5 Must-Have Features in a Bronze Figure Sculpture

Sculpture cast in bronze

Bronze is the most popular metal for cast metal sculptures; a cast bronze sculpture is often called simply "a bronze". It can be used for statues, singly or in groups, reliefs, and small statuettes and figurines, as well as bronze elements to be fitted to other objects such as furniture. It is often gilded to give gilt-bronze or ormolu.

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Common bronze alloys have the unusual and desirable property of expanding slightly just before they set, thus filling the finest details of a mould. Then, as the bronze cools, it shrinks a little, making it easier to separate from the mould.[1] Their strength and ductility (lack of brittleness) is an advantage when figures in action poses are to be created, especially when compared to various ceramic or stone materials (such as marble sculpture). These qualities allow the creation of extended figures, as in Jeté, or figures that have small cross sections in their support, such as the equestrian statue of Richard the Lionheart.[2]

But the value of the bronze for uses other than making statues is disadvantageous to the preservation of sculptures; few large ancient bronzes have survived, as many were melted down to make weapons or ammunition in times of war or to create new sculptures commemorating the victors, while far more stone and ceramic works have come through the centuries, even if only in fragments. As recently as several life sized bronze sculptures by John Waddell were stolen, probably due to the value of the metal after the work has been melted.[3]

Material

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There are many different bronze alloys. Typically modern bronze is 88% copper and 12% tin.[4] Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4'5% tin are used to make coins and a number of mechanical applications. Historical bronzes are highly variable in composition, as most metalworkers probably used whatever scrap was on hand; the metal of the 12th-century English Gloucester Candlestick is bronze containing a mixture of copper, zinc, tin, lead, nickel, iron, antimony, arsenic with an unusually large amount of silver ' between 22.5% in the base and 5.76% in the pan below the candle. The proportions of this mixture may suggest that the candlestick was made from a hoard of old coins. The Benin Bronzes are really brass, and the Romanesque Baptismal font at St Bartholomew's Church, Liège is described as both bronze and brass.

In the Bronze Age, two forms of bronze were commonly used: "classic bronze", about 10% tin, was used in casting; and "mild bronze", about 6% tin, was hammered from ingots to make sheets. Bladed weapons were mostly cast from classic bronze, while helmets and armour were hammered from mild bronze. According to one definition, modern "statuary bronze" is 90% copper and 10% tin.[2]

History

[edit] Further information: History of sculpture

The great civilizations of the old world worked in bronze for art, from the time of the introduction of the alloy for tools and edged weapons. Dancing Girl from Mohenjo-daro, belonging to the Indus Valley Civilisation and dating back to c.' BCE, is perhaps the first known bronze statue.[5] Life-sized bronze statues in Ancient Greece have been found in good condition; one is the seawater-preserved bronze Victorious Youth that required painstaking efforts to bring it to its present state for museum display. Far more Roman bronze statues have survived.

The ancient Chinese knew both lost-wax casting and section mould casting, and during the Shang dynasty created large numbers of Chinese ritual bronzes, ritual vessels covered with complex decoration, which were buried in sets of up to 200 pieces in the tombs of royalty and the nobility. Over the long creative period of Egyptian dynastic art, small lost-wax bronze figurines were made in large numbers; several thousand of them have been conserved in museum collections.

The Nuragic civilization in the Mediterranean island of Sardinia produced a large number of small bronze statues, known as bronzetti (Nuragic bronze statuettes), starting from the 12th century BCE.[6]

The 7th-8th century Sri Lankan Sinhalese bronze statue of Buddhist Tara, now in the British Museum, is an example of Sri Lankan bronze statues.

