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Contenido archivado el 2024-05-29

Developing new analytical techniques and materials for monitoring and protecting metal artefacts and monuments from the Mediterranean region

Final Report Summary - PROMET (Developing new analytical techniques and materials for monitoring and protecting metal artefacts and monuments from the Mediterranean region)

The partners of the project PROMET developed new strategies to monitor the corrosion of metal objects using the state-of-the-art portable techniques of Laser- induced breakdown spectroscopy (LIBS) and micro-X-ray fluorescence (micro-XRF). Scientific studies are providing other and newer ways to obtain information about these unique collections from the past. At the same time, conservation scientists developed and tested new materials, corrosion inhibitors, coatings, and PVD and PECVD barrier films as alternative ways of better protecting metals collections, ways that are safer, more effective, reversible, and longer-lasting.

Technological innovation is considered as the key element to economic development in the Mediterranean region that requires cooperation. To this end, the project known as PROMET represents an endeavour which brings together specialists and end users in the field of preservation of cultural heritage so as to develop innovative strategies to protect, preserve, and interpret the material culture made of metals, which is in museums of the Mediterranean basin.

To fulfil this curatorial mission, a good knowledge of the composition and manufacturing techniques of the finds of the collections is necessary. This information is of primary importance for improving the archaeological / historical data of the collection and for conducting a survey of the metallic collections to allow the set up of a coherent conservation and restoration policy.

Taking into account environmental data and the degradation mechanisms, the curator will need to develop and implement a coherent conservation plan. In some cases, the plan shows that conservation-restoration work must take place because of the presence of active corrosion on a metal object. At the end of the conservation treatment, a protection coating must be applied to slow down the exchange between the metal and the corrosive media. With the application of these protective coatings, a maintenance programme must be set up to prevent the degradation of the coating and to determine the time between two applications.

Unfortunately, most of those methods cannot be easily pursued because of the lack of qualified persons in the institutions; consequently, it is difficult to have a coherent conservation and restoration policy since, except for large institutions, the curator has to hire a Conservator-restorer (C-R) as well as search for funds.

The collections studied in the PROMET project are collections of different metals.

Archaeological collections
A1.Silver alloy collection from the Egyptian museum of Cairo
A2.Copper alloy and iron collection from the Archaeological museum of Ancient Messene, Greece
A3.Silver alloy Coins from the national Roman museum, Italy
A4. Copper alloy collection from the museum of Umm Qais, Jordan
A5. Copper alloy collection from the Rabat archaeological museum of Morocco
A6.Iron alloy collection from the villages of Calatrava la Vieja and El Saucedo, Spain
A7.Copper and iron alloys collection from excavations in the Syrian Arab Republic
A8.Copper alloy and iron collection from the Van museum in Eastern Anatolia, Turkey.

Historical collections
H1.Silver alloy collection from the museum of technology of Athens, Greece
H2.Silver Abbasid coins from the museum of Jordanian heritage
H3.Steel armour of the knights from St. John, grand masters palace, Malta.

Accelerated corrosion tests in climatic chambers and through electrochemical measurements were particularly useful to compare the efficiency of the different protection systems under study. Such an approach was essential to discard most of the traditional protection systems commonly used in northern countries in the conservation field (Renaissance and Cosmoloid waxes, cellulose nitrate, polyvinyl acetate), adapt the composition of the innovative CIs / coatings developed to meet the needs of the Mediterranean region and study the effect of corrosion stimulators. The latter must definitely be removed (through stabilisation treatments) so as to prevent any rapid failure of the protection systems applied.

Natural ageing of the most effective coatings / CIs on naturally aged coupons (reflecting better the alteration of real objects) confirmed some of the results already obtained through short-term testing and further improved the assessment of the protection systems under study. Paraloid B72 gave poor results on steel coupons where filiform corrosion developed.

