Noninvasive analysis of low-contrast images on ancient textiles: the case of the Shroud of Arquat

We present the results of the first in-depth measurements of the linen cloth of the shroud of Arquata, a precious copy of the Shroud of Turin, which dates back to 1653. The measurements aimed at finding the nature of the faint and low-contrast body impressions on the linen cloth, which are not produced by drawings or paintings as in the other copies of the Shroud of Turin. In general, the optical analysis and the imaging of low-contrast stains on ancient textiles is a complex task, due to the irregular surface and the influence of spectrum, position and uniformity of the illuminating source on colour accuracy and rendition. A correct evaluation requires a multidisciplinary approach. We used noninvasive technolo- gies, including imaging topological radar, laser induced fluorescence, absolute diffused reflectance and absorption spectra, which were previously used to study frescoes, paintings, antique papers, but were never exploited on ancient textiles. The combined results of our measurements and data elaboration allowed identifying the origins of the body impressions, of the stains simulating blood and of the other marks embedded on the linen cloth. Our results can be used to plan the proper long-term conservation of the linen cloth and of marks on it.