Preservation Briefs: 27 The Maintenance And Repair Of Architectural Cast Iron

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Preservation Briefs 27, National Park Service, Pad
Metal Materials
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The link immediately below connects to the latest version of Preservation Brief 27:

John G. Waite, AIA
Historical Overview by Margot Gayle

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
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The preservation of cast-iron architectural elements, including
entire facades, has gained increasing attention in recent years as
commercial districts are recognized for their historic significance
and revitalized.  This Brief provides general guidance on
approaches to the preservation and restoration of historic cast

Cast iron played a preeminent role in the industrial development of
our country during the 19th century.  Cast iron machinery filled
America's factories and made possible the growth of railroad
transportation.  Cast iron was used extensively in our cities for
water systems and street lighting.  As an architectural metal, it
made possible bold new advances in architectural designs and
building technology, while providing a richness in ornamentation.

This age-old metal, an iron alloy with a high carbon content, had
been too costly to make in large quantities until the mid-18th
century, when new furnace technology in England made it more
economical for use in construction.  Known for its great strength
in compression, cast iron in the form of slender, non-flammable
pillars, was introduced in the 1790s in English cotton mills, where
fires were endemic.  In the United States, similar thin columns
were first employed in the 1820s in theaters and churches to
support balconies.

By the mid-1820s, one-story iron storefronts were being advertised
in New York City.  Daniel Badger, the Boston foundryman who later
moved to New York, asserted that in 1842 he fabricated and
installed the first rolling iron shutters for iron storefronts,
which provided protection against theft and external fire.  In the
years ahead, and into the 1920s, the practical cast-iron storefront
would become a favorite in towns and cities from coast to coast.
Not only did it help support the load of the upper floors, but it
provided large show windows for the display of wares and allowed
natural light to flood the interiors of the shops.  Most
importantly, cast-iron storefronts were inexpensive to assemble,
requiring little on-site labor.

A tireless advocate for the use of cast iron in buildings was an
inventive New Yorker, the self-taught architect/engineer James
Bogardus.  From 1840 on, Bogardus extolled its virtues of strength,
structural stability, durability, relative lightness, ability to be
cast in almost any shape and, above all, the fire-resistant
qualities so sought after in an age of serious urban
conflagrations.  He also stressed that the foundry casting
processes, by which cast iron was made into building elements, were
thoroughly compatible with the new concepts of prefabrication, mass
production, and use of identical interchangeable parts.

In 1849 Bogardus created something uniquely American when he
erected the first structure with self-supporting, multi-storied
exterior walls of iron.  Known as the Edgar Laing Stores, this
corner row of small four-story warehouses that looked like one
building was constructed in lower Manhattan in only two months.
Its rear, side, and interior bearing walls were of brick; the floor
framing consisted of timber joists and girders.  One of the
cast-iron walls was load-bearing, supporting the wood floor joists.
The innovation was its two street facades of self-supporting cast
iron, consisting of multiples of only a few pieces-- Doric-style
engaged columns, panels, sills, and plates, along with some applied
ornaments.  Each component of the facades had been cast
individually in a sand mold in a foundry, machined smooth, tested
for fit, and finally trundled on horse-drawn drays to the building
site.  There they were hoisted into position, then bolted together
and fastened to the conventional structure of timber and brick with
iron spikes and straps.

The second iron-front building erected was a quantum leap beyond
the Laing Stores in size and complexity.  Begun in April 1850 by
Bogardus, with architect Robert Hatfield, the five-story Sun
newspaper building in Baltimore was both cast-iron-fronted and
cast-iron-framed.  In Philadelphia, several ironfronts were begun
in 1850: The Inquirer Building, the Brock Stores, and the Penn
Mutual Building (all three have been demolished).  The St. Charles
Hotel of 1851 at 60 N. Third Street is the oldest ironfront in
America.  Framing with cast-iron columns and wrought-iron beams and
trusses was visible on a vast scale in the New York Crystal Palace
of 1853.

