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Project ID: ICA3-CT-2002-10030
Financiado con arreglo a: FP5-INCO 2
País: Greece

Innovative conservation treatments for bronze archaeological artefacts via plasma

The hydrogen glow discharge plasma treatment is based on the reduction of the corrosion products by reactive reducing species, such as hydrogen atoms, in H2 glow discharge plasma of low pressure and temperature. Soil agglomerates and encrustations on the corrosion layer of the treated objects become brittle and can be easily removed mechanically by the conservators. In addition, the phases containing chloride ions can be destabilized and the chloride can be removed to the gaseous phase of the discharge

The optimisation of the hydrogen glow discharge plasma treatment (in terms of time, temperature and pressure), depends on the nature of the metal that is going to be treated and is strongly related to the nature of the corrosion products, the morphology and the thickness of the corrosion layer. Hydrogen glow discharge plasma can be safely employed for the treatment of Cu-based artifacts within the temperature range of 170oC to 240oC and for up to 6.5hours of duration, without altering the metallographic characteristics of the metal itself. The mechanism of plasma reduction involves the destabilization of the chloride corrosion products by the plasma atmosphere, the chloride removal into the gaseous phase of the discharge and the concomitant reduction of the corrosion products. Gas, temperature and time are the key parameters of the plasma process.

The treatment with 100% hydrogen plasma at temperatures ranging from 170oC to 240oC and for 1.5h to 6.5h duration can be characterized as mild having as priority the preservation of the metallographic characteristics of the metal itself. The prolonged treatment at higher temperature removes efficiently but not completely the chloride contaminants and gradually reduces the copper oxides, cuprite and tenorite back to copper metal. The complete removal of the large amount of the basic copper trihydroxychlorides that were introduced on the surface of the specimens by chemical corrosion is practically impossible, keeping in mind that the accelerated corrosion led to the complete coverage of the surface of the samples with the chloride corrosion products; despite that, longer treatments at higher temperatures can be more effective but not recommended because they can affect the integrity of the treated object. The plasma treatment at all conditions is successful in decreasing the density of the corrosion products and therefore facilitates subsequent mechanical cleaning. The gradual elimination of the chlorine containing corrosion products in favour of the formation of more stable species and sometimes even the complete reduction back to copper metal is proportional to the duration of the plasma treatment. It should be taken into account that a partial reduction is preferable to a complete reduction, as it does not make the object too brittle.

There is an indisputable change in the original colour of the samples, where the greenish coloration disappears completely after the plasma treatment. The colour monitoring of the treated samples in all cases revealed that the initial dark brown colour of the shorter treatments eventually turns to more shiny red at longer treatments, due to the formation of cuprite (Cu2O) and the subsequent reduction back to copper metal. Apart from the aesthetic reasons that should be considered there is also a question about the stability of the newly formed surface.

This technique should be considered as a good practice for the removal of the soil agglomerates and the free-chlorine corrosion products from objects with warty corrosion, as the encrustations become brittle and can be quite easily removed mechanically in order to clean down to the level of the rest of the patina. It should be taken into account that when the corrosion layer contains chlorine species, this process should be followed by complementary treatment, which will properly isolate and seal the surface from any contact with air and moisture.

That is because the presence of the disfiguring light green corrosion excrescences does not necessarily imply that they are unstable but it is the ability of the unreacted cuprous chloride to lie dormant until exposed to the atmosphere. As quoted previously, the plasma treatment creates minuscule cracks and therefore may trigger the bronze disease by creating paths for air and moisture to get in contact with any unreacted chlorides. The primary disadvantage of this conservation technique, although preliminary results have been encouraging, is the high cost of the equipment and the limitations in the size of the artefacts that can be treated imposed by the reaction jar.

Reported by

National Technical University of Athens, School of Chemical Engineering, epartment of Material Science
9, Iron Polytechniou Str.
15780 Athens
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