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The Maintenance and Repair of
Architectural Cast Iron
John G. Waite, AIA
Historical Overview by Margot Gayle
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 iron.
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 (Fig. 1).
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, nonflammable 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-1820's, 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 onsite 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 Manhrattan
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 (cover photo and Fig. 2). 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 (Fig. 3)
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 iron-fronts were begun in 1850: The Inquirer Building, the Brock
Stores, and the Penn Mutuai Building (all three have been demolished).
The St. Charles Hotel of 1851 at 60 N. Third Street is the oldest iron-front
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 iron-fronts 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 (Fig.
1). Regrettably, a large proportion of iron-fronts 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
(Fig. 4). 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 (Fig. 5). With such widespread
demand, many American foundries that had been casting machine parts,
bank safes, iron pipe, or cookstoves added architectural iron departments
(Fig. 6). 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
treatments.
What is Cast Iron?
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 phosphorus.
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.
Maintenance and Repair
Many of the maintenance and repair techniques described in the 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 treatment.
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.
Deterioration
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
(Figs. 7, 8).
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 rust.
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 (Fig. 9). 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 process.
Condition Assessment
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 a 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 (Fig. 10). 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 (Fig. 11). 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.
Cleaning and Paint
Removal
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 (Fig. 12).
There are a number of techniques that can be used to remove paint
and corrosion from cast iron:
Hand scraping, chipping, and wire brushing are the most common and
least expensive methods of removing paint and light rust from cast iron
(Fig. 13a, b). 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 buildup
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 buildup, 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 lightly
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 buildup (Fig. 13). 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 (Fig. 14). For these
reasons, field application of alkaline paint removers and acidic cleaners
is not generally recommended.
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.
Painting and Coating
Systems
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.
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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.
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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 methods.
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.
Selection of Paints and Coatings
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 anticorrosive 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 (Fig. 13c). 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.
Application Methods
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.
Caulking,
Patching, and Mechanical Repairs
Most architectural cast iron is made of many small castings assembled
by bolts or screws (Fig. 16a). 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.
Water that penetrates the hollow parts of a cast-iron architectural
element causes rust that may streak down over other architectural elements.
The water may freeze, causing the ice to crack the cast iron. Cracks
reduce the strength of the total castiron assembly and provide another
point of entry for water. Thus, it is important that cracks be made
weathertight by using caulks or fillers, depending on the width of the
crack.
Filler compounds containing iron particles in an epoxy resin binder
can be used to patch superficial, nonstructural cracks and small defects
in cast iron. The thermal expansion rate of epoxy resin alone is different
from that of iron, requiring the addition of iron particles to ensure
compatibility and to control shrinkage. Although the repaired piece
of metal does not have the same strength as a homogeneous piece of iron,
epoxy-repaired members do have some strength. Polyester-based putties,
such as those used on auto bodies, are also acceptable fillers for small
holes.
In rare instances, major cracks can be repaired by brazing or welding
with special nickel-alloy welding rods. Brazing or welding of cast iron
is very difficult to carry out in the field and should be undertaken
only by very experienced welders.
In some cases, mechanical repairs can be made to cast iron using iron
bars and screws or bolts. In extreme cases, deteriorated cast iron can
be cut out and new cast iron spliced in place by welding or brazing.
However, it is frequently less expensive to replace a deteriorated cast-iron
section with a new casting rather than to splice or reinforce it. Cast-iron
structural elements that have failed must either be reinforced with
iron and steel or replaced entirely.
A wobbly cast-iron balustrade or railing can often be fixed by tightening
all bolts and screws. Screws with stripped threads and seriously rusted
bolts must be replaced. To compensate for corroded metal around the
bolt or screw holes, new stainless steel bolts or screws with a larger
diameter need to be used. In extreme cases, new holes may need to be
tapped.
The internal voids of balusters, newel posts, statuary, and other
elements should not be filled with concrete; it is an inappropriate
treatment that causes further problems (Fig. 15). As the concrete cures,
it shrinks, leaving a space between the concrete and cast iron. Water
penetrating this space does not evaporate quickly, thus promoting further
rusting. The corrosion of the iron is further accelerated by the alkaline
nature of concrete. Where cast-iron elements have been previously filled
with concrete, they need to be taken apart, the concrete and rust removed,
and the interior surfaces primed and painted before the elements are
reassembled.
Duplication and Replacement
The replacement of cast-iron components is often the only practical
solution when such features are missing, severely corroded, or damaged
beyond repair, or where repairs would be only marginally useful in extending
the functional life of an iron element (Fig. 16).
Sometimes it is possible to replace small, decorative, nonstructural
elements using intact sections of the original as a casting pattern.
For large sections, new patterns of wood or plastic made slightly larger
in size than the original will need to be made in order to compensate
for the shrinkage of the iron during casting (cast iron shrinks approximately
1/8 inch per foot as it cools from a liquid into a solid). Occasionally,
a matching replacement can be obtained from the existing catalogs of
iron foundries. Small elements can be custom cast in iron at small local
foundries, often at a cost comparable to substitute materials. Large
elements and complex patterns will usually require the skills and facilities
of a larger firm that specializes in replication.
