RICERCA ITALIANA     

Novel technologies for assessing the level of deterioration of metals

Università degli Studi di Roma "La Sapienza"

Abstract

The cultural heritage suffer several methods of deterioration and its preservation cannot be based only on the experience of restorers, but must be based on suitable, validated scientific methodologies. In particular, metallic artefacts of archaeological and historic-artistic interest are subject to the interaction with the environment and quite often, when they are excavated if archaeological, or when they are restored if outdoors monuments, they are strongly modified with respect to their original manufacturing in their chemical composition or in their surface micro-structure and even in their bulk. Since the natural environments (atmosphere, water, soil) are complicated systems, changing with time, the interpretation of the full corrosion processes is very complicated and, even if they are well known, they give different results in different artefacts. Due to this situation it is very hard to find out general methodologies for assessing the level of deterioration of artefacts before restoration or their control for a preventive preservation.
The aim of this project is to determine some research methodologies, if possible not destructive, otherwise slightly invasive, for assessing the level of preservation of an artefact. We do not expect to find a single, general method, good for every metallic object. In this project we shall consider only copper alloys and precious metal alloys, that form two large groups of strong interest for the cultural heritage.
In the first step of the project metal artefacts will be chosen. This selection will be done by all the UO's with the collaboration of experts of the superintendence's, museums, restorers, ... The latter work with the cultural heritage and are familiar with the problems arising from the preservation. Then the UO POLITO and UO ISMN will produce reference samples with composition and morphological properties similar to those of the chosen artefacts; the reference samples will be aged artificially simulating natural processes, then they will be restored and protected. The reference samples, during the different stages of these processes, will be given to the other UO to be analysed with techniques either to be developed for the project or to be optimised. The UO UNIROMA1 will make a device for laser ablation with in situ monitoring using portable XRF, XRD, Raman and colorimetric equipments: the aim is to study the efficiency of the portable instruments for the diagnosis of the level of degradation; laser ablation is necessary for studying the degradation in depth from the surface; these results will be compared with the external monitoring. For acquiring more precise information, the samples, that have undergone laser ablation, will be studied also with microscopic and nonoscopic techniques of analysis available at the UO ISMN. The UO UNIPA will follow a similar approach, but it will focus on other parameters characterising the damaged artefact: its roughness or the distribution and concentration of paramagnetic ions, etc. The UO UNISS will study precious alloys, in particular gilded copper, used in the pre-Columbian South America, with the aim of improving the knowledge of their properties, of the production technologies and of the preservation methods. For this research the UO UNISS will develop a XRF profilometric device. The UO POLITO, who is expert in the electrochemistry required for studying the corrosion and preservation processes, will produce reference samples, as said above. In addition it will apply innovative techniques, in particular electrochemical impedance spectroscopy for evaluating the barrier effect of the surface layers on the reference samples before and after ageing, before and after restoration, by using a portable instrument, developed by the UO POLITO, implemented in order to widen the range of applicability. In addition, the UO POLITO, during the PECVD processes, will perform an accurate investigation by means of emission spectroscopy to study the main emitting species and how they are correlated to chemical and protective properties of the natural or artificial layers.
The investigations performed on the reference samples with many different techniques by the UO's will be compared with each other in order to find out general methodologies suitable to the assessment of the preservation state of metallic artefacts. The methodologies will be validated applying them to selected cultural heritage. During the project the collaboration with superintendence's, museums, restorers, ... will continue for updating us with new problems arising from the most recent archaeological findings. <<<

Principal Investigator

Mario Piacentini Università degli Studi di ROMA "La Sapienza"

Research Objectives

The electrochemical and physical processes related to the corrosion and to the deterioration of metal cultural heritages are well known and widely studied. Nevertheless, each object or monument is different from the others, since its conservation state strongly depends on the environment in which it has been buried or exposed to the weather during centuries. In fact, the environmental factors contribute in a decisive way to the processes of corrosion and deterioration of the artifact; they can also act in a different way in different points of the object either because it is made of different metal or because different areas have been exposed in a different way to the action of the external agents. For example, it is not uncommon to find in an excavation objects similar, close to each other but with a very different conservation state. For these reasons, it is very important for the restorers of metal artifacts to know their grade of deterioration. Even if an expert restorer is able to sense it with his “eye”, is more and more fundamental to carry out a diagnosis with well tested scientific methodologies, in order to avoid operations traumatic for the artifact and to successively preserve it in an adequate way for its future conservation.
The first objective of the research is the study of the deterioration processes to which metal objects of archaeological and historic-artistic interest are subject. Due to the great amount of different typologies of metal artifacts, we will restrict our work to artifacts made of copper alloys and noble metals.
These two categories are very important, since copper and its alloys have been the first metals employed since the earliest times; they are not only at the basis for the realization of tools for work and domestic use but, in some cases, they constituted a system of semi-precious alloys (the Roman horicalcus) and, finally, they are at the basis for all the bronze monumental statuary. Concerning the noble metals, the ancient jewelry represents one of the most refined and delicate forms of art, which is at the present still imitated; beside the jewelry there are also ancient coins, whose study is necessary in order to reach a deep knowledge of the social and economical situation in a specific historical period. The innovation of this project consists in the fact that the study is carried out putting together different competences: researchers working in the fields of electrochemistry, surfaces, metals and materials. The joined work, carried out with different techniques, has the aim of determine which techniques are able to provide information on the degradation state of the artifact in the less invasive way.
The second objective is the determination of the best methodologies which allow to preserve the artifact after the restoration, both studying the passivation processes of its surface and the protective coatings which are applied, and monitoring the condition of the artifact with periodical controls of its conservation state.
The third objective is the study of the ancient metallurgical technologies. This objective results from the need of having test samples that reproduce ancient alloys, worked following the ancient techniques, on which we can perform all the researches without the risk of damaging real ancient artifacts. <<<