In the second half of the 19th century, the United States was in an
era of tremendous economic and territorial growth.  The use of iron
in commercial and public buildings spread rapidly, and hundreds of
iron-fronted buildings were erected in cities across the country
from 1849 to beyond the turn of the century.  Outstanding examples
of ironfronts exist in Baltimore, Galveston, Louisville, Milwaukee,
New Orleans, Philadelphia, Richmond, Rochester (N.Y.), and
especially New York City where the SoHo Cast Iron Historic District
alone has 139 iron-fronted buildings.  Regrettably, a large
proportion of ironfronts nationwide have been demolished in
downtown redevelopment projects, especially since World War II.

In addition to these exterior uses, many public buildings display
magnificent exposed interior ironwork, at once ornamental and
structural.  Remarkable examples have survived across the country,
including the Peabody Library in Baltimore; the Old Executive
Office Building in Washington, D.C.; the Bradbury Building in Los
Angeles; the former Louisiana State Capitol; the former City Hall
in Richmond; Tweed Courthouse in New York; and the state capitols
of California, Georgia, Michigan, Tennessee, and Texas.  And it is
iron, of course, that forms the great dome of the United States
Capitol, completed during the Civil War.  Ornamental cast iron was
a popular material in the landscape as well, appearing as fences,
fountains with statuary, lampposts, furniture, urns, gazebos,
gates, and enclosures for cemetery plots.  With such widespread
demand, many American foundries that had been casting machine
parts, bank safes, iron pipe, or cookstoves added architectural
iron departments.  These called for patternmakers with
sophisticated design capabilities, as well as knowledge of metal
shrinkage and other technical aspects of casting.  Major companies
included the Hayward Bartlett Co. in Baltimore; James L. Jackson,
Cornell Brothers, J. L. Mott, and Daniel D. Badger's Architectural
Iron Works in Manhattan; Hecla Ironworks in Brooklyn; Wood & Perot
of Philadelphia; Leeds & Co., the Shakspeare (sic) Foundry, and
Miltenberger in New Orleans; Winslow Brothers in Chicago; and James
McKinney in Albany, N.Y.

Cast iron was the metal of choice throughout the second half of the
19th century.  Not only was it a fire-resistant material in a
period of major urban fires, but also large facades could be
produced with cast iron at less cost than comparable stone fronts,
and iron buildings could be erected with speed and efficiency.  The
largest standing example of framing with cast-iron columns and
wrought iron beams is Chicago's sixteen-story Manhattan Building,
the world's tallest skyscraper when built in 1890 by William
LeBaron Jenney.  By this time, however, steel was becoming
available nationally, and was structurally more versatile and
cost-competitive.  Its increased use is one reason why building
with cast iron diminished around the turn of the century after
having been so eagerly adopted only fifty years before.
Nonetheless, cast iron continued to be used in substantial
quantities for many other structural and ornamental purposes well
into the 20th century: storefronts; marquees; bays and large window
frames for steel-framed, masonry-clad buildings; and street and
landscape furnishings, including subway kiosks.

The 19th century left us with a rich heritage of new building
methods, especially construction on an altogether new scale that
was made possible by the use of metals.  Of these, cast iron was
the pioneer, although its period of intensive use lasted but a half
century.  Now the surviving legacy of cast-iron architecture, much
of which continues to be threatened, merits renewed appreciation
and appropriate preservation and restoration treatment.


Cast iron is an alloy with a high carbon content (at least 1.7% and
usually 3.0 to 3.7%) that makes it more resistant to corrosion than
either wrought iron or steel.  In addition to carbon, cast iron
contains varying amounts of silicon, sulfur, manganese and

While molten, cast iron is easily poured into molds, making it
possible to create nearly unlimited decorative and structural
forms.   Unlike wrought iron and steel, cast iron is too hard and
brittle to be shaped by hammering, rolling, or pressing.  However,
because it is more rigid and more resistant to buckling than other
forms of iron, it can withstand great compression loads.  Cast iron
is relatively weak in tension, however, and fails under tensile
loading with little prior warning.

The characteristics of various types of cast iron are determined by
their composition and the techniques used in melting, casting, and
heat treatment.  Metallurgical constituents of cast iron that
affect its brittleness, toughness, and strength include ferrite,
cementite, pearlite, and graphite carbon.  Cast iron with flakes of
carbon is called gray cast iron.  The "gray fracture" associated
with cast iron was probably named for the gray, grainy appearance
of its broken edge caused by the presence of flakes of free
graphite, which account for the brittleness of cast iron.  This
brittleness is the important distinguishing characteristic between
cast iron and mild steel.

