Archaeological Iron Conservation Colloquium 2010

iron-colloquium.abk-stuttgart.de

Abstract
AI

This research investigates the application of gas plasmas for the conservation of archaeological metals, focusing particularly on iron, silver, and lead artifacts. The study finds that pure hydrogen plasma treatments are effective in optimizing radical concentration and reducing corrosion layers, facilitating subsequent mechanical cleaning. Additionally, the research presents a novel combination of plasma treatment and alkaline sulphite desalination, resulting in long-term stability of treated artifacts.

Figures (8)

But the problem to stabilize iron finds is still with us, washing with water as Krause recommended does not help very much. The alkaline sulphite treatment described by North and Pearson was a big step forward for individual finds without organic remains. For dealing with the tons of iron excavated every year many conservation labs found it too complicated. While working on a book on iron conservation (Scott and Eggert 2009) it became apparent that there might be ways to simplify the treatment (no heating, diluted solutions, alternatives to sulphite to keep the oxygen out). That was start of a research project, generously funded by the Deutsche Bundesstiftung Umwelt (DBU) who also supports this colloquium to share its results with the international conservation community. The idea to compare variations was straight forward: It's not the amount of chloride extracted into the solution, but the chloride left behind in the object which needs to be measured (Eggert and Schmutzler 2009). Fortunately, the Romans left so many nails in the soil of Baden-Wiirttemberg that the Landes- amt fiir Denkmalpflege (LAD Baden-Wiirttemberg), cooperation partner for the research project and co-organiser of this conference, was able to provide us with enough samples. Experimentally, measuring chloride in iron turned out much more complicated than expected from the literature... gerhard.eggert@abk-stuttgart.de; b.schmutzler@abk-stuttgart.de — But the problem to stabilize iron finds is still with us, washing with water as Krause recommended does not help very much. The alkaline sulphite treatment described by North and Pearson was a big step forward for individual finds without organic remains. For dealing with the tons of iron excavated every year many conservation labs found it too complicated. While working on a book on iron conservation (Scott and Eggert 2009) it became apparent that there might be ways to simplify the treatment (no heating, diluted solutions, alternatives to sulphite to keep the oxygen out). That was start of a research project, generously funded by the Deutsche Bundesstiftung Umwelt (DBU) who also supports this colloquium to share its results with the international conservation community. The idea to compare variations was straight forward: It's not the amount of chloride extracted into the solution, but the chloride left behind in the object which needs to be measured (Eggert and Schmutzler 2009). Fortunately, the Romans left so many nails in the soil of Baden-Wiirttemberg that the Landes- amt fiir Denkmalpflege (LAD Baden-Wiirttemberg), cooperation partner for the research project and co-organiser of this conference, was able to provide us with enough samples. Experimentally, measuring chloride in iron turned out much more complicated than expected from the literature... [email protected]; [email protected]

but with reduced object temperatures (about 90 °C) during treatment.

Optimal settings are then assumed to be also valid for the treatment of metal artefacts. A group of 20 Roman nails is divided into small groups and differently treated. The effects of hydrogen-argon plasma, pure hydrogen plasma and heat treatment in vaccum are compared. Metallographic samples from each of the nails are taken both before and after drying and/or plasma treatment and observed under a scanning electron microscope. It is shown that only the plasma treatment leads to the desired separation between the original surface layer of the object and outer corrosion layers. Mechanical removal of outer corrosion layers can thereby be more easily and precisely carried out. A separation of layers due to dehydration can be ruled out. The facilitation of mechanical cleaning after plasma reduction is furthermore shown in a double-blind study. ruled out. The facilitation of mechanical cleaning after plasma reduction is furthermore shown The effect of plasma treatment on the iron corrosion products haematite, magnetite, goethite and akaganeite is measured by micro-Raman spectroscopy. Measurements on the powdered samples are carried out before and after the plasma treatment. The reduction of particular corrosion products in surface layers can be confirmed. The penetration of the plasma into different corrosion layers is investigated on archaeological artefacts. Corrosion products from stratigrafically cleaned areas as well as metallographic samples are investigated by means of Raman spectroscopy both before and after plasma treatment. Chemical reduction during treatment was confirmed to be active only at the surface layer, the more deeply situated corrosion layers having not been penetrated. and akaganeite is measured by micro-Raman spectroscopy. Measurements on the powdered