First Results

There are different results that can arise from the proposed research, some concern the instrumental aspects and the development of advanced diagnostic methodologies, others are related to the possibility of monitoring the metal artifacts using standard procedures. Several of the techniques that will be used are at the present already employed for the study of metals, nevertheless all of them require a better optimization that, in some cases, implies significant instrumental changes.
The best aiming of all the techniques is certainly one of the principal expected results. Some instruments should be developed during the project and we hope that they will be significantly innovative for the study and the monitoring of ancient metal objects.
It is a common conviction among the specialists of this field that it is necessary to let the practice of conservation evolve towards investigation procedures that control and/or prevent the deterioration of the artifacts. The systematic study of the monitoring procedures most efficient for the different typologies of artifacts, a work that will occupy a great part of the second year of the project, is one of the most significant aspects of the project, also with respect to the possibility of a development of the metal conservation field toward procedures more modern and effective.
The secondary deliverables of the project are numerous and different. Certainly, the proposed research can contribute to improve the restoring techniques by using lasers, which are starting to be commonly used on different materials, but less on metals. The improvement of the techniques, which is one of the aims of the project can be useful to bring out new portable instrumentations with performances better than the ones available at the present. The SME could than be interested in the project in order to take its results in time. <<<

Timescale

24 months

National and international background

Modern science and technology are very important for a better knowledge of art and of cultural heritage. As an example we recall the use of thermo-luminescence and of radiocarbon dating methods in archaeometry, as well as the use of X-rays for investigating possible hidden layers in an art work. In spite of that, the application of modern science and technology in art conservation and archaeology is rather limited in comparison to the applications in other fields, such as medicine.
Let us consider medicine. Many years ago, an expert physician performed his diagnosis considering the symptoms shown by the patient and on external observations; however nowadays he can use more and more refined analytical techniques, specialised to the disease to be diagnosed: high contrast local radiographs, computer aided tomography, nuclear magnetic resonance, ecographies, etc.
Also the cultural heritage artefacts can be considered "patients" that can suffer several "diseases"; thus, the diagnostics either preceding a restoration operation or for preservation cannot be based only on the eye of the restorers, even if they are very expert, but it must be based on suitable, tested scientific methodologies.
The highly complex and diverse nature of the objects involved creates the need for an interdisciplinary approach, combining the expertise of chemists, physicists and engineers to overcome these obstacles and to expand the scientific and technological tools available for the preservation of cultural artefacts. For this reason the necessity of a multidisciplinary research is emerging for a better understanding of the fundamental aspects. There is also the requirement of a higher level of scientific education and of awareness for the cultural heritage. The aim is to overcome the obstacles to the use of scientific and technological instruments for the preservation of art works.
Nowadays, objects of art and antiquity may be diagnosed and treated using a variety of modern technologies, including X-Ray fluorescence (XRF), Raman spectroscopy, Laser Induced Breakdown Spectroscopy (LIBS), Small Angle X-Ray Scattering (SAXS), Electron Paramagnetic Resonance (EPR), X-Ray CT imaging, infrared imaging, electron microscope analysis (SEM), etc. The proliferation of these novel technological tools has already revolutionized medicine, making easer the establishment of scientific methods to replace previous reliance on speculation and deduction. A similar trend has appeared, although with some delay, in the fields of art preservation and validation as well as in archaeology.
Restricting ourselves to the field of metallic cultural heritage, the metallic artefacts of archaeological and artistic-historic interest interact with the environment and are subjected to modifications that involve the bulk and the surface layers. On several artefacts, in the excavations sites or during the restoration activities, noteworthy variations in the chemical composition and/or in the microstructure are detected. For example, in a copper alloy humidity can impoverish one of the elements present in the alloy: this process starts from the sample surface, then grows inside the material and, sometimes, it can affect the total thickness of the metal. Since the natural environments (atmosphere, water, soil) in contact with the artefacts are complex and subjected to variations during time, the interpretation of the overall long-period corrosion process is complex. The atmospheric corrosion is almost always a uniform corrosion; it occurs in presence of a layer of electrolyte, water or rain, on the surface of the metallic structure exposed to the atmosphere, whose aggressivity is increased by the presence of pollutant agents, such as sulphur dioxide, present in urban areas, and powders that act as condensation centres; chloride ions, present in sea-side atmospheres, accelerate noticeably the corrosion phenomena with R.U. higher than 70%.
