The existence of a market for any kind of valuable object almost always encourages the production of counterfeits. It happens with drugs, banknotes, and designer handbags. It also happens with works of art. But whereas counterfeiting banknotes or other documents has always been considered a crime, attitudes toward art forgery have changed greatly over time, as Jonathon Keats and Thierry Lenain explain in their recent books. Keats provides a succinct, intelligent, and very readable summary of the subject, concentrating on some of the most famous modern art forgers, while Lenain, in a notably learned and wide-ranging text, goes into more detail and is more concerned with the broader implications of his topic.

It is often said that art forgery has existed as long as the demand for works of art, but this is not strictly true. There is no clear evidence that art forgeries as such existed in the ancient world. There were plenty of collectors, but they seem to have found copies just as desirable as originals. Even the presence of a signature was not necessarily taken as an indication that the object in question had been made by that artist. The notion of art forgery, as we understand it today, seems to require the idea that originals possess certain qualities not found even in the best copies. It also requires the presence of an expert with the ability to distinguish between the two; but such expertise does not seem to have existed in antiquity.

We get no closer to the modern idea of art forgery in the early or later Middle Ages, when almost all painting and sculpture was meant to serve a religious function. Works of art were commissioned directly from the artist, usually for a specific location (often a church), and were seldom bought and sold. But there was one class of art object for which there was a huge demand, namely miraculous images. The most famous of these originated in the eastern Mediterranean and were supposedly representations of Christ, either made by Christ himself or based on the imprint of his features left on a cloth, or portraits of the Madonna traditionally attributed to Saint Luke. All of them, apart from the Turin shroud, whose established history begins relatively late, were recorded in various versions throughout Europe and the Middle East.

With such images, one might suppose that authenticity was a key issue. But to complicate matters, according to legend a copy of the main image of Christ, known as the Mandylion, was created miraculously shortly after the original. It too supposedly had miraculous powers, as did other copies made subsequently in various ways. The potency of the image therefore did not reside in a single version, but in several. The situation with images of the Madonna was equally confusing,…