Microbiologically influenced corrosion of archaeological artefacts: characterisation of iron(II) sulfides by Raman spectroscopy

Abstract

Archaeological objects are exposed to the action of micro-organisms when they lay in a biologically active environment. The presence of iron sulfides in the corrosion system testifies in most cases that the degradation was influenced by sulfate-reducing bacteria. Iron sulfides and other iron/sulfur-containing compounds were detected by micro-Raman spectroscopy in rust layers of archaeological ferrous objects and in wet wooden fragments contaminated by iron, extracted from ancient wrecks. Although mackinawite (FeS) is very reactive towards oxygen, this phase was observed in each sample. Its crystallisation levels and oxidation states could be differentiated. Greigite (Fe 3 S 4 ) was also identified by means of X-ray diffraction, used when possible as a complementary analytical tool. Known as the result of the oxidation of mackinawite, greigite was likely to form during the experiments carried out without any protection against air. It was then possible to describe the formation and oxidation processes of iron sulfides in archaeological iron artefacts or in organic materials contaminated by iron.

The transformation of mackinawite into greigite studied by Raman spectroscopy

“…3(a) must be attributed to greigite, in agreement with a previous work dealing with archaeological artefacts. [37] The same experiment was performed many times, and always led to a partial transformation of Fe II S into Fe 3 S 4 . The evolution of the pH of the suspension was followed with time.…”

“…Incidentally, the Raman spectrum of synthetic samples of greigite could be obtained for the first time, which confirmed previous analysis performed on archaeological artefacts. [37] Experimental Synthesis of iron sulfides Fe(II) sulfides were precipitated from FeSO 4 · 7H 2 O and Na 2 S· 8-9H 2 O solutions. The Fe(II) sulfate was provided by Aldrich and verified to be of 99% minimum purity.…”

Raman spectroscopy in art and archaeology

“…The presence of iron sulphides in the corrosion system testifies in most cases that the degradation was influenced by sulphate-reducing bacteria. Iron sulphides and other iron/sulphur containing compounds (mackinawite, FeS and greigite, Fe 3 S 4 , mainly) were detected by Rémazeilles et al [9] in rust layers of archaeological ferrous objects and in wet wooden fragments contaminated by iron, extracted from ancient wrecks. The detection of Fe(III)-containing mackinawite or greigite indicates that the archaeological object was exposed in the burial to a small amount of oxygen; this testifies on one hand that the object settled in anoxic conditions and on the other hand that it suffered from microbiologically influenced degradation.…”

Hydrogen Sulphide Corrosion of Carbon and Stainless Steel Alloys Immersed in Mixtures of Renewable Fuel Sources and Tested Under Co-processing Conditions

“…On the other hand, iron sulphide in a form close to mackinawite [42] might keep evolving in the early stages on 1.0425, St35.8 and 1.4541 as wavenumbers of moderate intensity at 321, 317, and 315 cm -1 , respectively, were detected. They usually appear as intense overlapping peaks between 310 and 320 cm -1 depending on the degree of crystallinity [43]. Thus, this seems to concur with greater reaction rates leading to a lower proportion of crystallinity in the product as the rate of nucleation overtakes to a level of far beyond orientation and growth.…”