UNDP/IND/85/028 Assignment Rep0 rt Studies on Deterioration and Conservation of Metals

FOR INTERNAL USE ONLY: NOT FOR GENERAL DISTRIBUTION

I N D I A

Conservation of Metals

by

S. Turgoose

This report represents the views of the author and not necessarily those of Unesco

UNITED NATIONS EDUCATIONAL,, SCIENTIFIC AND CULTURAL ORGANIZATION (Unesco)

Paris, I987

Serial No. FMR/CC/CH/87/283(UNDP)

UNDP/IND/85/028 Assignment Report (Turgooce) FMR/ C C / CH/ 8 7 / 2 8 3 (UNDP ) Paris, 22 December 1987

Table of Contents

Page - I. INTRODUCTION

II. TRAINING

General lectures

Detailed lectures/discussions

III. DEVELOPMENT OF RESEARCH ACTIVITIES IN METAL CONSERVATION AT NRLC

Aims of research

Current infrastructure

Current research and expertise

Current conservation problems

Suggestions for research in metals conservation

IV. RECOMMENDATIONS

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UNDP/IND/85/028 - Studies on Deterioration and Conservation of Metals

Assignment Report

I. INTRODUCTION

1. The consultant was sent by Unesco to the National Research Laboratory for Conservation of Cultural Property, Lucknow (NUC) under Project IND/85/028, "Studies in Deterioration and Conservation of Metals", as a consultant in metals conservation.

2. The terms of reference were :-

"to advise and train in the field of metals conservation".

3. On arrival in Lucknow the detailed programme was discussed with the staff of NRLC. The two major aims identified were to provide training in the deterioration and conservation of archaeological metal artefacts and to advise on future research activities in metal conservation.

4. The agreed programme of training consisted of :-

(a) some general lectures to NRLC staff and to students undertaking the six-month conservation course run by NRLC, and

(b) detailed and specific lectures and discussions with members of the Metals Research Division, NRLC.

5. In addition, the consultant was requested to present a paper on the recent developments in the conservation of iron at the XVIIIth National Conservation Seminar, organized by the Indian Association for Study of Conservation and held at Lucknow during the period of the mission.

II. TRAINING

General lectures

6. The general lectures given to staff of NiUC and conservation students were :

(a) The Corrosion of Archaeological Iron with Particular Reference to Post-Excavation Processes - 2 lectures - Introduction to corrosion

. electrochemical nature of corrosion

. anodic and cathodic reactions

- Formation of corrosion products on iron - Thermodynamics of corrosion product formation - Corrosion processes

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- Chloride ions in corroded artefacts . chloride species . reasons for presence

- Post-excavation reactions . oxidation . corrosion

- Nature of species responsible for deterioration - Prevention of deterioration by control of environment . anaerobic storage

theory methods and practical considerations

. "dry" storage

theory definition of "safe" relative humidity methods and practical considerations

- Comparison of storage methods (b) The Conservation of Iron

- Scope of conservation - Aims of Conservation - Washing methods for stabilization . scientific basis . specific current methods, including

NaOH alkaline sulphite bo il ing water

. comparison of current methods mechanisms effectiveness practical considerations effects on other materials

- Possible future developments in iron conservation

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(c) The Application of Corrosion Inhibitors in Conservation

- Principles of corrosion inhibition . anodic and cathodic inhibitors

- Factors affecting corrosion inhibitors - Considerations in the inhibition of corroded metals - The use of inhibitors in acid cleaning . methods . mechanisms

- Inhibitors in the washing of iron . problems . possibilities

- Benzotriazole treatment of copper alloys . inhibitive effects . chemical effects

- Possible further applications of inhibitors Detailed lectures/discussions

7. upon the above general lectures in much more detail and in addition other subject areas were covered. The courses given were :-

Specific lectures to the staff of the Metals Research Division expanded

(a) Principles of Corrosion and Corrosion Prevention:

8. The aim of this was to present the basic principles of corrosion processes and possible preventative measures. The information contained provided a scientific basis for the subsequent lectures and discussions on the corrosion of specific metals and possible conservation methods for them.