From the ninth through the thirteenth century the Chola dynasty in South India represented the pinnacle of bronze casting in India.[7]

Process

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Making bronzes is highly skilled work, and a number of distinct casting processes may be employed, including lost-wax casting (and its modern-day spin-off investment casting), sand casting and centrifugal casting. The term "bronze" is also applied to metal sculptures made by electrotyping (or galvanoplasty), although these sculptures are typically pure copper and their fabrication does not involve metal casting.[8]

Lost wax method

[edit] Main article: Lost wax casting

In lost-wax or investment casting, the artist starts with a full-sized model of the sculpture, most often a non-drying oil-based clay such as Plasticine model for smaller sculptures or for sculptures to be developed over an extended period (water-based clays must be protected from drying), and water-based clay for larger sculptures or for sculptures for which it is desired to capture a gestural quality ' one that transmits the motion of the sculptor in addition to that of the subject. A mould is made from the clay pattern, either as a piece mould from plaster, or using flexible gel or similar rubber-like materials stabilized by a plaster jacket of several pieces. Often a plaster master will be made from this mould for further refinement. Such a plaster is a means of preserving the artwork until a patron may be found to finance a bronze casting, either from the original moulds or from a new mould made from the refined plaster positive.

Once a production mould is obtained, a wax (hollow for larger sculptures) is then cast from the mould. For a hollow sculpture, a core is then cast into the void, and is retained in its proper location (after wax melting) by pins of the same metal used for casting. One or more wax sprues are added to conduct the molten metal into the sculptures - typically directing the liquid metal from a pouring cup to the bottom of the sculpture, which is then filled from the bottom up in order to avoid splashing and turbulence. Additional sprues may be directed upward at intermediate positions, and various vents may also be added where gases could be trapped. (Vents are not needed for ceramic shell casting, allowing the sprue to be simple and direct). The complete wax structure (and core, if previously added) is then invested in another kind of mould or shell, which is heated in a kiln until the wax runs out and all free moisture is removed. The investment is then soon filled with molten bronze. The removal of all wax and moisture prevents the liquid metal from being explosively ejected from the mould by steam and vapour.

Students of bronze casting will usually work in direct wax, where the model is made in wax, possibly formed over a core, or with a core cast in place, if the piece is to be hollow. If no mould is made and the casting process fails, the artwork will also be lost. After the metal has cooled, the external ceramic or clay is chipped away, revealing an image of the wax form, including core pins, sprues, vents, and risers. All of these are removed with a saw and tool marks are polished away, and interior core material is removed to reduce the likelihood of interior corrosion. Incomplete voids created by gas pockets or investment inclusions are then corrected by welding and carving. Small defects where sprues and vents were attached are filed or ground down and polished.

• A model of an apple in wax. This is the first step of the lost-wax casting process in bronze
• Liquid bronze at  °C is poured into the dried and empty casting mould
• The sculpture of the apple, just extracted from its mould. Below is the funnel through which the bronze was poured (upside down). This is the last step of the lost-wax casting process

Creating large sculptures

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For a large sculpture, the artist will usually prepare small study models until the pose and proportions are determined. An intermediate-sized model is then constructed with all of the final details. For very large works, this may again be scaled to a larger intermediate. From the final scale model, measuring devices are used to determine the dimensions of an armature for the structural support of a full-size temporary piece, which is brought to rough form by wood, cardboard, plastic foam, and/or paper to approximately fill the volume while keeping the weight low. Finally, plaster, clay or other material is used to form the full-size model, from which a mould may be constructed. Alternatively, a large refractory core may be constructed, and the direct-wax method then applied for subsequent investment. Before modern welding techniques, large sculptures were generally cast in one piece with a single pour. Welding allows a large sculpture to be cast in pieces, then joined.

Finishing

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After final polishing, corrosive materials may be applied to form a patina, a process that allows some control over the colour and finish.

Another form of sculptural art that uses bronze is ormolu, a finely cast soft bronze that is gilded (coated with gold) to produce a matte gold finish. Ormolu was popularized in the 18th century in France and is found in such forms as wall sconces (wall-mounted candle holders), inkstands, clocks and garnitures. Ormolu wares can be identified by a clear ring when tapped, showing that they are made of bronze, not a cheaper alloy such as spelter or pewter.