Other forms of alterations were observed on copper and silver-based alloys. NTUA suggested that nano-alumina pigments are added to the acrylic resin, as this seems to decrease the risk of alteration on silver-based coupons; however, more testing is required to further support these preliminary results, especially on copper and iron-based alloys.

As regards innovative protection systems, it was found that PVD coatings, as prepared by DMME, should not be considered for steel objects. PECVD coatings gave interesting results on bronze and silver-based alloys. Concerning PVD coatings, more research is needed on the colour effects produced during the application and the reversibility of the coatings with time; the use of colourless and reversible PECVD coatings may be proposed if the required equipment is locally available. More research is needed to determine the durability of the coating.

A good replacement for Paraloid B-72 for temporary protection on partly oxidised historic steel objects was carboxylatation treatment (HC10 + H2O2 or NaBO3). Other CIs tested (OTH, FPTS and NaC10) were apparently less effective and would require more testing. Additional work would also be needed to confirm the efficiency of FPTS and bitriazoles CIs on copper-based objects and the application protocol of NaC7 and NaC10 on these materials should improve to prevent the whitening of the metal surface. Poligen ES 91009 could be suggested for the long term protection of historic steel objects. The role of CI additives should be studied further as well as the durability and reversibility of the coating alone or with the CI additives tested. Since the failure of these coatings was observed on oxidised copper-based objects an alternative could be the application of silane A, although its reversibility is problematic. This coating seemed to be an option for silver-based objects as well.

When testing the most promising protection systems on real artifacts and comparing them to the traditional Paraloid B72, we found out that Paraloid B72 should not be fully removed from the list of possible protection systems to use in Mediterranean countries. It should be applied, though with care (by C-Rs and after a good cleaning of the metal surface), and efforts should be made to improve the conditions of exposure of the objects protected (limitation of the fluctuations of environmental parameters). We confirmed though that the addition of alumina nano-particles should be beneficial (at least in the case of silver-based objects).

The good behaviour of NaC10 CI (for historic copper-based objects) and carboxylatation solutions (for slightly oxidised iron-based objects) for short-term protection was confirmed. FPTS CI gave good results on archaeological objects (even active ones) except when these contain Ag, in which case rapid tarnishing was observed. NaC10 should not be considered though on active archaeological objects. These new temporary protection systems are easy to repair and reversible in ethanol. Still they should be further investigated to define precisely their field of application, durability and reversibility beyond several months of application. If applicable, they could be used during transfer of collections from one building to another where uncontrolled environments are involved, or to objects on loan for temporary exhibition that require specific protection during transport.

We also confirmed that Poligen ES 91009 alone or with CI additives and silane A were good candidates for long-term protection. Poligen ES 91009 systems were suitable for historic iron-based objects, but should not be considered for active archaeological objects.

Furthermore, the conditions of their application to copper-based objects should be further defined. Silane A might be an interesting alternative here that could also be applied to silverbased objects. In both cases, the removal protocol should be further studied to make sure that it does not provoke any harm to the metal surface. If applicable, these new protection systems would solve the problem of the safe storage or exhibition of large metal collections exposed to uncontrolled conditions in museums of the Mediterranean countries.

The PROMET project was too short to study the combination effect of some particularly effective coatings and additional CIs. Based on the good results obtained for Paraloid B72, such testing should be carried out in future projects. Furthermore, at the time of writing this chapter, the assessment of the results for all the work performed, such as the assessment of natural ageing of protection systems on naturally aged coupons using reflectance FTIR, had not been completed.

It is too early to draw definitive conclusions on the real efficiency of the protection systems proposed by the PROMET partners. As stated before, the conservation community tends to be very conservative, and perhaps rightfully so, since innovative protection systems not tested enough and applied immediately to the field might provoke some new damages in the long-term that did not occur during our project because of its limited duration. Still, some PROMET partners actively work in the conservation field and will hopefully be the promoters of the methodology followed in the project and of future testing experiments on the different protection systems suggested.