Leonardo's Lost Princess by Peter Silverman

within two hundred years.
    The most famous and controversial case of carbon-14 testing involved the Shroud of Turin, the cloth that is alleged to have been the burial shroud of Jesus. In 1988, carbon testing revealed that the age of the cloth was medieval, which means it could not have belonged to Jesus. That might have settled the matter once and for all, but there was so much interest in the Shroud of Turin, and so much passion among true believers about proving its authenticity, that speculation raged about possible explanations for the “false” result.
    In 2005, Raymond N. Rogers, a highly respected chemist and a fellow of the Los Alamos National Laboratory, revealed in a scientific journal that the entire cloth was much older than the test sample—at least twice as old, and possibly two thousand years old. The explanation: the corner that was tested had been subject to mending and thus contained newer material. Rogers’s discovery did not stop the controversy, and studies of the Shroud of Turin continue. 13
    We knew that carbon-14 testing could not authoritatively establish that our portrait had been created in the fifteenth century, much less in Leonardo’s lifetime. But it could contradict the Christie’s attribution of the nineteenth century, and that’s what we were after. The carbon dating of the vellum showed that the drawing had with great probability been done between 1440 and 1650, which was within the perimeters of a fifteenth-century authentication. It was important evidence, although carbon dating alone wasn’t proof. Art forgers were known to use materials from a desired era. For example, Hans van Meegeren, who created phony Vermeers, used canvas from the seventeenth century. Nevertheless, the carbon dating was a first positive result in a long checklist, and Cotte and his technicians began their study in earnest.
    Cotte’s scientific method did not ignore the processes of art anthropology; it just made it easier to view the evidence. Much of Cotte’s investigation involved the details of style, fashion, and era as well as a comparative analysis with Leonardo’s other signature works. Science does not replace aesthetic sense, historical study, or any of the other expert means of authenticating works of art. It merely adds another layer of proof.
    Cotte began by digitizing the portrait at a resolution of 1,570 pixels per millimeter, an extraordinary level of definition. At this resolution, the slightest nuances and tiniest details––the craquelure (network of fine cracks on the surface), the grainy surface texture of chalk or graphite, and even fingerprints––were perfectly visible.
    In a single scanning session, which lasted only one hour, the work was measured and captured in thirteen spectral bands, and the multispectral camera recorded and generated approximately 24 gigabytes of digital data. Cotte’s goal was to produce an image that contained additional information: what could be seen beneath the various paint layers, since many pigments that are opaque to ordinary visible light are transparent to infrared. Cotte explained:
    The study or analysis of images of such ultra high resolution and high definition gives the researcher a considerable weapon––a trump card, if you will. A few hours spent before a large computer screen, armed with images produced by a multispectral camera, can be invaluable, complementing the evidence obtained in a traditional conservation studio. The high degree of sophistication and extreme precision of multispectral images enable an in-depth study of the object’s physical characteristics, reducing the need for further direct contact, since the work is handled only once in the initial scanning session. 14
    The results:
Perfectly clear, ultra-high-resolution images
Normalized (standardized) colors with a unique level of accuracy
A broad spectral range, with infrared and raking light infrared images extended to 1,050 nanometers (instead of the 850 nanometers