Gallery

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• Bronze statuette of a horse, solid cast, early 5th century BC, Argive workshop.
• The Orator, c.'100 BC, an Etrusco-Roman bronze statue depicting Aule Metele (Latin: Aulus Metellus), an Etruscan man wearing a Roman toga while engaged in rhetoric; the statue features an inscription in the Etruscan alphabet
• Brunswick Lion, c.', in Brunswick, Germany
• Lady Tholose, Jean Rancy, -50, in Toulouse, France
• Patinated bronze (above) and ormolu (below) Empire style clock, c. , by Pierre-Philippe Thomire
• Prospect Park War Memorial, Augustus Lukeman, , in Prospect Park, Brooklyn
• Grodan (The Frog), Paris , by Per Hasselberg. Cast in bronze for Rottneros Park near Sunne in Värmland, Sweden
• Jeté, Enzo Plazzotta, , at Millbank in Westminster, England, illustrates the material's capabilities
• Balance by David Ascalon shows how reactive chemicals applied to the metal create a marbleized blue surface

Sculptors

[edit] Main article: List of sculptors

See also

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• Bronze and brass ornamental work

References

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Bibliography & Further reading

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• Scholten, Frits (). European sculpture and metalwork. New York: The Metropolitan Museum of Art. ISBN .
• Weinryb, Ittai, The Bronze Object in the Middle Ages (Cambridge, ).
• Dafas, K. A., . Greek Large-Scale Bronze Statuary: The Late Archaic and Classical Periods, Institute of Classical Studies, School of Advanced Study, University of London, Bulletin of the Institute of Classical Studies, Monograph, BICS Supplement 138 (London).

Introduction

The following guidelines provide general information on the characteristics and common uses of bronze and identify typical problems associated with the material. See also 'Checklist for Inspecting Bronze Failures'.

References

• Gayle, M., Look, D. and Waite, J. Metals in America's Historic Buildings: Uses and Preservation Treatments. Washington, DC: Department of the Interior, National Park Service, .

• Preservation Science. 'Preventing Galvanic Corrosion. By Choosing the Right Materials'. Web. .

• Weaver, M. Conserving Buildings: Guide to Techniques and Materials (1st Edition). New York: Wiley, .

• Zahner, L. W. Architectural Metal Surfaces. New York: Wiley, .

Introduction

Bronze is an alloy of copper which can vary widely in its composition. It is often used where a material harder than copper is required, where strength and corrosion resistance is required and for ornamental purposes. The variations in bronze (both in proportion and elemental composition) can significantly affect its weathering characteristics. 'True' bronze is a combination of approximately 90% copper (Cu) and 10% tin (Sn), however there are three major classes or types of 'bronzes' used in sculpture and construction. They are:

1. Statuary Bronze - approximately 97% copper (Cu), 2% tin (Sn) and 1% zinc (Zn); this composition is the closest to 'true' bronze.

2. Architectural Bronze - actually more of a 'leaded brass', this composition is commonly composed of approximately 57% copper (Cu), 40% zinc (Zn) and 3% lead (Pb).

3. Commercial Bronze - composed of approximately 90% copper (Cu) and 10% zinc (Zn).

Traditionally, a copper alloy which contains zinc is a 'brass'; a copper alloy which contains tin (not exceeding 11%) is a 'bronze'. Bronze composition may vary significantly however, and contemporary bronzes are typically copper alloys which may contain silicon (Si), manganese (Mn), aluminum (Al), zinc (Zn) and other elements, with or without tin (Sn).