Compared with cast iron, wrought iron is relatively soft,
malleable, tough, fatigue-resistant, and readily worked by forging,
bending, and drawing.  It is almost pure iron, with less than 1%
(usually 0.02 to 0.03%) carbon.  Slag varies between 1% and 4% of
its content and exists in a purely physical association, that is,
it is not alloyed.  This gives wrought iron its characteristic
laminated (layered) or fibrous structure.

Wrought iron can be distinguished from cast iron in several ways.
Wrought-iron elements generally are simpler in form and less
uniform in appearance than cast iron elements, and contain evidence
of rolling or hand working.  Cast iron often contains mold lines,
flashing, casting flaws, and air holes.  Cast iron elements are
very uniform in appearance and are frequently used repetitively.
Cast iron elements are often bolted or screwed together, whereas
wrought iron pieces are either riveted or forge-molded (heat
welded) together.

Mild steel is now used to fabricate new hand-worked metal work and
to repair old wrought-iron elements.  Mild steel is an alloy of
iron and is not more than 2% carbon, which is strong but easily
worked in block or ingot form.  Mild steel is not as resistant to
corrosion as either wrought iron or cast iron.


Many of the maintenance and repair techniques described in this
Brief, particularly those relating to cleaning and painting, are
potentially dangerous and should be carried out only by experienced
and qualified workmen using protective equipment suitable to the
task.  In all but the most simple repairs, it is best to involve a
preservation architect or building conservator to assess the
condition of the iron and prepare contract documents for its

As with any preservation project, the work must be preceded by a
review of local building codes and environmental protection
regulations to determine whether any conflicts exist with the
proposed treatments.  If there are conflicts, particularly with
cleaning techniques or painting materials, then waivers or
variances need to be negotiated, or alternative treatments or
materials adopted.


Common problems encountered today with cast iron construction
include badly rusted or missing elements, impact damage, structural
failures, broken joints, damage to connections, and loss of
anchorage in masonry.

OXIDATION, or rusting, occurs rapidly when cast iron is exposed to
moisture and air.  The minimum relative humidity necessary to
promote rusting is 65%, but this figure can be lower in the
presence of corrosive agents, such as sea water, salt air, acids,
acid precipitation, soils, and some sulfur compounds present in the
atmosphere, which act as catalysts in the oxidation process.
Rusting is accelerated in situations where architectural details
provide pockets or crevices to trap and hold liquid corrosive
agents.  Furthermore, once a rust film forms, its porous surface
acts as a reservoir for liquids, which in turn causes further
corrosion.  If this process is not arrested, it will continue until
the iron is entirely consumed by corrosion, leaving nothing but

GALVANIC CORROSION is an electrochemical action that results when
two dissimilar metals react together in the presence of an
electrolyte, such as water containing salts or hydrogen ions.  The
severity of the galvanic corrosion is based on the difference in
potential between the two metals, their relative surface areas, and
time.  If the more noble metal (higher position in electrochemical
series) is much larger in area than the baser, or less noble,
metal, the deterioration of the baser metal will be more rapid and
severe.  If the more noble metal is much smaller in area than the
baser metal, the deterioration of the baser metal will be much less
significant.  Cast iron will be attacked and corroded when it is
adjacent to more noble metals such as lead or copper.

GRAPHITIZATION of cast iron, a less common problem, occurs in the
presence of acid precipitation or seawater.  As the iron corrodes,
the porous graphite (soft carbon) corrosion residue is impregnated
with insoluble corrosion products.  As a result, the cast-iron
element retains its appearance and shape but is weaker
structurally.  Graphitization occurs where cast iron is left
unpainted for long periods or where caulked joints have failed and
acidic rainwater has corroded pieces from the backside.  Testing
and identification of graphitization is accomplished by scraping
through the surface with a knife to reveal the crumbling of the
iron beneath. Where extensive graphitization occurs, usually the
only solution is replacement of the damaged element.

