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Historic Preservation - Technical Procedures

Spectitle:

Bronze: Characteristics, Uses And Problems

Procedure code:

0501003S

Source:

Outdoor Sculpture Manual - Center For Public Buildings

Division:

Metals

Section:

Metal Materials

Last Modified:

08/09/2012

Details:

Bronze: Characteristics, Uses And Problems



BRONZE:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of bronze and identifies typical problems
associated with the material.  See also 05010-02-S for guidance on
inspecting bronze failures.

 

REFERENCES:  

Martin Weaver, Conserving Buildings (Revised Edition). New York: J.Wiley & Sons, 1997.

M.Gayle, D. Look, J. Waite. Metals in America's Historic Buildings. Washington, DC: National Park Service, 1992.

L. W. Zohner.  Architectural Metals.  New York: J. Wiley & Sons, 1995.


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 effect 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",
         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).

Bronze in its "raw" state is a "pinkish", 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, and

    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, and

    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 their protection by 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
sulphurization.  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.

Corrosion may have many causes and symptoms, 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.  

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

    b.   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.  

    STAGE 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.

    STAGE 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.

    STAGE 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.

    STAGE 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.

    STAGE 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.

SULPHURIZATION:

Bronze also reacts with many atmospheric pollutants, especially
sulphur compounds, which are normally found in the atmosphere as
sulphur 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 sulphurization 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 even, bright 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 sulphides and
sulphates will continue to form in the presence of moisture and
atmospheric sulphur 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 sulphurization 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.

Excretion can also build up in sheltered areas, providing
concentrations of damaging chemical agents of deterioration.  In
some cases, animals or birds can even nest in these sheltered
areas.

GALVANIC CORROSION:

Localized corrosion can occur when two dissimilar metals are
brought into contact.  Only one of the metals will deteriorate and
that one will be the less stable one which is lower on the
elemental chart.  As an example, if bronze or copper is brought
into contact with iron, the iron will frequently begin to corrode.
The process can continue until the iron is totally eliminated.  The
National Institute of Standards and Technology (NIST) - formerly
known as The National Bureau of Standards (NBS) - in the
publication "Corrosion -Facts for the Consumer", prepared a diagram
to show the susceptibility of various metals to corrosion.  Metals
were ranked from least to most corrosive in the following order:

    1 - GOLD
    2 - GRAPHITE
    3 - SILVER
    4 - STAINLESS STEELS
    5 - BRONZE, COPPER, BRASS
    6 - TIN
    7 - LEAD
    8 - LEAD-TIN SOLDERS
    9 - CAST IRON, ORDINARY STEEL
    10 - ALUMINUM
    11 - ZINC
    12 - MAGNESIUM

In this series these are compared with Hydrogen as a standard or
zero point.  Metals less active ( 1-5 above) are called "noble" and
metals which are more active ( 6-12  above) are called "ignoble" or
"base" metals. Galvanic corrosion is caused by an electric
potential, between two dissimilar metals, in the presence of
moisture.  A flow of metallic ions moves from one to the other at
a rate determined by the "potential difference" between the two
metals in the activity series.  The movement of these metallic ions
represents a physical loss of metal from the source.  It may
continue unchecked until the source metal is completely gone.  The
farther apart two metals are in the list above, the more likely the
metal, with the higher number, is to corrode.

Galvanic corrosion typically occurs where dissimilar metals are
used as connectors or parts of the armature.  It can be stopped by
replacing the dissimilar metal with stainless steel or a connector
of the same metal as the ornament.  NIST suggests that when two
dissimilar metals must be in contact with one another, the risk of
corrosion can be substantially reduced by painting the metal with
the lower number, with a good quality paint.

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.

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 very exposed, unprotected 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.

                         END OF SECTION
 


bronze, bronze characteristics, bronze problems