Characterisation of pigments and corrosion patinas by means of micro-Raman spectroscopy

New applications of micro-Raman spectroscopy are presented here for the investigation of colouring agents in a wax model of plant and of corrosion products on metal artworks. A late 1700 wax model of Camellia japonica L. was studied in order to characterise the nature of white, red and green colours of petals and leaves. White and red colours of petals were identified as pigments of baryte and a mixture of cinnabar and carmine lake, respectively. Fragments taken from leaves with different green hues were also studied; only yellow grains were isolated in this case which were identified as orpiment and yellow chrome. The applicability of micro-Raman spectroscopy to the study of corrosion products on metal objects is presented here for iron archaeological artefacts and for lead samples taken from the cupolas of the Consolata Sancturay in Turin. Analysis of blackish and reddish corrosion patinas entirely covering the archaeological iron objects led to the identification of magnetite and maghemite, goethite and lepidocrocite, respectively. As to lead fragments, the main corrosion product was lead sulphate, and minor amounts of lead carbonate, nitrate and oxide (litharge) were also found.     

A Study on Corrosion Processes of Archaeological Glass from the Valencian Region (Spain) and its Consolidation Treatment

The combined use of scanning electron microscopy/energy dispersive X-ray microanalysis (SEM/EDX) and square wave voltammetry (SQWV), is used for studying the corrosion processes that have taken place in buried glass from different archaeological sites in the Valencian Region (Spain). The procedures permit a parallel investigation of morphology and chemical composition. Determination of the chemical composition of the glasses and their alteration crust and identification of the elements responsible for the colour has been also carried out using energy dispersive X-ray microanalysis. The electrochemical response of samples attached to paraffin-impregnated graphite electrodes reveals the presence of different iron and manganese oxide species in the browning areas of the corroded glass. Image analysis applied to microphotographs obtained by means of SEM led to the determination of morphological parameters concerning the corrosion phenomena occurring on the surface of the fragments such as thickness of the corrosion layer and its laminated structure. Additionally, measurement of the thickness of the film of polymer used as coating in the consolidation treatments has been carried out using cryo-scanning electron microscopy (cryo-SEM) combined with image analysis.     

‘One‐Touch’ Voltammetry of Microparticles for the Identification of Corrosion Products in Archaeological Lead

Voltammetry of microparticles is applied to the identification of lead corrosion products by means of an essentially non‐invasive ‘one‐touch’ technique based on the use of graphite pencil. This methodology permits the mechanical attachment of few nanograms of sample from the surface of lead archaeological artefacts to a paraffin‐impregnated graphite electrode, which, upon immersion in aqueous electrolytes, provides distinctive voltammetric responses for litharge and cotunnite‐ anglesite‐, cerusite‐based corrosion products. The reported method is applied to the identification of corrosion products in archaeological lead pieces from different Iberian sites in Valencia (Spain).     

Minerals associated with artists’ paintings and archaeological iron objects

This article provides a short review of mineral-based pigments used in paintings with examples drawn from technical studies of selected historic paintings. Pigments such as azurite, natural ultramarine, orpiment, and clay earth pigments have been identified. Some examples will also be given of particular case studies which describe the alteration of selected pigments and consequences of these interactions. The second theme shows how use has been made of such interactions in evaluating the effects of environmental impact on paintings and reference is made to previous studies and the application of paint films as dosimeters. Accelerated ageing and site exposure studies are reported, and results provide information on pigment binder interactions. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TG) have been used to characterise the behaviour of pigments in binding media and to assist in characterising samples from wall paintings. Reference is also made to the occurrence of iron-oxide based minerals present as corrosion products in archaeological iron objects. Examples are given of objects from two archaeological sites in England, the Anglo-Saxon burial site Sutton Hoo in Suffolk, and the burial site in Wetwang, East Yorkshire. It will be shown that post excavation changes occur in the objects and this information is used to inform preventive conservation of these objects, in storage and in display.     