Castings may also be fractured or flawed as a result of
imperfections in the original manufacturing process, such as air
holes, cracks, and cinders, or cold shuts (caused by the "freezing"
of the surface of the molten iron during casting because of
improper or interrupted pouring).  Brittleness is another problem
occasionally found in old cast-iron elements. It may be a result of
excessive phosphorus in the iron, or of chilling during the casting


Before establishing the appropriate treatment for cast-iron
elements in a building or structure, an evaluation should be made
of the property's historical and architectural significance and
alterations, along with its present condition.  If the work
involves more than routine maintenance, a qualified professional
should be engaged to develop an historic structure report which
sets forth the historical development of the property, documents
its existing condition, identifies problems of repair, and provides
a detailed listing of recommended work items with priorities.
Through this process the significance and condition of the cast
iron can be evaluated and appropriate treatments proposed.  For
fences, or for single components of a building such as a facade, a
similar but less extensive analytical procedure should be followed.

The nature and extent of the problems with the cast-iron elements
must be well understood before proceeding with work. If the
problems are minor, such as surface corrosion, flaking paint, and
failed caulking, the property owner may be able to undertake the
repairs by working directly with a knowledgeable contractor.  If
there are major problems or extensive damage to the cast iron, it
is best to secure the services of an architect or conservator who
specializes in the conservation of historic buildings.  Depending
on the scope of work, contract documents can range from outline
specifications to complete working drawings with annotated
photographs and specifications

To thoroughly assess the condition of the ironwork, a close
physical inspection must be undertaken of every section of the iron
construction including bolts, fasteners, and brackets. Typically,
scaffolding or a mechanical lift is employed for close inspection
of a cast-iron facade or other large structures.  Removal of select
areas of paint may be the only means to determine the exact
condition of connections, metal fasteners, and intersections or
crevices that might trap water.

An investigation of load-bearing elements, such as columns and
beams, will establish whether these components are performing as
they were originally designed, or the stress patterns have been
redistributed.  Areas that are abnormally stressed must be examined
to ascertain whether they have suffered damage or have been
displaced.  Damage to a primary structural member is obviously
critical to identify and evaluate; attention should not be given
only to decorative features.

The condition of the building, structure, or object, diagnosis of
its problems, and recommendations for its repair should be recorded
by drawings, photographs, and written descriptions to aid those who
will be responsible for its conservation in the future.

Whether minor or major work is required, the retention and repair
of historic ironwork is the recommended preservation approach over
replacement.  All repairs and restoration work should be
reversible, when possible, so that modifications or treatments that
may turn out to be harmful to the long-term preservation of the
iron can be corrected with the least amount of damage to the
historic ironwork.


When there is extensive failure of the protective coating and/or
when heavy corrosion exists, the rust and most or all of the paint
must be removed to prepare the surfaces for new protective
coatings.  The techniques available range from physical processes,
such as wire brushing and grit blasting, to flame cleaning and
chemical methods.  The selection of an appropriate technique
depends upon how much paint failure and corrosion has occurred, the
fineness of the surface detailing, and the type of new protective
coating to be applied.  Local environmental regulations may
restrict the options for cleaning and paint removal methods, as
well as the disposal of materials.

Many of these techniques are potentially dangerous and should be
carried out only by experienced and qualified workers using proper
eye protection, protective clothing, and other workplace safety
conditions.  Before selecting a process, test panels should be
prepared on the iron to be cleaned to determine the relative
effectiveness of various techniques. The cleaning process will most
likely expose additional coating defects, cracks, and corrosion
that have not been obvious before.

There are a number of techniques that can be used to remove paint
and corrosion from cast iron:

least expensive methods of removing paint and light rust from cast
iron.  However, they do not remove all corrosion or paint as
effectively as other methods. Experienced craftsmen should carry
out the work to reduce the likelihood that surfaces may be scored
or fragile detail damaged.

LOW-PRESSURE GRIT BLASTING (commonly called abrasive cleaning or
sandblasting) is often the most effective approach to removing
excessive paint build-up or substantial corrosion.  Grit blasting
is fast, thorough, and economical, and it allows the iron to be
cleaned in place.  The aggregate can be iron slag or sand; copper
slag should not be used on iron because of the potential for
electrolytic reactions.  Some sharpness in the aggregate is
beneficial in that it gives the metal surface a "tooth" that will
result in better paint adhesion.  The use of a very sharp or hard
aggregate and/or excessively high pressure (over 100 pounds per
square inch) is unnecessary and should be avoided. Adjacent
materials, such as brick, stone, wood, and glass, must be protected
to prevent damage.  Some local building codes and environmental
authorities prohibit or limit dry sandblasting because of the
problem of airborne dust.