In its 'raw' state, bronze is a semi-pink or salmon-colored metal; however it is rarely seen in its pure state. Bronze usually exhibits some patination or corrosion so that its color normally ranges from lime green to dark brown. Exposed bronze undergoes continuous change and progresses through several predictable 'stages' of oxidation and corrosion. The stages of bronze corrosion vary in duration and time of onset, based on many factors, including:

1. Composition of the bronze

2. Patination or other protective treatments applied at the foundry

3. Weather

4. Location and exposure to rain, sun, and other climatic conditions

5. Atmospheric pollutants

6. Scheduled maintenance/cleaning

7. Adjacent materials including residual core materials

Typical Uses

Statuary bronze is typically used in outdoor sculpture. Its forms are almost limitless since it may be cast in any shape for which a mold can be devised. The most common types of forms include the human figure, landscapes, battle scenes, animals, weapons, decorative elements such as stars, rosettes, etc., and plaques.

Architectural bronze is typically used for:

1. Door and window frames

2. Door and window hardware

3. Mail boxes and chutes

4. Trim or rails

5. Furniture hardware

As a general rule, architectural applications seek to preserve the natural, highly polished 'pinkish' finish of raw bronze, in contrast to the patination of outdoor sculpture/ornament. This is achieved by the frequent polishing and oiling of bronze/brass decorative and structural elements, or the application of clear lacquers which must be renewed on a periodic basis.

Problems and Deterioration

Bronze has good resistance to:

1. Industrial, rural and marine atmospheres

2. Weak acids if suitably shielded with appropriate protective coatings.

Bronze has poor resistance to:

1. Ammonia

2. Ferric and ammonia compounds

3. Cyanides

4. Urban pollution

5. Acid rains

6. Bird droppings

Problems may be classified into two broad categories: 1) Natural or inherent problems based on the characteristics of the material and the conditions of the exposure, and 2) Vandalism and human- induced problems.

Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely.

Natural or Inherent Problems

Bronze, like cast iron, is a manufactured product. Copper is extracted from natural ores and alloyed with tin to create a metal which does not exist in nature. Many of the inherent problems relate to the normal physical process of the bronze 'returning to nature', i.e. to the most stable states of its components.

Additionally, most outdoor bronze is erected with a foundry applied patina of some type. The actual surface patina could be one of dozens of different composites as a result of the foundry applied finishes. Each of these finishes may react differently with the environment and result in different corrosion types and rates.

Regardless of which finish exists, the bronze will begin the deterioration process described below, where the surface will be subjected to the alteration of the patina through oxidation and sulfurization. Patinated and protected surfaces will resist the effects of exposure more than bare metal; therefore, such pieces will maintain their original appearance longer and exhibit changes more slowly.

Corrosion

Corrosion of one form or another is the chief cause of the deterioration of metals, including statuary and architectural bronze. The degree of corrosion which occurs, and the corrosion by-products which result, are affected by several factors including bronze composition or formulation, environmental conditions and adjacent materials.

While the composition of bronze does affect the rate of corrosion, it has been generally recognized that composition is one of the least significant factors in bronze deterioration. The existence of chemicals in the atmosphere, such as chlorine, sulfur, and nitrogen oxides, in the presence of moisture, is the most significant cause of bronze deterioration.

There are numerous causes and symptoms of corrosion, including:

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1. Uniform Oxidation or Corrosion: Corrosion attacks the metal surface evenly.

2. Pitting: Attacks the metal surface in localized areas.

3. Selective Attack: When a metal is not homogenous throughout, certain areas may be attacked in preference to others.

4. Erosion: When a corrosion-resistant oxide layer is removed and the bare metal beneath corrodes.

5. Oxygen Cell Corrosion (or Atmospheric Corrosion): The most common form of corrosion; Moisture containing environmental gases (carbon dioxide, oxygen, sulfur compounds, soot, fly ash, etc.) produces chemical corrosion on the metal.

6. Galvanic Corrosion: The increased corrosion of a metal due to its contact with another metal, or in some cases, the same metal.

   1. Galvanic corrosion causes extensive deterioration to the attacked metal(s), and in turn the corrosion products stain and streak the adjacent surfaces.

   2. It is an electrolytic reaction. For this to occur, there must be an anode (negatively charged area), a cathode (positively charged area), and an electrolyte (conducting medium). The electrolyte can be rainwater, condensation, acid, alkali, or a salt. The formation of an anode and a cathode may occur due to the presence of impurities, difference in work hardening, or local differences of oxygen concentration on the surface.