Dating Archaeological Strata in the Magna Mater Temple Using Solid‐state Voltammetric Analysis of Leaded Bronze Coins

The application of solid state electrochemistry techniques for dating archaeological strata using lead‐containing bronze coins is described. The proposed methodology was applied to samples coming from the Roman archaeological site of Magna Mater Temple (Rome, Italy) occurring in different strata dating back between the second half and the end of the 4^th century A.D. and the 20^th century. The voltammetric signatures of copper and lead corrosion products in contact with aqueous acetate buffer, as well as the catalytic effects produced on the hydrogen evolution reaction, were used for establishing the age of different strata and dating coins belonging to unknown age. Voltammetric data were consistent with a theoretical approximation based on a potential rate law for the corrosion process.     

Application of Modified Tafel Analysis to the Identification of Corrosion Products on Archaeological Metals Using Voltammetry of Microparticles

An operational modification of Tafel analysis, applied to the intermediate region of square wave voltammetric curves, devoted to the identification of corrosion products on archaeological metal, is described. This is based on the voltammetry of microparticles methodology using conventional abrasive conditioning of the electrode, as well as ‘one‐touch’ and layer‐by‐layer techniques. The proposed methodology is applied to the identification of copper and silver corrosion products in mediaeval silver‐copper coins from the Libertad street hoard in Valencia (Spain).     

Multianalytical study of corrosion layers in some archaeological copper alloy artefacts

Corrosion layers in some copper and bronze archaeological objects from Haft Tappeh archaeological site, southwest Iran, were studied. For this purpose, optical microscopy, scanning electron microscopy with energy dispersive X‐ray microanalysis, micro‐Raman spectroscopy and X‐ray diffraction methods were applied to observe corrosion stratigraphy and their characteristics as well as identification of chemical composition and phase determination of different corrosion layers. Based on optical and electron microscopy, three different corrosion strata were identified in cross section of different metallic objects including various red, green, white‐grey powdery and dark internal compact layers. Scanning electron microscopy with energy dispersive X‐ray microanalysis on different corrosion layers revealed that Cu, Sn and Cl are the main elements in the chemical composition of different layers. Tin‐rich phases were detected in white‐grey and dark layers that may be formed because of the internal oxidation of tin as well as the decuprification (selective dissolution of copper) phenomena occurring during long‐term burial period in the soil. Also, the XRD and micro‐Raman spectroscopy results proved that the main corrosion products are nantokite (CuCl), copper trihydroxychlorides and copper oxides. The combination of these analytical methods allows us to explore the surface and internal corrosion layers of the archaeological copper and bronze samples, and major interest is on studying their chemistry, microstructural properties and corrosion stratigraphy. Copyright © 2015 John Wiley & Sons, Ltd.     

Integrated analytical methodologies for the study of corrosion processes in archaeological bronzes

The investigations on structure and micro-chemical composition of archaeological metal alloys are needed in archaeometry. The aim of this study is devoted both to acquire information about their provenance and production technology, and to improve our understanding about the corrosion processes. In this paper we present the study of the corrosion phenomena of bronze samples, laboratory-made according to binary, ternary and quaternary alloys typical of Roman archaeometallurgical production through an integrated methodology based on the use of non or micro invasive physical techniques. Among the analysed samples, two were artificially aged through burial in the archaeological site of Tharros, along the west coast of Sardinia (Italy). The corrosion products, typical of the bronzes in archaeological sites near the sea, have been characterized by non invasive and micro-destructive measurements. In particular, the corrosion patinas were examined through optical microscopy, scanning electron microscopy and microanalysis, X-ray fluorescence and laser ablation spectroscopy. The use of integrated technologies allowed us to determine both the elemental composition and surface morphology of the patina, highlighting the correlation between patina nature and chemical composition of the burial context. Moreover, data obtained by the laser-induced breakdown spectroscopy along the depth profile on the samples, have yielded information about the stratigraphic layers of corrosion products and their growth. Finally, the depth profiles allowed us to verify both the chemical elements constituting the patina, the metal ions constituting the alloy and the occurrence of migration phenomena from bulk to the surface.     