WET SANDBLASTING is more problematic than dry sandblasting for
cleaning cast iron because the water will cause instantaneous
surface rusting and will penetrate deep into open joints.
Therefore, it is generally not considered an effective technique.
Wet sandblasting reduces the amount of airborne dust when removing
a heavy paint build-up, but disposal of effluent containing lead or
other toxic substances is restricted by environmental regulations
in most areas.

FLAME CLEANING of rust from metal with a special multi-flame head
oxyacetylene torch requires specially skilled operators, and is
expensive and potentially dangerous.  However, it can be very
effective on light-to-moderately corroded iron.  Wire brushing is
usually necessary to finish the surface after flame cleaning.

CHEMICAL RUST REMOVAL, by acid pickling, is an effective method of
removing rust from iron elements that can be easily removed and
taken to a shop for submerging in vats of dilute phosphoric or
sulfuric acid.  This method does not damage the surface of iron,
providing that the iron is neutralized to pH level 7 after
cleaning.  Other chemical rust removal agents include ammonium
citrate, oxalic acid, or hydrochloric acid-based products.

CHEMICAL PAINT REMOVAL using alkaline compounds, such as methylene
chloride or potassium hydroxide, can be an effective alternative to
abrasive blasting for removal of heavy paint build-up.  These
agents are often available as slow-acting gels or pastes.  Because
they can cause burns, protective clothing and eye protection must
be worn.  Chemicals applied to a non-watertight facade can seep
through crevices and holes, resulting in damage to the building's
interior finishes and corrosion to the backside of the iron
components.  If not thoroughly neutralized, residual traces of
cleaning compounds on the surface of the iron can cause paint
failures in the future.  For these reasons, field application of
alkaline paint removers and acidic cleaners is not generally

Following any of these methods of cleaning and paint removal, the
newly cleaned iron should be painted immediately with a
corrosion-inhibiting primer before new rust begins to form.  This
time period may vary from minutes to hours depending on
environmental conditions. If priming is delayed, any surface rust
that has developed should be removed with a clean wire brush just
before priming, because the rust prevents good bonding between the
primer and the cast iron surface and prevents the primer from
completely filling the pores of the metal.


The most common and effective way to preserve architectural cast
iron is to maintain a protective coating of paint on the metal.
Paint can also be decorative, where historically appropriate.

Before removing paint from historic architectural cast iron, a
microscopic analysis of samples of the historic paint sequencing is
recommended.  Called paint seriation analysis, this process must be
carried out by an experienced architectural conservator.  The
analysis will identify the historic paint colors, and other
conditions, such as whether the paint was matte or gloss, whether
sand was added to the paint for texture, and whether the building
was polychromed or marbleized.  Traditionally many cast iron
elements were painted to resemble other materials, such as
limestone or sandstone.  Occasionally, features were faux-painted
so that the iron appeared to be veined marble.

Thorough surface preparation is necessary for the adhesion of new
protective coatings.  All loose, flaking, and deteriorated paint
must be removed from the iron, as well as dirt and mud,
water-soluble salts, oil, and grease.  Old paint that is tightly
adhered may be left on the surface of the iron if it is compatible
with the proposed coatings.  The retention of old paint also
preserves the historic paint sequence of the building and avoids
the hazards of removal and disposal of old lead paint.

It is advisable to consult manufacturer's specifications or
technical representatives to ensure compatibility between the
surface conditions, primer and finish coats, and application

For the paint to adhere properly, the metal surfaces must be
absolutely dry before painting.  Unless the paint selected is
specifically designed for exceptional conditions, painting should
not take place when the temperature is expected to fall below 50
degrees Fahrenheit within 24 hours or when the relative humidity is
above 80 per cent; paint should not be applied when there is fog,
mist, or rain in the air.  Poorly prepared surfaces will cause the
failure of even the best paints, while even moderately priced
paints can be effective if applied over well-prepared surfaces.


