Bronze: Characteristics, Uses And Problems

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, 1992.

• Preservation Science. “Preventing Galvanic Corrosion. By Choosing the Right Materials”. Web. 2008.

• Weaver, M. Conserving Buildings: Guide to Techniques and Materials (1st Edition). New York: Wiley, 1997.

• Zahner, L. W. Architectural Metal Surfaces. New York: Wiley, 2004.

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:

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.