Archaeopolymetallurgical study of materials from an Iberian culture site in Spain by scanning electron microscopy with X-ray microanalysis, chemometrics and image analysis

Archaeometallurgical materials from “La Bastida de Moixent”, a site in Valencia (Spain), from the second Iberian iron age (4th Century B.C.) have been studied using metallographic techniques, microanalysis, chemometrics and image analysis. The materials come from various phases of iron production and cupellation of argentiferous lead to obtain silver. Scanning electron microscopy (SEM) is used to determine the morphological, microstructural and topographic characteristics of the samples. Image analysis was used to obtain a numeric estimate of the main components in these materials. X-ray microanalysis (SEM/EDX) provides qualitative and quantitative information about the elements in the sample. The semiquantitative results have been treated by chemometric tools such as Principal Component Analysis (PCA) to group the different archaeological materials. These analyses provide information on the lead and iron metallurgical processes, such as silver production by cupellation used in the period. Macroscopic and microscopic analyses indicate a high quality, acceptably uniform manufactured product, indicating broad technical skill in the later metallurgical process of transformation and refinement of these materials to obtain ingots and manufactured products to trade.     

Iron Forms Fe(II) and Fe(III) Determination in Pre-Roman Iron Age Archaeological Pottery as a New Tool in Archaeometry

This article presents studies on iron speciation in the pottery obtained from archaeological sites. The determination of iron forms Fe(II) and Fe(III) has been provided by a very simple test that is available for routine analysis involving the technique of molecular absorption spectrophotometry (UV–Vis) in the acid leachable fraction of pottery. The elemental composition of the acid leachable fraction has been determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Additionally, the total concentration of the selected elements has been determined by X-ray fluorescence spectrometry with energy dispersion (EDXRF). The results of the iron forms’ determinations in archaeological pottery samples have been applied in the archaeometric studies on the potential recognition of the pottery production technology, definitely going beyond the traditional analysis of the pottery colour.     

Monitoring stabilizing procedures of archaeological iron using electrochemical impedance spectroscopy

A methodology for monitoring washing procedures applied to stabilize archaeological iron is described. It is based on the combination of voltammetry of microparticles (VMP) with electrochemical impedance spectroscopy (EIS). A semi-empirical approach is used where the impedances at low and high frequencies were related with the fraction areas of passive and corrosion layers generated during the stabilizing treatment, the thickness, and the porosity of the corrosion layer. The variation of such parameters with the time of washing was determined from EIS data for four types of desalination procedures using concentrated NaOH and/or Na₂SO₃ aqueous solutions on archaeological iron artifacts. After 2 months of treatment, EIS data indicate that an essentially identical “stable” state was attained in all cases, as confirmed by the formation of a passive magnetite layer identified in VMP measurements while the rate of variation of corroded surface and porosity at short washing times varied significantly from one stabilization procedure to another.     

A mechanistic model for the metabolism of corrosion products and of biological trace elements in metallosis tissue based on results obtained by activation analysis

Instrumental neutron activation analyses have been carried out on tissues adjacent and distant from metal implants. From the time-dependence of the concentrations of the corrosion products some conclusions have been drawr on the routes of metal transport from the implant. Additionally, the behaviour of haematoma iron has been shown as well as the dependence of the zinc concentrations on different biochemical processes, during implantation and then at the following stages of the development of the metallosis tissue. This work was supported by the Bundesministerium für Forschung und Technologie, the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie.     

Use of thermomagnetometry in the study of iron-containing pyrotechnic systems

Thermal analysis is routinely used to characterize pyrotechnic fuels, oxidants and fuel/oxidant mixtures [1]. Thermomagnetometry (TM) can provide additional information if the magnetic properties of the materials change during reaction. TG, TM and DTA results for the iron/potassium permanganate, iron/barium peroxide, and iron/strontium peroxide systems as loose powders or pressed pellets indicate predominantly solid-gas mechanisms for reactions in these systems.     

Mössbauer and X-ray analysis of corrosion products from surface equipment of oil production well

Wellhead equipment corrosion samples including internal surface of choke were analyzed by means of Mössbauer spectroscopy and X-ray diffractometry. The samples include some impurities of quartz, dolomite and calcite. The resulted analysis shows some iron oxides and oxyhydroxides as surface equipment corrosion products.     

