Risultati finali
The production of 13 not commercially available reference Cu-based reference alloys characterised by chemical composition and metallurgical features similar to those of the ancient alloys is important for its possible use to test the innovative conservation materials and methods.
The chemical composition of the reference alloys is ascribed to the following material classes: Cu-Sn, Cu-Pb, Cu-Sn-Pb, Cu-Fe and Cu-Zn-Sn-Pb and covers the major part of the family of Cu-based alloys used in ancient times.
These materials can be used also as standard materials to optimise the analytical performances of portable or laboratory diagnostic equipments and at present are not commercially available.
New corrosion resistant coatings for the protection of bronze archaeological artefacts, by using clean and environmentally friendly processes, have been developed and optimised. The coatings deposition has been performed by PECVD technique (plasma enhanced chemical vapour deposition) in a home-made reactor with hexamethyldisiloxane-oxygen-argon mixtures of different compositions and at different input power values. The deposition has been carried out, in a first step, on Cu-based reference alloys produced by ISMN-CNR with microchemical and microstructural features similar to the ancient alloys, as-received and aged in different soils, and after the optimisation, in a second step, on archaeological bronze specimens, after restoration.
The optimisation of the PECVD treatment has been performed by changing the different parameters and has been accompanied by an accurate study of both the plasma phase and the characteristics of the deposits obtained; this information, in addition to assuring the exploration of all the possible operative conditions turns out to be indispensable for the reproduction of the results, for successive process scale-up and for the in situ process control. Starting from inorganic precursors (silane) or organic (organosilicons), it is possible to deposit thin film of SiO2-like containing variable quantities of carbon, hydrogen, and eventually nitrogen (if contained in the monomer); when these molecules are immersed in the plasma phase, they fragment due to electron impacts, generating ions and highly reactive radicals, which may react in a gas phase, producing films more or less thin (0.01 - 5 micron), with variable composition and property.
The reaction occurs at a relatively low temperature, 20 - 70°C and the fragments formed by the organosilicon precursors, on the whole, may react with possibly added oxidants (generally O2), both in the gas phase (homogeneous oxidation), and on the surfaces on which the organosilicon film grows (etherogeneous oxidation). Naturally, with the proper variations in the conditions of the process (substrate temperature, specimens position in the reactor, oxidant concentration) it is possible to vary with continuity the chemical composition and the properties of the deposit. The low temperatures and the low oxidant concentrations promote the production of deposits with high organic component, flexible, slightly hard, permeable to gases and to water, and at times, not very stable. The high temperatures, the ionic bombardment and the massive oxidant addition generate inorganic film (SiO2-like): stable, non porous, slightly permeable to water and to gases, hard, chemically resistant.
Even the temperature and the difference of potential between the bulk of plasma and surfaces are very important, since they induce the breakage of the bond Si-H, Si-CH and C-H; they increase cross-linking and reduce the tenor of carbon and hydrogen.
The elevated compactness, low porosity, hardness and low permeability to water and gases of SiOx deposits obtained in this project revealed them definitely suitable for the protection from corrosion of metals. Moreover the reversibility, due to the possibility of removal by means of plasma treatment and the good aesthetic appearance render them suitable also for cultural heritage artefacts of artistic, historical and archaeological interest.
The noticeable scientific and industrial interest of the PECVD technique is a highly versatile technique that can be employed for the production of a variety of coatings, on a avariety of substrates, from metals to textiles, paper and polymers, with a wide range of properties simply by means of a proper selection of the experimental conditions.
As part of the programmed tasks in the framework of the EFESTUS program regarding the identification of the chemical and physical degradation mechanism of the archaeological artefacts, it arose the need to analyse the effect of the atmospheric environmental conditions found in Museums and storage facilities, where such artefacts were going to be exhibited or kept, presumably for a long time.
In order to provide a set of data that is both locally meaningful and analytically representative, and to obtain significant information about the atmospheric conditions and their effect on the metallic materials of chose, the Atmospheric Environment Control Stations (AECS, or CS) were developed.
Their final objective is to identify the environmental elements responsible for the corrosion processes going on in very specific locations, so allowing preventive measures to be taken accordingly.
Each control station is composed by the following elements:
- One set of metallic reference probes (10mm diameter disks) in a sample panel, to be exposed to the atmosphere. The amount of probes will depend on how many different materials are to be tested and how many exposition periods are programmed.
- One High Sensitivity Atmospheric Corrosion Probe
- One digital data logger that measures and registers environmental temperature and humidity
- Passive Samplers to gather information about the solid particles in suspension in the atmosphere and the concentration of various gaseous contaminants of choice (SO2, NO2, Organic Acids&).
All of them assembled in a methacrylate frame.
As a whole, the CS is adaptable and relatively inconspicuous, so it can be set anywhere, including the interior of museum's exhibition cases, with a minimal disruption of the visual arrangement of the exhibit.
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.
A new standardised method for producing degraded Cu-based alloys via an accelerated procedure based on chemical and burial treatments has been developed and optimised.
The starting Cu-based alloys are characterised by a chemical and metallurgical features similar to those used in ancient times and after the accelerated degradation process are characterised by a corrosion products structure i.e. the patinas, similar to those grown on archaeological artifacts.
These materials are not commercially obtainable and can be used for evaluating the effectiveness of conservation materials and methods as well as for anticorrosion studies.
The standardised method has been proposed on the base of the results achieved in the framework of the EFESTUS project. The results have shown that the bronze archaeological artefacts suffer of an intense and selective dissolution of copper that in many cases induce the formation of a stratified structure constituted by an external zone of Cu (II) compounds also enriched by the soil elements and an internal zone with a cuprous oxide layer (Cu2O) and a noticeable amount of copper chloride at the interface between cuprite and Cu-base remaining alloy matrix.
The role of the cuprite layer is considered acting as an electrolytical membrane allowing the transport of anions such as Cl- and O2- inward and cuprous ions outward. Indeed, the presence of copper chlorides in the archaeological artefacts indicates a noticeable transportation of chlorides from the soil trough the permeable corrosion product layers to the internal zone and remaining Cu-base matrix. The accumulation of chloride ions can be interpreted as an autocatalytic reaction that facilitates the oxidation of copper resulting also in an accumulation of chloride ions and in the formation of cuprite and cuprous chloride.
On the base of this information the method has been designed, optimised and tested and the corrosion products structure successfully compared with the corrosion products structure grown on the archaeological artefacts.
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