All the tests, with the exception of the 75% RH tests at 3000 and 6000ug/m’, produced more akaganeite. Geothite was obvious in the infra-red spectrum at 3000ug/m3 and 75% and was the major product at 6000ug. Close examination of the spectrum also revealed some geothite at 1500 ug/m3. To take this into account, samples were taken from the 75-76% tests and analysed with TGA to quantify the amounts of akaganeite and geothite present. These results are shown as 75-76% total above. The formation of goethite at 75% RH and higher acid concentration is possibly due to the decreased pH during Fe(II)Cl oxidation (Misawa 1974). An alternative explanation could be initial formation of akaganeite, which is then dissolved by the lower pH solution and re-precipitates as geothite.

As can be seen, the 30% and 55% thresholds appear to hold with real objects.

Fig. 1: Raman spectrum of the green/yellowish powdery corrosion product of iron identified as ammoniojarosite A green/yellowish powdery corrosion product was observed on some archaeological iron objects in storage at the museum as well as on freshly excavated iron objects. The Raman spectrum of the corrosion products (Figure 1) indicated that it is probably a member of the jarosite group, i.e. MFe3(SO4)2(OH)s compounds, where M = K, NHg, Na, etc. It was identified as ammoniojarosite, NH4Fe3(SO4)2(OH)5 by a combination of X-ray fluorescence spectroscopy (XRF) and X-ray diffraction (XRD).

References morphology needs further research. cryastals that often caused the flaking of archeological iron (Fig. 2 & 3). The RH of 54% wa: achieved by a saturated Mg(NO3)2.6H20 solution. Comparison of iron coupons exposed respectively, to the nitrate and glycerol solution which was used to achieve the same RH leve showed that the nitrate seemed to accelerate the formation and growth of the elongatec akaganeite crystals. This raises the following questions: are other pollutants such as SO2, NO: harmful to iron objects? Is the formation of elongated cerytalline akaganeite, which is oftet observed on archaeological iron, promoted by the presence of nitrate in the enviroment? Th« effect of RH levels and pollutants including nitrogen oxides and nitrate on the corrosior morphology needs further research.

Table 1 : Iron/sulphur containing compounds In our study, corrosion layers of 16" century old nails coming from a terrestrial site (Glinet, France) and of Roman ingots excavated from a wreck off Les Saintes Marie de la Mer (France) were analysed by combining several methods: scanning electron microscopy (SEM) coupled with X-ray microanalysis and Raman micro-spectroscopy. In each sample, mackinawite was detected, and its three physico-chemical forms could be observed. The corrosion layers were mainly composed of iron(II) compounds such as siderite (FeCOs), chukanovite (Fe2(OH)2CO3) and -Fe.(OH)3Cl, confirming the anoxic conditions of both burial environments. Such conditions are favourable for SRB development. As far as the eee EE See, on ee ote en RN ete eee A ee: ee yeneral oxidation processes of iron sulphides are widely studied and several mechanisn leduced from works realised on laboratory systems and synthetic products are proposed | iterature. Parameters like the oxic/anoxic nature of the environment, sulphur supply v ydrogen sulphide production, temperature, humidity, pH, etc. seem to determine or athway or another, leading from mackinawite either to the formation of pyrite or to tk lecomposition into sulphur and iron(III) oxyhydroxides. A recent work focused on tk nalysis of synthetic mackinawite by Raman spectroscopy demonstrated that this phase ca resent three physico-chemical states (nanocrystalline, well crystallised and Fe(III ontaining mackinawite), each of them being differentiated by a characteristic spectrui Bourdoiseau 2008).

Key takeaways

The penetration of the plasma into different corrosion layers is investigated on archaeological artefacts.
The influence of the plasma treatment on frequently occurring iron corrosion products could be made apparent in surface layers.
The experiments have used iron and iron (II) chloride mixtures as model systems.
The conversion of akaganeite to other iron minerals will almost certainly release chloride back into the object as no other iron mineral contains the same concentration of chloride.
Post excavation corrosion of archaeological iron has been widely studied and most of the research has been focused on akaganeite and iron oxyhydroxides.