7. Stress Corrosion Cracking: Attacks areas in a metal which were stressed during metal working.

8. Humidity, temperature and condensation: Affect the rate of corrosion; in a marine environment, aerosols can deposit chloride and other salts which will accelerate the rate of atmospheric corrosion.

The bronze corrosion process goes through five predictable stages. The specific results of each stage can differ due to combinations of atmospheric elements, bronze composition, patination, and other protective treatments such as waxing, oiling or lacquering.The five stages are:

1. Induction is when normal oxidation takes place, normally producing the dark brown copper oxide film which can be a protective barrier against future pollutants. The actual film composition is dependent upon the type and concentration of pollutants in the atmosphere, upon the duration of exposure, and upon the relative degree and duration of wetness on the surface. High concentrations of sulfides in the atmosphere can dramatically alter the result of stage 1, producing less protective, even potentially damaging films. The rate of oxidation can also have an effect on long term durability of the surface finish; oxides formed over longer time periods seem much more resistant to deterioration.

2. The conversion of the topmost metallic surface to copper sulfate normally begins to occur on surfaces with the most severe exposure, such as horizontal surfaces. Oxygen deprivation and deposition of particulates and moisture create a catalytic situation where electrolytic reactions occur. (This is the same principle as a battery, where the charged ions move from a positive to a negative pole.) The visual symptom of this phase is the formation of thin, light green patches on the more exposed areas.

3. Run-off streaking and scab formation occurs at a slower rate than the two previous stages but the consequences are significant. Copper sulfates and sulfides may have been formed during the earlier stages, yet the degree of solubility of these compounds may vary widely. It is during Stage 3 that the familiar streaking and uneven discoloration may occur due to differential weathering of the corrosion by-products. This erosion can continue until uneven blackish areas or island- like scabs are present on the surface.

4. Pitting may spread around the black scab formation; the pitting can also continue to spread below what appears to be a stable surface. Pitting is generally caused and accelerated by microscopic particles of chlorides deposited from the air, and if chlorides are present below a crust or a barrier coating, the corrosion can continue unchecked and invisible to casual observation.

5. Complete conversion of all exposed surfaces to the bright blue-green copper sulfate is the final stage of corrosion. The result is the familiar solid green bronze with the lime- green color and a matte texture. This condition is sometimes misperceived as the desirable end condition, but it is actually a phase of active corrosion.

Oxidation

Unprotected areas of raw bronze will oxidize, or combine with oxygen present in the air, resulting in a thin film of copper oxide along the surface of the exposed bronze. The resulting appearance is a flat, dark brown surface. The most common example to which most users can relate is the process of oxidation of a copper penny. The specular (shiny) finish of a new penny is familiar, as is the shift to the dark, red-brown finish as the surfaces oxidize over time.

This normal process of oxidation is a form of corrosion. The resultant oxide film is less reactive than raw bronze and forms a stable, protective barrier with a greatly reduced rate of oxidation.

Sulfurization

Bronze also reacts with many atmospheric pollutants, especially sulfur compounds, which are normally found in the atmosphere as sulfur dioxide and hydrogen sulfide. Both are produced in industrial manufacturing processes. Concentrations of these gasses are generally greater in or near urban and industrial areas; therefore higher rates of corrosion can normally be expected in such areas. The initial symptom of sulfurization is the appearance of patches of light green primarily on exposed surfaces. This usually begins on horizontal surfaces which receive the greatest exposure to rains and water run-off.

A general layer of surface corrosion can eventually spread over the entire metallic surface, resulting in an overall bright green surface. The uniform green surface is often accepted by the general public, and others, as protective and the normal state of bronze. This is a misconception, and one which has probably resulted in the public acceptance of appearances which are actually symptoms of corrosion and deterioration. The sulfides and sulfates will continue to form in the presence of moisture and atmospheric sulfur compounds. The presence of green corrosion products on the bronze is always an indication of active corrosion. The pattern and result of this process will vary based upon several environmental factors such as wind, rain, pollutants, patina, and the nature of previous corrosion.

