"BECAUSE the X-ray machine can penetrate the surface of a painting without doing any damage, it has long been an indispensable tool for art historians. Layers of paint on canvas (including the liberal amounts of white lead used by old masters to lighten their pigments) absorb X rays in varying amounts, thus producing on a negative a revealing shadowgraph. To the trained art scholar's eye, an X ray of a painting can often reveal its whole history, from the first unseen priming coat the artist put on the canvas, through the artist's corrections...

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