Study of corrosion in archaeological gilded irons by Raman imaging and a coupled scanning electron microscope–Raman system

Abstract

In this work, analytical and chemical imaging tools have been applied to the study of a gilded spur found in the medieval necropolis of Erenozar (Bizkaia, Spain). As a first step, a lot of portable equipment has been used to study the object in a non-invasive way. The hand-held energy-dispersive X-ray fluorescence equipment allowed us to characterize the artefact as a rare example of an iron matrix item decorated by means of a fire gilding technique. On the other hand, the use of a portable Raman system helped us to detect the main degradation compounds affecting the spur. Afterwards, further information was acquired in the laboratory by analysing detached fragments. The molecular images obtained using confocal Raman microscopy permitted us to characterize the stratigraphic succession of iron corrosions. Furthermore, the combined use of this technique with a scanning electron microscope (SEM) was achieved owing to the use of a structural and chemical analyser interface. In this way, the molecular characterization, enhanced by the magnification feature of the SEM, allowed us to identify several micrometric degradation compounds. Finally, the effectiveness of one of the most used desalination baths (NaOH) was evaluated by comparing its effects with those provided by a reference bath (MilliQ). The comparison proved that basic treatment avoided any side effects on the spur decorated by fire gilding, compensating for the lack of bibliographic documentation in this field. This article is part of the themed issue ‘Raman spectroscopy in art and archaeology’.

Development of a novel method for the in-situ dechlorination of immovable iron elements: optimization of Cl− extraction yield through experimental design

“…By taking into consideration the FWHM of the main peak detected at 26.8°2theta, the mean crystallite size was calculated to be around 80 nm. This value was found to be in accordance with the MCL values calculated from the XRD analysis of real akaganeite-rich rust samples collected from iron archaeological artefacts deeply analyzed in previous works (between 20 and 100 nm) 15 , 65 .…”

“…Among them akaganeite (FeO 0.883 (OH) 1.167 Cl 0.167 ), which chemical structure is characterized by tunnels (filled by chloride ions) parallel to the c-axis of the tetragonal lattice, tends to form low density rust layers 12 , whose high fragility facilitates cracking and exfoliation phenomena 13 , 14 . If not treated, akaganeite formation triggers a cyclical mechanism that often ends with the total consumption of the iron core 15 . Beyond the deterioration of iron elements directly exposed to the external environment, chlorides can penetrate porous concrete thus reaching steel reinforcing bars (rebar) 16 – 18 .…”

FTIR spectroscopic semi-quantification of iron phases: A new method to evaluate the protection ability index (PAI) of archaeological artefacts corrosion systems

“…With regards to archaeological iron artefacts resumed from oxic environments, the corrosion system is generally mainly composed of magnetite (Fe 3 O 4 ), goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and akaganeite (β-FeOOH). In this context, is it well known that magnetite and goethite are stable compounds that help the preservation of findings, whereas lepidocrocite and akaganeite can be considered as degradation accelerators, as seen in the literature [5][6][7].…”

Chemical study of degradation processes in ancient metallic materials rescued from underwater medium

“…This could mean that the treatment has not reacted properly with the material of the archaeological piece or the loss of the applied protector layers, probably due to the incompatibility between materials and the aggressive environment of the building, set in front of the sea and affected by the continuous marine aerosols. Anyway, it is demonstrated that the best treatment for an iron buried tool exposed to the atmosphere is that which accelerates the transformation of lepidocrocite/akaganeite into goethite …”