9. The course included consideration of the following subjects :

- Electrochemical nature of corrosion - Electrode potentials and electrochemical series - Galvanic series - Thermodynamics of corrosion

. E-pH and Pourbaix diagrams

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- Kinetics of corrosion Tafel equation concentration polarization Evans' diagrams

- Localized corrosion differential aeration p it t ing galvanic effects

- Corrosion of alloys deal loy ing preferential corrosion of selected phases

- Corrosion in natural environments soils marine environments atmospheric corrosion microbial corrosion of buried and immersed metals

- Principle of corrosion prevention by modifying the environment

deaeration dess icat ion

classif ícation mechani sms effect of corrosion products on effectiveness factors influencing effectiveness

mechanisms of action

by corrosion inhibitors

by coatings

(b) Corrosion, Corrosion Product Formation and Conservation of Selected Metals:

10. The following factors were considered for metals of archaeological interest, i.e. iron, copper alloys, silver, lead and tin :

- The structure of corroded artefacts thermodynamic model of corrosion product structure kinetic effects compounds present composition of included solutions

- Corrosion processes in corroded artefacts location of anodic and cathodic sites

- Post-excavation processes oxidation of species present further corrosion

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- Reasons for instability of excavated material - Prevention of deterioration by control of environment - Cleaning of archaeological metals

acid cleaning methods complexing agents

- Stabilization treatments theory practice advantages and disadvantages of individual methods comparison of methods

11. The above emphasized the current position as regards treatments, the scientific basis of these, and the areas in which further improvements might be made. As such it provided a basis for discussion of possible research areas as described later.

(c) Application of Instrumental Techniques in Conservation :

12. These lectures concentrated on the techniques available to NRLC. The aim was to increase awareness of the potential applications of techniques in the field of conservation, and to highlight some of the problems that may be encountered in applying the techniques to corroded metal artefacts. discussed included :

The techniques

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- X-ray diffraction - X-ray fluorescence - atomic absorption - inf ra-red spectroscopy - optical microscopy - SEMIEDAX

(d) Methodology of Research in Conservation :

, The aim of these lectures/discussions was to i lustrate the processes involved in undertaking the problem-solving type of research involved in metal conservation. Areas covered included :

- how to break down a conservation problem into individual scientific questions; how to devise experiments to answer these questions; -

- the selection of relevant experimental techniques;

- the analysis of experimental results.

These lectures took the form of case studies, using the areas identified below as being possible research topics.

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III. DEVELOPMENT OF RESEARCH ACTIVITIES IN METAL CONSERVATION AT NRLC

Aims of research

14. conservation several factors were considered important.

15. They should utilize facilities currently available, or through this project shortly to be available, at NEC.

In initial discussions of possible future research activities in metal

16. Where possible, they should build upon past or current research activities and utilize the current expertise of the staff of NRLC.

17. They should be related to conservation problems existing in India and throughout South and South-East Asia.

18. The search for new conservation procedures should take account of the facilities available to conservators throughout the region.

19. In the course of the mission other desirable objectives became apparent. These were to provide the staff of NRLC with exposure to as wide a range of problems, materials and conservation treatments as possible (see paragraph 25).

20 0 As a basis, the present positions as regards. the above need to be considered.

Current infrastructure

21. As a result of this and previous programmes NRLC is, or shortly will be, equipped with a wide range of major instrumental facilities. relevance to research in metal conservation include :

Those of particular

- X-ray diffractometer - X-ray fluorescence spectrometer - Atomic absorption spectrometer - Inf ra-red spectrometer - SEM/EDAX (not yet installed) - Ion analysis equipment - Optical microscopes 22. In addition, NRLC has excellent library facilities. The library is clearly well financed and the selection of material has been made with care.

Current research and expertise

23. The major part of the research currently being carried out by the Metals Research Division of NRLC relates to composition, metallurgical structure and thus to technology of Indian metal artefacts. There are, however, some interesting aspects of this work which have direct relevance to conservation and could be extended in this direction (see paragraphs 35 and 36).