The types of paints available for protecting iron have changed
dramatically in recent years due to Federal, state, and local
regulations that prohibit or restrict the manufacture and use of
products containing toxic substances such as lead and zinc
chromate, as well as volatile organic compounds and substances (VOC
or VOS). Availability of paint types varies from state to state,
and manufacturers continue to change product formulations to comply
with new regulations.

Traditionally, red lead has been used as an anti-corrosive pigment
for priming iron.  Red lead has a strong affinity for linseed oil
and forms lead soaps, which become a tough and elastic film
impervious to water that is highly effective as a protective
coating for iron.  At least two slow-drying linseed oil-based
finish coats have traditionally been used over a red lead primer,
and this combination is effective on old or partially-deteriorated
surfaces.  Today, in most areas, the use of paints containing lead
is prohibited, except for some commercial and industrial purposes.

Today, alkyd paints are very widely used and have largely replaced
lead-containing linseed-oil paints.  They dry faster than oil
paint, with a thinner film, but they do not protect the metal as
long.  Alkyd rust-inhibitive primers contain pigments such as iron
oxide, zinc oxide, and zinc phosphate.  These primers are suitable
for previously painted surfaces cleaned by hand tools.  At least
two coats of primer should be applied, followed by alkyd enamel
finish coats.

Latex and other water-based paints are not recommended for use as
primers on cast iron because they cause immediate oxidation if
applied on bare metal.  Vinyl acrylic latex or acrylic latex paints
may be used as finish coats over alkyd rust-inhibitive primers, but
if the primer coats are imperfectly applied or are damaged, the
latex paint will cause oxidation of the iron. Therefore, alkyd
finish coats are recommended.

High-performance coatings, such as zinc-rich primers containing
zinc dust, and modern epoxy coatings, can be used on cast iron to
provide longer-lasting protection. These coatings typically require
highly clean surfaces and special application conditions which can
be difficult to achieve in the field on large buildings.  These
coatings are used most effectively on elements which have been
removed to a shop, or newly cast iron.

One particularly effective system has been first to coat
commercially blast-cleaned iron with a zinc-rich primer, followed
by an epoxy base coat, and two urethane finish coats.  Some epoxy
coatings can be used as primers on clean metal or applied to
previously-painted surfaces in sound condition.  Epoxies are
particularly susceptible to degradation under ultraviolet radiation
and must be protected by finish coats which are more resistant.
There have been problems with epoxy paints which have been
shop-applied to iron where the coatings have been nicked prior to
installation.  Field touching-up of epoxy paints is very difficult,
if not impossible.  This is a concern since iron exposed by
imperfections in the base coat will be more likely to rust and more
frequent maintenance will be required.

A key factor to take into account in selection of coatings is the
variety of conditions on existing and new materials on a particular
building or structure.  One primer may be needed for surfaces with
existing paint; another for newly cast, chemically stripped, or
blast-cleaned cast iron; and a third for flashings or substitute
materials; all three followed by compatible finish coats.


Brushing is the traditional and most effective technique for
applying paint to cast iron.  It provides good contact between the
paint and the iron, as well as the effective filling of pits,
cracks, and other blemishes in the metal.  The use of spray guns to
apply paint is economical, but does not always produce adequate and
uniform coverage.  For best results, airless sprayers should be
used by skilled operators.  To fully cover fine detailing and reach
recesses, spraying of the primer coat, used in conjunction with
brushing, may be effective.

Rollers should never be used for primer coat applications on metal,
and are effective for subsequent coats only on large, flat areas.
The appearance of spray-applied and roller-applied finish coats is
not historically appropriate and should be avoided on areas such as
storefronts which are viewed close at hand.


Most architectural cast iron is made of many small castings
assembled by bolts or screws.  Joints between pieces were caulked
to prevent water from seeping in and causing rusting from the
inside out.  Historically, the seams were often caulked with white
lead paste and sometimes backed with cotton or hemp rope; even the
bolt and screw heads were caulked to protect them from the elements
and to hide them from view. Although old caulking is sometimes
found in good condition, it is typically crumbled from weathering,
cracked from the structural settlement, or destroyed by mechanical
cleaning.  It is essential to replace deteriorated caulking to
prevent water penetration.  For good adhesion and performance, an
architectural-grade polyurethane sealant or traditional white lead
paste is preferred.


Last Reviewed 2012-09-05