Analysis of Roman-Hispanic Archaeological Ceramics using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a useful tool for characterization of archaeological samples because it involves little or no sample preparation, and it allows on-site rapid and nondestructive analysis. In this study, ceramics from the archaeological site of Labitolosa (Huesca) were analyzed qualitatively and quantitatively. Furthermore, a protocol is described for the determination of iron in archaeological materials by interpolating calibration graphs and standard addition. Moreover, the potential of LIBS for direct quantitative analysis of iron in real archaeological materials was evaluated, and optimum analytical conditions were determined. The reproducibility of LIBS spectra from different archaeological samples was investigated as a function of the number of laser shots. The results were compared with those of a reference material.     

Corrosion behaviour of soldered joints of magnesium alloys and dissimilar materials

In the last decade magnesium based materials have been used in many industrial applications. Problems arise from joining technologies and the corrosion behaviour of joined parts. The present study shows results of a new flux free soldering technology and the study of the corrosion properties of these soldered samples. Joints have been obtained by soldering of magnesium alloy AZ31 samples among each other or to aluminum alloy AlMgSi1 as well as to unalloyed, low-carbon steel. Different corrosion protection layers have been applied by thermal spraying. The corrosive potential of different soldered samples has been examined using current density-potential measurements. The corrosion behaviour of coated samples has been studied by salt spray testing in comparison to uncoated samples.     

Spectrophotometric determination of iron and reducing agents with PPTS, a new water-soluble ferroin-type chromogen of superior sensitivity

A sulphonated derivative of 3-(4-phenyl-2-pyridyl)-5-phenyl-1,2,4-triazine has been prepared, its structure identified, and its chelation products with iron(II) and copper(I) identified and characterized. The water-soluble compound, referred to as PPTS, has been applied to the determination of iron in various types of samples and the spectrophotometric determination of trace quantities of certain reductants.     

Applications of SNMS in archaeometry

The recently developed High Frequency Mode HFM of electron gas SNMS allows investigations on insulating samples with the well known advantages of the SNMS Direct Bombardment Mode DBM for the analysis of conducting samples. HFM has been applied to analyses of different historic ceramic and glass samples in order to demonstrate the possibilities of SNMS in this field. It is shown that manufacturing places of ceramic samples could be distinguished by SNMS mass spectra ("fingerprints"). Furthermore questions of the constituents of colour remains on a painted ceramic ("Kaisermedaillon") could be answered by our SNMS analyses. SNMS investigations have been also applied to corrosion phenomena on different glass samples.     

Raman spectroscopic analysis of the Maya wall paintings in Ek'Balam, Mexico

Raman spectroscopy has been applied to the examination of wall painting fragments from the archaeological site of Ek'Balam (Yucatán, Mexico). Thirty-three samples have been studied, all originating from room 23 of the Acropolis, and being representative of the painting technique at Ek'Balam during the late Classic Maya period. Several pigments such as haematite, calcite, carbon, cinnabar and indigo were identified in these samples. The latter pigment was presumed to be present as 'Maya blue', which is an intercalation product of indigo and palygorskite clay. The observed Raman spectra are reported and some band assignments have been made. This survey is the first Raman spectroscopic examination of a whole set of pigments in archaeological Maya wall painting fragments.     

XPS imaging investigations of pitting corrosion mechanisms in Inconel 600

The surface chemistry associated with pitting corrosion on a nickel‐based alloy, Inconel 600, has been studied using imaging x‐ray photoelectron spectroscopy. The irregularity of the sample surfaces necessitated a more elaborate background correction than is usually applied in such imaging. Areas of active pitting were found to contain corrosion products that are high in chromium oxide and depleted in nickel and iron. Sites of anodic activity on the surface were able to be defined more clearly using principal component analysis. Elements that are dissolved preferentially are deposited cathodically within well‐defined regions whose distance from the anodic pit appears related to the open‐circuit potential. Copyright © 2005 John Wiley & Sons, Ltd.