Differential weathering due to winds, rain and surface orientation can result in uneven corrosion with patterns of green streaking on a dark blackish surface.

The process of sulfurization is complicated by two factors, both of which result in aesthetically unacceptable appearances; appearances which are generally perceived as neglect and deterioration. Uneven black and green streaking of bronzes is one of the most disfiguring problems which can occur with bronze. Random dark (black) and light (green) streaks follow the contours downward, resulting in distracting visual patterns with no relationship to the form or texture of the surface of the work. The artistic details which give form and definition to the bronze become extremely obscured by streaking which results from two phenomena:

1. Differential solubility of the corrosion products, and

2. Electrochemical processes between the dark (black) and light (green) areas.

The streaking of bronze indicates a differential corrosion of the bronze which will be permanently disfiguring. Two different surface corrosion products are dissolving at significantly different rates. The geological analogy is the formation of canyons by the erosion of the land surface. Where such corrosion has already occurred, conservation techniques are likely to be required. Early indications of streaking should be given serious attention in the inspection process, and called to the attention of the Regional Historic Preservation Officer (RHPO) at the earliest possible time.

Bronze Disease

Bronze disease is the result of exposure to chlorine compounds which can come from any saline source, such as contact with saline soils, atmospheric pollutants or airborne salt spray near bodies of salt water. The chlorine reacts with the copper in bronze to form copper chloride. The primary symptom is pitting, and the process can proceed unchecked below apparently sound patinas, or protective coatings.

The copper chloride is relatively unstable and the only way to arrest the continuing corrosion is the complete removal of the chlorides using electrochemical methods. All such methods of chloride removal are advanced conservation techniques requiring the employment of a skilled professional.

Core Migration

Bronze is cast in a foundry process which consists of the pouring of molten bronze into a mould containing a central core. Frequently this core material is gypsum or plaster of Paris, and occasionally portions of the core are left inside the casting. It is possible for the core material to migrate through the casting wall over time and appear on the exterior surface of the bronze.

The removal and repair of core migration problems is not a maintenance procedure and will require an 'existing conditions analysis' supporting a proposed conservation treatment. The RHPO should be notified of the problem following its identification. The most common symptom is the appearance of whitish spots, which gradually enlarge, in the bronze surface.

Pitting

Corrosion of bronze, unlike that of natural stones, is in part an electro-chemical phenomenon. Points of negative electrical potential called cathodes and points of positive potential called anodes form on the bronze. In the presence of moisture, the corrosion process is driven by an electrical differential between the two points. This process can occur at a highly accelerated rate.

An electric potential can develop between both large and small areas. Atmospheric pollutants, especially chlorides, can be deposited on the surface of bronze. Tiny 'islands' of corrosion can form, rapidly eroding/converting away the bronze metal and resulting in tiny voids or pits in the surface of the bronze. Pits may begin small and increase in size due to the continued electrochemical action and deposition within the pits. This may continue as long as moisture is present.

Pitting may be pinpoint or broad, as in patterns of deep etching created by differential erosion. (Also see: Bronze Disease)

Bird Droppings

Bird, or other animal, droppings may collect on the surface of bronze and (because of the acidic nature) may accelerate localized corrosion and deterioration. Droppings can also build up in sheltered areas, providing concentrations of damaging chemical agents of deterioration.

Galvanic Corrosion

Galvanic corrosion, also known as dissimilar metal corrosion, occurs when two dissimilar metals are brought into contact with one another. One of the metals will corrode, and the other will remain intact. As an example, if bronze is brought into contact with iron, the iron will frequently begin to corrode. Galvanic corrosion is caused by an electric potential between two dissimilar metals in the presence of water or moisture, where the water's electrolytes allow the flow of metallic ions from the more active metal, or the anode, to the more noble metal, or the cathode. The movement of these metallic ions represents a physical loss of metal from the metal being corroded. It can continue until the source metal is completely gone.

