“It works this way. Suppose an artist used lead white in several paintings. Now if the lead white were absolutely pure it would contain only lead, carbon, oxygen, and hydrogen. But the lead white the artist used would also contain very small quantities of other elements, these trace elements that I spoke of. In that particular batch of lead white, certain trace elements will be present in a certain quantity. The kind and amount of the trace elements will be present in that exact pattern only in that batch of lead white. “Now suppose you analyze the lead white from several paintings that you know were painted by that particular artist, and you find that there is silver, mercury, antimony, tin, and barium in every one of the samples. Also, each of these elements is always present in a certain concentration. Suppose also, that you have a painting which looks like it was painted by that particular artist but you’re not quite sure. “Well, if you take a sample of lead white from that unknown painting and you find that the pattern of impurities is the same as in the paintings you knew were genuine, then the ‘fingerprints’ match. The chances of duplicating impurities of this kind by pure accident are extremely small, just about as small as the chances of finding two people with the same fingerprints. That’s why we call this a ‘fingerprint method’.” “That sounds like a good idea,” said Harley. “Who thought it up?” x = one part per million (ppm) A known Rembrandt. x x x x x x x x x x x x x x x x x x x x x x x x x x x x x silver chromium zinc manganese iron cobalt Unknown painting A x x x x x x x x x x x x x x x x x x x x x x x x x x x x silver chromium zinc manganese iron cobalt Unknown painting B