Synchrotron-Based Phase Mapping in Corroded Metals: Insights from Early Copper-Base Artifacts

Abstract

The detailed description of corrosion processes in ancient and historical metal artifacts currently relies on the indepth study of prepared cross sections. The in-plane elemental and phase distributions can be established from a combination of light and electron microscopy characterization. Here, we show that high-resolution virtual sectioning through synchrotron X-ray microcomputed tomography allows a precise noninvasive 3D description of the distribution of both internal and external mineral phases in whole objects. In fragments of early copper artifacts (third−second millennium BC) from Southern Mesopotamia and the Indus valley, this approach provided essential clues on long-term corrosion processes. Major and minor phases were identified through semiquantitative evaluation of attenuation coefficients using polychromatic X-ray illumination. We found evidence for initially unidentified phases through statistical processing of images. We discuss interpretation of the distribution of these phases. A good correlation between the corrosion phases identified by CT and by invasive BSE-SEM is demonstrated. In addition to the stratigraphy of the copper corrosion compounds, we examine and discuss the variations observed in the attenuation coefficients of Cu(I) phases. Semiquantitative synchrotron X-ray microtomography phase mapping requires no specific sample preparation, in particular polishing or surface finishing, and any material tearing or displacement is avoided. We also provide evidence for the noninvasive observation of phases rapidly altered upon preparation of real cross sections. The method can be applied when cross-sectioning even of minute fragments is impossible.

Deciphering the Chemistry of Cultural Heritage: Targeting Material Properties by Coupling Spectral Imaging with Image Analysis

“…μCT can be used to determine the 3D distribution of compounds when the corresponding linear attenuations are sufficiently distinct to be matched to individual compounds. 3,48 Spectral imaging provides an essential complement by making it possible to collect spectra (or data of higher dimensionality) carrying speciation information in each voxel. A typical example is presented by Gueriau et al 49 and Davesne et al, 25 who used spectral imaging data to assess local redox conditions associated with the fossilization of ∼100 Ma fishes from Morocco at different time intervals.…”

“…60 For example, the regular observation of a very low intensity FT-IR signal in a specific layer of the stratigraphy of a musical instrument was used to validate jointly the existence of this layer and the presence of this specific signature (attributed to an iron-containing clay). 61 Li et al 48 discovered an unforeseen minor mineral phase in the 3D μCT image of a heavily corroded copper-based metal slab from the archeological site of Nausharo. This result was obtained by iteratively subtracting the contributions from the previously identified phases from the histogram of the gray values of the image.…”

“…Beyond point analysis, single-channel images (e.g., tomographic volumes, which are gray-level images characterizing the attenuation of the material in each voxel) already provide facies that are highly characteristic of certain dynamic processes (nucleation and growth, transport, drying, etc.). μCT can be used to determine the 3D distribution of compounds when the corresponding linear attenuations are sufficiently distinct to be matched to individual compounds. , Spectral imaging provides an essential complement by making it possible to collect spectra (or data of higher dimensionality) carrying speciation information in each voxel. A typical example is presented by Gueriau et al and Davesne et al, who used spectral imaging data to assess local redox conditions associated with the fossilization of ∼100 Ma fishes from Morocco at different time intervals.…”

A comprehensive strategy for exploring corrosion in iron-based artefacts through advanced Multiscale X-ray Microscopy

“…In the last few years, X-ray imaging, i.e, radiography and tomography, has been exploited in archaeometry to investigate depth corrosion, volumes, morphology and to detect cracks and other structural defects non-destructively and in three dimensions (3D) 8 – 20 . X-ray Microscopy ( XRM) had been already used in a huge variety of materials applications, such as lithium-ion batteries, aerospace, additive manufacturing, electronics and semiconductors engineering 21 , 22 due to its capability to investigate specimens at their microstructural level with resolution down to the micro- and nano-meter scale in a completely non-invasive way, allowing to understand the failure mechanisms and the properties of materials 23 .…”

Probing Trace Elements in Human Tissues with Synchrotron Radiation

“…For the past 70 years, synchrotron-based research has transformed the conventional thinking of what is experimentally possible in large realms of the natural and applied sciences: material science and engineering, condensed matter physics, chemistry and biology-related sciences, medicine, geology, environmental sciences, and public health. More recently, paleontological, archeological, anthropological, and cultural studies also utilized SR [11][12][13]. In a review paper published almost 15 years ago, Bilderback et al [5] acknowledged the success of the third generation SR applications to biological sciences as summarized in their brief research highlights subsection titled "Structural Biology".…”