Soutenance mémoire
Outdoor copper alloy sculpture
Preserving and conserving outdoor monuments is often a complex operation : being artworks, they both have an artistic and historic character, but as those artefacts are also part of the public heritage, they are supposed to take part in everyone’s day-life. By commemorating people, events or ideas, by decorating fountains, parks, streets or buildings, outdoor monuments are often a symbol to which people may be closely linked. They belong to the public, playing a major role in the history of the city and being an expression of art and civilization. As these monuments are dedicated to “live” along with people, they obviously degrade with time. As such, they need to be regularly maintained and cared for to remain attractive and highly visible.
Alterations to the surface are strongly dependent on the geographical environment of a sculpture. Artworks exposed to a marine atmosphere would not behave in the same way than the same sculpture exposed to a rural or an industrial environment. Humidity, sun, salts, winds, pollutants, temperature are uncontrollable external factors that strongly interact with copper alloys monuments and are responsible for the type and rate of degradation and corrosion. Direct location (for example if it is placed near the ground, part of a fountain, etc) or internal factors such as surface details, shape or metal combinations have to be considered as they can generate microclimates which can highly damage a monument.
Most common copper alloys used for outdoor monuments
Copper and its alloys have been commonly used to create sculpture since Antiquity, as craftsmen gradually became aware of the improved properties and workability provided by copper alloys. While Egyptians used to mix copper and lead to produce their sculptures, Greek and Roman civilizations preferred tin and zinc to lead. The craftsmen of the European Middle Ages usually chose tin and only a very small amount of lead and zinc. In the 19th Century, chemists considered pure tin bronze as the best choice to achieve the most aesthetically pleasing surfaces. Nevertheless, craftsmen or sculptors’ individual habits often prevailed and choices were mainly dictated by aesthetic tastes or local economy.
These variations in alloying metals can be explained by the fact that copper alloys show various mechanical, physical and chemical properties, depending on the alloys compositions and percentage.
While the addition of tin (Sn) reduces the melting point of copper, increases hardness, resistance to corrosion and improves the polishing quality of the metal, the presence of zinc (Zn) improves the pouring properties during the casting process. Adjunction of lead (Pb) also lowers the melting point of the alloy and increases the sharpness of the figure, allowing very fine details to be designed. Depending on the provenance and the period, being natural inclusions coming from the ore or voluntary adjunctions in the alloy, copper alloys may also contain traces of nickel (Ni), silver (Ag), antimony (Sb), arsenic (As), iron (Fe), bismuth (Bi), sulphur (S) or aluminium (Al). It is commonly admitted that, compared to other metals used for outdoor exposure (iron, lead), copper alloys generally show good resistance to corrosion in outdoor environments.
The distinction between the various copper alloys isn’t very accurate. It has to be underlined that usually, conservators use the general term “bronze sculpture” to assemble the large group of outdoor copper alloy sculpture. As most outdoor monuments are cast in bronze, this habit is not totally wrong.
It can be explained by the fact that systematic elemental analysis of the metal constituting sculptures is expensive and not always necessary to achieve good conservation interventions. Therefore, metallographic analyses are not always done. But other copper alloys can be used for casting.
Laser cleaning mechanisms
Laser cleaning is the result of interactions between laser light and the material to be cleaned, which shows its own chemical and physical properties. Each dirt layer or corrosion product has its own characteristic absorption spectrum which would make it react differently depending on the wavelength to which it is exposed. Each laser provides a different type of interaction determined by its wavelength. Wavelength influences the optical penetration depth, while pulse length determines the heat penetration depth.
As discussed earlier, laser cleaning of a dirty surface is effective if the dirt highly absorbs the laser energy while the underlying surface, the object, reflects most of it. For example, when a piece of white marble, covered with a thick black pollution crust, is laser cleaned at 1064 nm, the black deposit will absorb most of the energy from the beam while the marble will absorb much less.
This effect is important in that the photons, or light particles in the laser beam, can distinguish through the different layers much better than more conventional techniques, like abrasive methods or chemicals, whose effects are less controllable and can lead to over-cleaning and damage to the object. The infrared Nd:YAG laser (1064 nm) is used all along this chapter as it is the most used type of laser in conservation and because laser cleaning at infrared wavelength was used during this research.
Laser interactions with metal
While laser cleaning is now widely used on a large range of materials, its interactions with metals still have not received the same amount of attention , probably because of the complex nature of the corrosion products of metals. For example, copper and copper alloys, constituting outdoor sculpture as well as archaeological objects, show a layered corrosion system which is often very intricate and unevenly superposed, more or less thick depending on the zones. The original surface has often disappeared and has been replaced by corrosion products, making it difficult to distinguish.
