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The rewards of artistic success are money and fame, but the perfectly successful faker is by definition unknown -- and probably not very well paid even for an unknown. There is no fame in forging, only the notoriety of failure, and there can be very few forgers whose personalities excite real curiosity (Thomas Chatterton and Hans van Meegeren are two obvious exceptions). To deceive today's ignorant, art-hungry mass audience with a mechanical reproduction is no big trick. It does not approach the achievement of the 19th century German goldsmith Reinhold Vasters, who produced large numbers of "Renaissance" cups, bowls and jewels that are, in their own way (if you look at them as obsessive tributes to a style), as remarkable as any of the Gothic architectural restorations that were being done in Europe at the time.
To complicate matters, there was even in the 19th century such a thing as "subversive" forgery. Louis Marcy, a.k.a. Luigi Parmeggiani, a brilliant faker of medieval and Renaissance caskets, jewelry and reliquaries whose works entered the major museums of Europe, was an anarchist who wrote magazine articles that reviled the capitalist art market -- and other forgers.
The Italian Renaissance was a tremendously fertile ground for 19th century fakers. Two in particular, Giovanni Bastianini (1830-68) and Alceo Dossena (1878-1937), filled English and American museums with fakes of Donatello, Antonio Rosselino and Desiderio da Settignano. Bastianini's "portrait" of Lucrezia Donati, mistress of Lorenzo the Magnificent, made art historians swoon with rapture; even after it was found to be a fake, the Victoria and Albert bought it, and for the same price as a real quattrocento bust. There are almost certainly quite a few unidentified Dossenas and Bastianinis gazing serenely at museumgoers today.
Presumably, there are not many competent Renaissance fakers left: the common heritage of training, which changed so little between the 15th and 19th centuries, is a thing of the past. Its survival -- along with poor records and primitive techniques of scientific scrutiny -- had made the 19th century a golden age of forgery. Today, with exhaustive documentation of works of art and better analytic tools -- carbon dating, fluoroscopy, ultraviolet and X rays and chemical spectroscopy for pigments -- it is harder for an "old" fake to pass muster but certainly not impossible. The forger's greatest ally is always the cupidity of the collector: people want to believe.
Yet the fake does tend to become a little more obvious with the passage of time because forgers, consciously or not, are apt to work in the style of their own day, and that style in the end becomes historical. One "Botticelli," The Madonna of the Veil, was made by a still unidentified Italian forger in the 1920s. No less a connoisseur than Roger Fry enthused over it. The only thing that gave the game away was the precocious eye of young Kenneth Clark, who thought the Madonna looked like a silent-movie actress -- as indeed she does. Then tests were made, and down it went.