Abstract
The presence of a corrosion layer on lead art and archæological objects can severely impede the interpretation of inscriptions, thus hampering our overall understanding of the object and its context. While the oxidation of lead that dominates corrosion may be chemically reversible via reduction, potentially providing some access to inscriptions otherwise obscured by time, corrosion damage is overall neither entirely reversible nor is the reduction process in all cases easy or feasible to carry out. In this study, by taking advantage of the unique penetration ability of terahertz radiation and the abundant frequency bands covered by a single-cycle terahertz pulse, we perform nondestructive terahertz multispectral imaging to look under the corrosion on a sixteenth century lead funerary cross (croix d’absolution) from Remiremont in Lorraine, France. The multispectral images obtained from various terahertz frequency bands are fed into a judiciously designed post-processing chain for image restoration and enhancement, thus allowing us for the first time to read obscured inscriptions that might have otherwise been lost. Our approach, which brings together in a new way the THz properties of the constituent materials and advanced signal- and image-processing techniques, opens up new perspectives for multi-resolution analysis at terahertz frequencies as a technique in archæometry and will ultimately provide unprecedented information for digital acquisition and documentation, character extraction, classification, and recognition in archæological studies.
Highlights
The presence of a corrosion layer on lead art and archæological objects can severely impede the interpretation of inscriptions, hampering our overall understanding of the object and its context
Cross-shaped plaques, cut from thin lead (Pb) sheets have been found at a number of sites in France, Germany, and England[1], with a concentration in Lorraine, France where the practice continued to modern times
Noninvasive and noncontact imaging modalities operating at various frequencies, such as infrared photography[5], multispectral imaging[6], as well as X-ray fluorescence imaging[7], are under active research to improve the legibility of ancient inscriptions. Such modalities are not suitable for the investigation of corroded Pb objects, because conventional optical frequencies cannot penetrate the corrosion layer; X-rays are strongly absorbed by Pb and may not be able to distinguish the corrosion products from the underlying Pb
Summary
The presence of a corrosion layer on lead art and archæological objects can severely impede the interpretation of inscriptions, hampering our overall understanding of the object and its context. Noninvasive and noncontact imaging modalities operating at various frequencies, such as infrared photography[5], multispectral (between 550 nm and 950 nm) imaging[6], as well as X-ray fluorescence imaging[7], are under active research to improve the legibility of ancient inscriptions Such modalities are not suitable for the investigation of corroded Pb objects, because conventional optical frequencies cannot penetrate the corrosion layer; X-rays are strongly absorbed by Pb and may not be able to distinguish the corrosion products from the underlying Pb. Recently, imaging with terahertz (THz) radiation[8] has attracted considerable attention due to its unique ability to penetrate many electrically insulating materials that are opaque to conventional optical frequencies. We investigate the potential of THz pulsed imaging in reflection mode to reveal the obscured inscriptions under corrosion on the sixteenth century Pb funerary cross. Our work brings together in a novel combination the science of THz-materials interactions with signal and image processing, to bear on a problem in archæological science
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