Characterization of an iron smelting slag from Zimbabwe by Raman microscopy and electron beam analysis

Abstract

A sample of archaeological iron slag from Northern Zimbabwe was characterised by Raman microscopy (RM) and electron beam analysis (EBA). The phases identified in a slag found at an ironmaking site not only reflect the chemical composition of the slag but can also provide crucial information regarding the reconstruction and interpretation of the metallurgical operations, such as the prevailing redox conditions in a furnace at the time of cooling. Free iron oxides, such as haematite, magnetite and wüstite, are important indicators of these redox conditions. But while classical techniques of phase identification can be used to identify the different iron oxides, they cannot distinguish between dissimilar oxidation states due to their non-stoichiometric nature, and also the effect of substituting elements in iron oxides. RM proved invaluable in both respects. By combining the results provided by RM and EBA, it was possible to identify (1) the free iron oxides, haematite and magnetite in the original ore, and magnetite and wüstite in the slag, (2) oxidised magnetite and wüstite, characterized by a shift to higher wavenumbers of their intense characteristic Raman bands and (3) Al 3+ -substituted magnetite and a solid solution between magnetite and hercynite (FeAl 2 O 4 ). This pilot study aims to develop a model that enables close identification of the redox conditions by analysing the free iron oxides from a variety of smelting processes. It is hoped that this will provide an independent and quantifiable criterion to distinguish smithing slags (more oxidising) from smelting slags (more reducing), and to understand better the actual smelting process that transforms highly oxidised iron ore to fully reduced iron metal.

Determination and imaging of binder remnants and aggregates in historic cement stone by Raman microscopy

“…Band broadening and shifts – especially of this band – are known to be an indicator for the integration of foreign elements into the crystal lattice . The shoulder from 670 to 770 cm −1 coincides with the main band of magnetite (Fe 3 O 4 ) that is centered at 670 cm −1 . In the literature, the appearance of this band in hematite spectra is attributed to the presence of a small portion of magnetite in hematite .…”

“…We are aware that the investigated slag particle is characterized by a high heterogeneity in both phase composition and oxidation states, which might seem surprising. On the other hand, it is known that slags produced under nonequilibrium conditions or were subject to post‐process alterations often exhibit a range of redox conditions over the distance of only a few millimeters . Further studies including imaging of large areas are needed to study this in more detail.…”

Graphene as a lubricant on Ag for electrical contact applications

“…The Raman spectrum from the steel/Ag track shows a number of peaks e.g., at *560, 650, and 1320 cm -1 . These peaks can be attributed to metal oxides such as Fe 2 O 3 which has been transferred from the steel ball to the Ag surface during the tribological test [20]. In contrast, the Raman spectrum from the steel/GL/Ag track shows clear peaks from graphene, similar to the Ag/GL/Ag case, but no metal oxide peaks.…”

Geochemistry of Ancient Metallurgy: Examples from Africa and Elsewhere

“…It exploits the phenomenon that as radiation passes through a transparent medium, a small proportion of the incident beam is scattered in all directions (Muralha et al, 2011;Vandenabeele, 2004). It exploits the phenomenon that as radiation passes through a transparent medium, a small proportion of the incident beam is scattered in all directions (Muralha et al, 2011;Vandenabeele, 2004).…”

“…It exploits the phenomenon that as radiation passes through a transparent medium, a small proportion of the incident beam is scattered in all directions (Muralha et al, 2011;Vandenabeele, 2004). The application of Raman spectrometry in combination with electron beam analysis illuminated the chemical composition of iron slags from northern Zimbabwe and showcased the prevailing redox conditions in a furnace at the time of cooling (Muralha et al, 2011). The difference in wavelength between the incident and scattered radiation is characteristic of the material responsible for scattering (Pollard et al, 2011).…”