Proc. Natl Acad. Sci. USA 111, 1708–1713 (2014)

Credit: © NORTH CAROLINA MUSEUM OF ART/CORBIS

The scientific study and conservation of our cultural heritage relies a great deal on modern imaging and spectroscopic techniques, especially on non-destructive methods that alleviate the need to take samples from valuable pieces of art. Tana Villafana and colleagues have now adapted 'femtosecond pump–probe microscopy' — a non-destructive biomedical imaging technique, typically applied in analysing skin tissue — for use in conservation science.

In pump–probe microscopy, molecules are electronically excited by a laser pulse train; their response is then probed by a second pulse train. If particular frequencies of the two pulses are chosen, the distribution of pigment molecules on the surface of a painting can be traced.

The authors applied their technique to a fourteenth-century painting, The Crucifixion (pictured) by Italian painter Puccio Capanna. Virtual cross-sections of the robes of Mary and of one of the 'floating angels' reveal differing compositions of the pigments — in particular, of natural ultramarine, which is obtained from the mineral lapus lazuli and in the fourteenth century was more expensive than gold.