24. Research in the field of metal conservation has included studies of the stabilization of iron artefacts by the use of anion exchange resins, and a study of a range of corrosion inhibitors, including natural products, for copper alloys. Both of these areas could be extended.

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I- 25. AS regards research experience in the area of metal conservation, it needs to be emphasized that several of the staff of the Metals Research Division have only recently joined NRLC, and whilst they are experimentally competent on the analytical equipment,they are still learning as regards the application of these techniques in the field of metal conservation, both in terms of the potential benefits that may arise and the problems that may occur. Also, for the same reason, there is a lack of experience in the methodology of the problem-solving type of research commonly encountered in metal conservation. The attainment of expertise in both these areas inevitably takes time but the process can be accelerated by obtaining first-hand experience of a wide range of systems and problems. Aspects of these areas were covered in both the formal lectures and in the detailed discussions of possible research projects. In addition, the recommendation in paragraph 37 relates to one way in which these learning processes can be accelerated.

',

Current conservation problems

26. Some of the major areas identified by the staff of NRLC where conservation problems exist in India were :

(a) the stabilization of archaeological iron;

(b) the prevention of deterioration of copper alloy artefacts by the process commonly known as "bronze disease", and

(c) the cleaning and surface treatment of historical ironwork.

27. In general, most of the current problems relate to iron or copper alloy artefacts and this is reflected in the specific research areas discussed.

Suggestions for research in metals conservation

28. medium and long-term projects. This is considered important for the reasons given in paragraph 25. The projects also cover a range of materiais and types of conservation procedure.

The possible research areas described below provide a combination of

29. These research projects were discussed in detail with the staff of NRLC. These discussions covered the reasons why further research is needed, the present position in these areas, the possible approaches to the problems and the types or experimental procedures that could usefully be employed.

30. additional major equipment. acquisition of minor itmes (e.g. in the price range of up to US$ 1,000 - 2,000) but it is not possible to identify specific needs at this stage.

The projects below could all be carried out without the need for any Individual projects may benefit from the

Washing methods for archaeological iron artefacts :

31. Over the last 10-15 years a large number of new washing methods have. been proposed and whilst some are reasonably effective in certain situatiorsnone has received universal approval. In recent years the scientific basis for the

washing process has come to be understood and there is a limited amount of evidence to suggest that comparatively simple solution treatments may be effective, For example, treatments involving cold, neutral but deaerated solutians need to be investigated, and the work previously carried out at NRLC on the use of anion exchange resins also needs extension and quantification. The work would involve a study of the kinetics of chloride removal and of the fraction of total chloride removed in a range of possible wash solutions, with comparisons with current commonly used methods such as the alkaline sulphite and sodium hydroxide treatments.

Treatment of corroded copper alloy artefacts :

32. treatment may not be the universal solution to the problem of bronze disease, especially when it is required to preserve the patina on the artefact. precise interactions between benzotriazole and artefacts are not clear, particularly as regards whether th,e beneficial effects arise from the action of benzotriazole as a corrosion inhibitor or from its chemical reactions with the corrosion products. The work proposed would involve a study of this and a comparison with the effects of other possible treatments. NRLC has identified several natural products with inhibitive properties equal to benzotriazole and it would be useful to study their effects on corroded artefacts. Another uncertainty in the treatment of 'bronze" disease is the influence of metal composition OR the processes occurring and ora the effectiveness of possible treatments. copper-tin alloys (bronzes) whereas many Indian and Asian artefacts are esentially copper-zinc alloys (brasses). The effect of alloy composition on corrosion product composition is of considerable importance in this project, but also has many other implications in conservation and is discussed as a separate possible research area (paragraph 36).

