art, so dealers have it "improved" and that was how Mr Aczel made most of his money. Should a painting be unsaleable because it represented an ugly woman, the ugly woman would become a pretty young girl. If it represented a saleable young man contemplating an unsaleable skull, the offending skull was changed into a brimming glass of wine, or some other object with commercially viable associations. A cat added to the foreground guaranteed the sale of the dullest landscape. Dogs and horses enlivened otherwise unsaleable pastures. Balloons floated into commercially deficient skies at once became immensely important (that is, expensive) documents in the history of aviation. Popular signatures came and unpopular signatures went. Sullen-faced individuals left our easels wreathed in smiles. Poppies bloomed in dun-coloured fields. Unknown sitters transformed themselves into illustrious statesmen, generals, admirals, actors, actresses, musicians, and men of letters. So, like Gilbert's king whose heart was twice as good as gold, we "... to the top of every tree promoted everybody".
And here's a long, rather grim 2001 NY Times magazine article, "A Crisis of Fakes: The Getty Forgeries," about a long-drawn out brouhaha at the Getty Center museum in LA. The Getty is the world's best-endowed museum, and by law, it must spend 5% of its wealth each year, so it is the number one target of forgers, which has led to a number of scandals. In this one, a curator noticed that a half dozen Old Master drawings expensively purchased by his predecessors looked like original Hebborns, which eventually led to his firing and his lawsuit against the Getty. It's not that fun of an article, though, because there's not enough Hebborn in it.
The most famous forgery case was fought out right after WWII, when art dealer and portraitist Han van Meegeren was put on trial in the Netherlands for having collaborated with the Nazis for trading a national treasure, an early Vermeer, to Hermann Goering for 200 lesser Dutch paintings. Van Meegeren's defense was that he was a national hero because he'd painted the Vermeer himself. So, he proposed he paint another Vermeer, and he was ultimately convicted only of fraud and sentenced to a year in prison for the various "Vermeers" he'd sold to others.
The Goering painting is quite ugly. Van Meegeren justified this by claiming it was an early Vermeer, before he'd developed his exquisite mature style. Van Meegeren had succeeded in getting some of his work validated by the leading Vermeer scholar by making it fit the art historians' pet theory of the time: that the young Vermeer had traveled to Italy and studied Caravaggio's paintings and been influenced to paint large religious paintings, which might eventually turn up. So, van Meegeren painted pictures that looked like the hypothetical missing link pictures.
Philosopher Denis Dutton argues that forgeries of Old Masters look too much like the art of their own time to survive detection for long -- that one of van Meegeren's Vermeer faces looks like Greta Garbo. Of course, this assumes we've unmasked all the old forgeries, which is a big If.
There's been a theory for a long time that the 1911 theft from the Louvre of the Mona Lisa (which was recovered in 1913) was part of an elaborate conspiracy to pawn off forged copies of the Mona Lisa on American robber barons who would think they were buying the real thing. But, I can't find much confirmation for that cool idea.
You'll notice that the topic of art forgery is more interesting to philosophers than to art historians, who would prefer not to think about it. Philosophers like to ask questions like, "If this small sketch was so beautiful it was worth a million dollars when it was a Raphael, why isn't it worth anything now that it's a Hebborn?" Works of art are the modern equivalent of medieval saints' relics, the remnants left behind by secular saints. You are paying to own something that was touched by Raphael.