Furthermore, the difficulty lies in the fact that some corrosion products have to be preserved while others have to be removed.
Because of that surface complexity, laser cleaning of metals is not self-limiting. Once the dirt / paint/ corrosion layer is removed from the surface, any further pulse would induce further cleaning and so, modifications of the corrosion layer or of the metal structure. If cleaning is not carried out carefully, over-cleaning may occur, leading to the removal of corrosion layers that should have to be preserved.
To minimise this effect, the properties of the laser as well as those of the material to be cleaned have to be considered carefully.
Laser cleaning of the copper alloy samples
Samples preparation
Metallic plates were prepared in order to have a material available on which to test the laser cleaning effect. The choice of a new material, instead of an original one such as samples from copper roofing or a sculpture, offered a few advantages.
First, the samples could be chosen as similar as those constituting the two laser-cleaned monument of Queen Victoria and Lord Nelson. Mainly, this avoided taking any material from the original monuments. But as the discoloration of oxidized copper had been observed by conservators on those two sculptures after laser cleaning, finding similar alloys could induce more realistic results after the same treatment. Moreover, it was interesting to study the particular behaviour of the different copper alloys after laser irradiation.
Queen Victoria being made of copper and Lord Nelson of brass, industrial alloys have been chosen, the most similar possible to those of the sculptures. Unfortunately, it has been extremely difficult to find alloys of the same exact composition. As the quantities requested for this research were very tiny, it has not been possible to have it cast by a foundry for that precise purpose. Hence, industrial wrought brass and copper were selected190. Moreover, a bronze cast ingot from an English foundry was used to complete the selection. This modern bronze is named gunmetal, which characteristic is the presence of some zinc in the copper-tin alloy. It was frequently used during the 19th and 20th centuries for maritime use and sculpture casting and hence, could bring complementary results. The second advantage of using new metal samples was the possibility they offered to create artificial corrosion layers. It could appear strange not to have chosen authentic pieces, with thick layers of corrosion products and natural concretions resulting from outdoor exposition. But the idea was to create individual and “pure” surface layers: non-corroded metal samples, cuprite (Cu2O) and brochantite (CuSO4.3Cu(OH)2) layers of corrosion. Cuprite and brochantite are commonly found on copper alloys having been exposed outdoors for a few decades. These separately corroded samples would allow us to study the effects of laser cleaning on both corrosion products, independently from one another and without the interferences brought by the usual pollutants coming from outdoor exposure, such as soot, salts, sand, organic deposits, grease, etc.
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Table des matières
1. Introduction
1.1. Presentation of the project and objectives
1.2. The National Conservation Centre (National Museums Liverpool)
1.3. Historical aspects of laser cleaning
1.4. General properties and main advantages of laser cleaning
1.5. Presentation of the different chapters
Part I: Copper alloys outdoor monuments
2. Outdoor copper alloy sculpture
2.1. Most common copper alloys used for outdoor monuments
2.1.1. Copper
2.1.2. Brass
2.1.3. Bronze
2.2. Technological aspects
2.2.1. Casting processes
2.2.1.1. Lost-wax process
2.2.1.2. Sand-casting process
2.2.1.3. Final treatments
2.2.2. Natural and artificial patination
2.3. Alterations factors
2.3.1. Environment
2.3.2. Water and relative humidity (RH)
2.3.3. Temperature
2.3.4. Pollutants
2.3.4.1. Gaseous pollutants
2.3.4.2. Particulate matter
2.3.5. Vegetation
2.3.6. Animals
2.3.7. Function and use
2.3.8. Vandalism
2.3.9. Design and metal structure particularities
2.4. Specific corrosion products