The wet corrosion is related to the water aggressivity that depends in turn on the temperature, pH, oxygen content, presence and amount of dissolved salts (calcium ions, sulphate ions, chloride ions, etc..) and on the presence of micro-organisms. The seawater is particularly aggressive for the presence of high chloride contents that lead to the onset of localized attacks.
The corrosion in soil depends on the content of soluble salts, gas, organic compounds micro-organisms and temperature. In particular the range of the water content is really wide near the soil surface, while under the freatic layer the pores are totally filled up of water. Dealing with the oxygen content, at high distance from the surface, the oxygen diffusion rate is very low and th corrosion rate is consequently very low, while near the surface they accelerate because of the higher aeration. Just upon the freatic layer, both the oxygen uptake and the conducibility are high enough for inducing an active corrosion on the artefacts.
The silver-based artefacts are subjected to degradation phenomena that involve, first of all, the surface, as tarnishing, that modifies the aesthetic appearance, and by selective corrosion phenomena that penetrate in teh interior of the artefact and preferentially corrode the copper present in the alloy, while silver oxidation rate is lower giving rise to oxychlorides, mainly in marine environments.
Very little information is available on the composition and the technology related to pre-Columbian Peru gold objects, since only few fragments could be analysed in Laboratory. We can have only an incomplete view of the technology of that time and location. Nevertheless it appears that the gold alloy preparation not only was very refined, but also differentiated: simple gold alloys, gilded copper, silver plated copper, and a special alloy called "tumbaga".
In recent years several studies have been carried on the degradation of metal artefacts, but not always tailored and reliable conservation methodologies have been developed. Bronzes, covered with a surface layer, called patina, of alteration products, may be subjected to dramatic corrosion phenomena that destabilize the artefacts. The patinas are constituted by different crystal species: oxides, sulphides, sulphates, chlorides/hydroxychlorides, carbonates, oxalates. They can have a different texture, for example they can be porous or very compact, and their thickness can change significantly in different points. The wide variety of the possible corrosion products and of the structures cause different appearances to the artefacts, consequently the intervention methodologies have to be specific and tailored. It is important to find proper evaluation techniques of the barrier effect of the patinas in order to evaluate the possibility of removing partially or totally the layers in order to stabilize the artefacts.
The techniques currently employed for studying the processes of deterioration of metals are: elemental analyses (XRF, LIDS,...), surface analyses (XPS, Auger,...), microscopy and metallographic microanalysis (SEM). Some techniques are not destructive, the others require invasive treatments of the sample. For example, optical and electronic microscopies for looking at the bulk structure of the alloy require thin sections of the metal. The overall picture of the available techniques seems satisfactory, however it will be useful to develop new technologies with which it becomes possible to follow accurately the processes and quantify their evolution. In fact, the above diagnostic techniques lack of a good spatial resolution. These studies rarely use techniques employed for advanced researches, such as the new surface or nanoscopic techniques, whereas the degradation processes occur at a nano- at most microscopic level. Thus, macroscopic methods often cannot give objective data on the nature of the causes and the processes of deterioration, do not allow a detailed definition of the mechanisms and the conditions of risk, do not allow the complete and detailed acquisition of those information contained on the surfaces or sub-surfaces of metals. Thus it is not possible to disclose the degradation processes that are either active or potentially active (i.e. that become active if the environment conditions change), so that it is not possible to plan actions for reducing or stopping the deterioration processes.
The protection of a metal artefact is a complex item that involves:
- the identification of the chemical and micro structural nature of the corrosion products finalized to the understanding of the degradation mechanisms and to the identification of the aggressive agents by means of chemical-physical analysis.
- the application of tailored conservation methodologies by means of the selection and utilization of techniques, protective materials or inhibitors that hinder a further degradation of the artefact.
It is a widespread opinion that the degradation processes are characteristic and well understood; however they appear differently in different artefacts. Thus finding out general methodologies for assessing the conservation level of an artefact is very difficult. The existence of a large number of artefacts both indoors (museums, houses, public rooms) and outdoors require efficient control procedures for the conservation. In situ control techniques are now being used in industries and in avionics, as well as in special monuments such as the Eiffel Tower. However these techniques are not yet fully aimed, since they are not developed for artefacts and ancient metal objects. In this particular sector it is necessary to develop a research program which defines the role that different techniques have within a synergic frame of tests for assessing the level of deterioration processes. For developing such control methodologies will be necessary to modify and to optimize the techniques for in situ analysis. <<<