In recent years there has developed an awareness that benzotriazole

The

Previous work at

Most work in Europe has involved either copper or

The selective chemical cleaning of gilt bronzes :

33. This is a shorter term study than the two previous ones and relates to a specific problem that currently exists. Many artefacts, originally gilded, now possess a thin fragile gold layer within the copper corrosion products. Mechanical cleaning would be likely to damage this layer but existing chemical cleaning methods would tend to remove all corrosion products and lead to detachment of the gilding. ones, on the dissolution of the individual components of the patina needs to be determined so that the outer copper (II) compounds can be removed without damage to the copper (I) oxide under the gilding.

The effect of cleaning solutions, including novel

Cleaning and surface treatment or historical ironwork :

34. Indian museums possess large collections of comparatively uncorroded .iron artefacts, e.g. arms. The cleaning of these and subsequent corrosion prevention presents a continuing problem. Recent work elsewhere has indicated that effective cleaning with minimal metal loss might be accomplished by the use of inhibited acids, but further work is needed particularly to accurately quantify the amount of corrosion that occurs during cleaning. Also the effectiveness of acid corrosion inhibitors is often temperature dependent and it needs to be

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(1 determined that these inhibitors remain effective under tropical conditions. As regards surface treatments inhibited waxes and oils have been used elsewhere but again their effectiveness needs to be assessed under the more severe

Also the physical characteristics of the coatings, e.g. softening temperature, need to be appropriate for the local climate.

t environmental conditions (temperature and humidity) that may occur in India.

Original surfaces in corwded artefacts :

35, Whilst this study is essentially metallographic or analytical it addresses a question that has been discussed by conservators for many years, namely, can the position of the original metal surface be located in the corrosion products? Work currently being carried out at NRLC has demonstrated that residual metallurgical structures can be identified in corrosion products, e.g. carbides in iron or remnant dendritic Structures in copper alloys. Clearly the limit of these features can also determine the limit of the original metal and in this study the correlation of the location of the original surface with structural or compositional changes in the corrosion products could be investigated.

The dependence of corrosion product composition on alloy composition :

36. In many alloys there is selective corrosion of one or more components. This means that the metal surface does not possessthe same cornposition as the original artefact, and in smaller artefacts the selective dissolution may mean that no part of the remaining metal has the original composition. products on such metals are often enriched in the elements depleted in the remaining metal core but there have only been a few attempts to quantify the relationship between the alloy composition and the corrosion product composition, and the influence of burial environment on this relationship. Building on the analytical studies of copper alloy artefacts being carried out at NRLC it would be useful to study this relationship for a range of copper alloys especially since in this system the nature of the copper (II) corrosion products gives an indication of the environment in which the corrosion occurred.

The corrosion

IV. RECOMMENDATIONS

37. To enhance the experience of the research scientists there should be a balance between short, medium and long term projects, with the initial emphasis being on the first and second. gations of specific problems of individual artefacts to one or two month research projects related perhaps to selection and testing of conservation procedures or to structure and composition of a particular type of artefact.

These would range from one or two day investi-

38. These short and medium term projects should be defined by conservators in discussion with the conservation scientists.

39. The type of work described in paragraph 37 above would benefit greatly from an increase in the quantity of material passing through NRLC for conservation. NRLC at present has links with many museums and efforts are being made to increase these and the amount of material received direct from excavations. These efforts are to be encouraged and will show benefits in research and also in the important teaching role undertaken at NRLC. However,

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significant increase in throughput of material would necessitate an increase in the number of conservators at NRLC. In this context it is worth remarking that the majority of high quality conservation research laboratories are based in organizations where the conservation scientists afe greatly outnumbered by the practising conservators. The wide range of conservation problems encountered in such a situation is invaluable in defining the nature of conservation problems and those areas in which long term research projects could usefully be carried out.

40. In view of the geographical location of N E C there will still be a need for exchange visits after the end of the current project. Visits to NRLC by international experts would usefully involve consultants whose main objective would be to discuss the current status of research programmes and to discuss and advise upon the future course of such programmes. Training visits abroad by NRLC staff should have two major objectives. First, they should provide an opportunity for NRLC staff to become acquainted with the application of instrumental techniques to a wide range of conservation problems and, second, they should provide training in methodology for research of a problem-solving rather than purely analytical nature.