2.4.1. Corrosion development on outdoor copper alloy sculpture
2.4.2. Oxides
2.4.3. Sulphates
2.4.4. Nitrates
2.4.5. Chlorides
2.4.6. Carbonates
2.4.7. Other compounds
2.5. Current conservation strategies
2.5.1. Ethical considerations
2.5.2. Direct interventions
2.5.2.1. Surface cleaning
2.5.2.1.1. Air-abrasive cleaning
2.5.2.1.2. Water pressure cleaning
2.5.2.1.3. Chemical cleaning
2.5.2.1.4. Laser cleaning
2.5.2.2. Artificial patination
2.5.2.3. Surface protection
2.5.2.4. Repair and restoration
2.5.3. Indirect interventions
2.5.3.1. Regular maintenance plan
3. Two case studies: the conservation of the monuments to Lord Nelson and Queen Victoria in the UK
3.1. General aspects
3.2. Monument to Lord Nelson
3.2.1. Historical aspects
3.2.2. Description and technological aspects
3.2.3. Alloy composition and corrosion products
3.2.4. Conservation state before laser cleaning
3.3. Statue of Queen Victoria
3.3.1. Historical aspects
3.3.2. Description and technological aspects
3.3.3. Alloy composition and corrosion products
3.3.4. Conservation state before laser cleaning
3.4. Conservation treatments of the Monuments to Lord Nelson and Queen Victoria
3.4.1. Laser cleaning
3.4.1.1. System employed
3.4.1.2. Results and observations
3.4.2. Other conservation treatments
3.4.3. Actual conservation state
Part II: Laser cleaning
4. Basics of laser cleaning technique
4.1. Properties of laser radiation
4.2. Laser system parameters
4.3. Laser cleaning mechanisms
4.3.1. Self-limiting process
4.3.2. Ablation and damage thresholds
4.3.3. Laser cleaning mechanisms
5. Laser interactions with metal
5.1. Temperature rise
5.2. Heat diffusion length and optical penetration depth
6. Laser cleaning of the copper alloy samples
6.1. Samples preparation
6.1.1. Artificial corrosion of the copper alloys samples
6.1.1.1. Artificial cuprite
6.1.1.2. Artificial brochantite
6.2. Laser settings
6.2.1. XY-axis table technique
6.2.2. Number of pulses per unit area calculation
6.3. Laser-cleaned samples: some remarks
Part III: Characterization of the interactions of laser radiation with copper alloys
7. Surface analyses
7.1. Optical analyses
7.1.1. Naked eye observations
7.1.1.1. Bare metal samples
7.1.1.2. Cuprite samples
7.1.2. Optical microscopy
7.1.2.1. Bare metal samples
7.1.2.2. Cuprite samples
7.1.2.2.1. Results
7.1.2.3. Cuprite cross-sections
7.1.2.3.1. Results
7.1.2.4. “Spot by spot” laser tests
7.1.3. Visible spectrophotometry
7.1.3.1. Extent of the discoloration
7.1.3.1.1. Cuprite-copper samples
7.1.3.1.2. Cuprite-brass samples
7.1.3.1.3. Cuprite-bronze samples
7.1.3.2. Reversibility process
7.1.3.2.1. Cuprite-copper samples
7.1.3.2.2. Cuprite-brass samples
7.1.3.2.3. Cuprite-bronze samples
7.2. Chemical analyses
7.2.1. Scanning Electron Microscope (SEM)
7.2.1.1. Bare metal samples
7.2.1.2. Cuprite samples
7.2.1.3. Cross-sections observations
7.2.2. Energy Dispersive X-Ray Spectroscopy (EDS)
7.2.2.1. Bare metal samples
7.2.2.2. Cuprite samples
7.2.2.3. EDS elemental mapping
7.2.3. X-Ray Diffraction (XRD)
7.2.3.1. Identification of the crystalline compounds
7.2.3.2. Determination of the crystalline structure
7.2.4. X-Ray Photoelectron Spectroscopy (XPS)
7.2.4.1. Qualitative results
7.2.4.1.1. Peaks identification
7.2.4.1.2. Superposition of the peaks
7.2.4.2. Quantitative results
7.2.5. Extra analysis: Diffuse and specular reflectance spectrometry
7.2.5.1. Measure of diffuse reflectance
7.2.5.2. Measure of diffuse and specular reflectance
7.2.5.3. Measure of specular reflectance
7.3. Synthesis of the results
7.3.1. Bare metal samples
7.3.2. Cuprite samples
7.4. Discussion
7.4.1. Possible interpretation
7.4.2. Influence of the results on the conservation practice of copper alloys with laser
8. Conclusion
9. Bibliography
9.1. Thematic bibliography
9.1.1. Laser cleaning and laser technology
9.1.1.1. Websites
9.1.2. Copper alloy outdoor sculpture
9.1.2.1. Websites
9.1.3. Artificial corrosion
9.1.4. Scientific analytical methods
9.1.4.1. EDS
9.1.4.2. Metallography
9.1.4.3. Optics
9.1.4.4. SEM
9.1.4.5. Spectrophotometry
9.1.4.6. XRD
9.1.4.7. XPS
10. Figures index
11. Tables index
12. List of units and symbols
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