Below, thirteen construction metals are ranked according to their susceptibility to corrosion, from most to least susceptible, or from active to noble. This type of ordered list is called a Galvanic Series chart.

The rate of the transfer of iron from the passive to the active metal is determined by the difference in electrode potential between the two metals. Therefore, the farther apart two metals are in the list below, the more likely the active metal (higher on the list) is to corrode.

1. Zinc

2. Aluminum

3. Galvanized streel

4. Cast iron, mild steel

5. Lead

6. Tin

7. Brass, bronze

8. Copper

9. Silver solder

10. Stainless steel

11. Silver

12. Graphite

13. Gold

Galvanic corrosion typically occurs where dissimilar metals are used as connectors or parts of a building's armature. It can be stopped by replacing the more active metal with a more noble metal such as stainless steel. When two dissimilar metals must be in contact with one another, the risk of corrosion can be substantially reduced by applying a coating to both of the materials but especially to the noble metal, or applying a sacrificial metallic coating that is more active than both of the metals.

The relative mass or sizes of the two metals in contact will also determine the rate at which galvanic corrosion occurs. As an example, in a bronze plaque with iron bolts, the bolts would corrode rapidly, but an iron plaque with bronze or copper bolts would exhibit a much lower, almost negligible, amount of galvanic corrosion as a result of its contact with the bolts. Therefore, bolts and other fasteners should be made of more noble metals where possible.

Erosion

Erosion or 'wearing away' of metal from the surface may be due to natural or environmental factors, or due to man-induced factors such as excessive handling or rubbing. Erosion due to human contact is by far the most serious problem, but erosion can occur due to the abrasive action of wind-driven pollutants.

Natural erosion will be a slow process and one which is, therefore, difficult to detect. It will be most obvious on outdoor bronze or in exposed locations. Industrial settings and areas where there are higher concentrations of airborne particulates, which can act as abrasives, also offer the possibility for higher rates of erosion. Natural, wind-driven abrasion will be generally so slow that it will be most apparent when comparing different exposures/orientations of bronze which has been in service for long periods. The differential loss of detail between protected and exposed surfaces will begin to be apparent over many years. Examination for this differential weathering should be part of any inspection.

Vandalism or Human-Induced Problems

Mechanical Deterioration (Purely Physical Processes)

1. Abrasion: Causes removal of the protective metal surface. Some metals such as zinc are relatively soft and therefore vulnerable to abrasion damage, especially in areas similar to roof valleys where the metal can be worn paper-thin.

2. Fatigue: Failure of metal that has been repeatedly stressed beyond its elastic limit, due to failure to provide necessary allowances for thermal expansion and contraction caused by temperature differences.

3. Creep: The permanent distortion of a soft metal which has been stretched due to its own weight. Thin areas of the metal will be among the first to fail. Can be found in lead sculptures which have inadequate or corroded internal armature.

4. Heat: Usually in the form of fire, will cause many metals to become plastic, distort, and fail.

5. Distortion: Permanent deformation or failure may occur when a metal is overloaded beyond its yield point because of increased live or dead loads, thermal stresses, or structural modifications altering a stress regime.

Connection Failure

1. Chemical and mechanical processes can cause the breakdown or reduced effectiveness of structural metal fixings such as bolts, rivets, and pins. Stress failure is often a contributor to breakdown situations. Iron connections which are water traps are particularly susceptible.

2. Most bronze corrosion can be characterized as 'general' or 'uniform' and 'pitting', with occasional signs of selective attack. Galvanic corrosion appears mostly in connection with pins, bolts, and replacement parts in different metal. Erosion is apparent most often in bronzes in fountains. Stress corrosion is less apparent in bronze than in brass, but could be a factor in some cases in bronze sculptures.

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