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The Casting Process
Architectural elements were traditionally cast in sand molds. The
quality of the special sands used by foundries is extremely important;
unlike most sands they must be moist. Foundries have their own formulas
for sand and its admixtures, such as clay, which makes the sand cohesive
even when the mold is turned upside down.
A two-part mold (with a top and a bottom, or cope and drag) is used
for making a casting with relief on both sides, whereas an open-top
mold produces a flat surface on one side (Fig. 17a). For hollow elements,
a third pattern and mold are required for the void. Many hollow castings
are made of two or more parts that are later bolted, screwed, or welded
together, because of the difficulty of supporting an interior core between
the top and bottom sand molds during the casting process.
The molding sand is compacted into flasks, or forms, around the pattern.
The cope is then lifted off and the pattern is removed, leaving the
imprint of the pattern in the small mold. Molten iron, heated to a temperature
of approximately 2700 degrees Fahrenheit, is poured into the mold and
then allowed to cool (Fig. 17b). The molds are then stripped from the
casting; the tunnels to the mold (sprues) and risers that allowed release
of air are cut off; and ragged edges (called "burrs") on the casting
are ground
smooth.
The castings are shop-primed to prevent rust, and laid out and preassembled
at the foundry to ensure proper alignment and fit. When parts do not
fit, the pieces are machined to remove irregularities caused by burrs,
or are rejected and recast until all of the cast elements fit together
properly. Most larger pieces then are taken apart before shipping to
the job site, while some small ornamental parts may be left assembled.
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Dismantling and Assembly
of Architectural Components
It is sometimes necessary to dismantle all or part of a cast-iron
structure during restoration, if repairs cannot be successfully carried
out in place. Dismantling should be done only under the direction of
a preservation architect or architectural conservator who is experienced
with historic cast iron. Extreme care must be taken since cast iron
is very brittle, especially in cold weather.
Dismantling should follow the reverse order of construction and re-erection
should occur, as much as possible, in the exact order of original assembly.
Each piece should be numbered and keyed to record drawings. When work
must be carried out in cord weather, care needs to be taken to avoid
fracturing the iron elements by uneven heating of the members.
Both new castings and reused pieces should be painted with a shop-applied
prime coat on all surfaces. All of the components should be laid out
and preassembled to make sure that the alignment and fit are proper.
Many of the original bolts, nuts, and screws may have to be replaced
with similar fasteners of stainless steel.
After assembly at the site, joints that were historically caulked
should be filled with an architectural-grade polyurethane sealant or
the traditional white lead paste. White lead has the advantage of longevity,
although its use is restricted in many areas.
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FLASHINGS
In some instances, it may be necessary to design and install flashings
to protect areas vulnerable to water penetration. Flashings need to
be designed and fabricated carefully so that they are effective, as
well as unobtrusive in appearance. The most durable material for flashing
iron is terne-coated stainless steel (Fig. 18). Other compatible materials
are terne-coated steel and galvanized steel; however, these require
more frequent maintenance and are less durable. Copper and lead-coated
copper are not recommended for use as flashings in contact with cast
iron because of galvanic corrosion problems. Galvanic problems can also
occur with the use of aluminum if certain types of electrolytes are
present.
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Substitute Materials
In recent years, a number of metallic and non-metallic materials have
been used as substitutes for cast iron, although they were not used
historically with cast iron. The most common have been aluminum, epoxies,
reinforced polyester (fiberglass), and glass fiber-reinforced concrete
(GFRC). Factors to consider in using substitute materials are addressed
in Preservation Briefs 16, which emphasizes that "every means of repairing
deteriorating historic materials or replacing them with identical materials
should be examined before turning to substitute materials."
Cast aluminum has been used recently as a substitute for cast iron,
particularly for ornately-detailed decorative elements. Aluminum is
lighter in weight, more resistant to corrosion, and less brittle than
cast iron. However, because it is dissimilar from iron, its placement
in contact with or near cast iron may result in galvanic corrosion,
and thus should be avoided. Special care must be taken in the application
of paint coatings, particularly in the field. It is often difficult
to achieve a durable coating after the original finish has failed. Because
aluminum is weaker than iron, careful analysis is required whenever
aluminum is being considered as a replacement material for structural
cast-iron elements.
Epoxies are two-part, thermo-setting, resinous materials which can
be molded into virtually any form. When molded, the epoxy is usually
mixed with fillers such as sand, glass balloons, or stone chips. Since
it is not a metal, galvanic corrosion does not occur. When mixed with
sand or stone, it is often termed epoxy concrete or polymer concrete,
a misnomer because no cementitious materials are included. Epoxies are
particularly effective for replicating small, ornamental sections of
cast iron. Since it is not a metal, galvanic action does not occur.
Epoxy elements must have a protective coating to shield them from ultraviolet
degradation. They are also flammable and cannot be used as substitutes
for structural cast-iron elements.
Reinforced polyester, commonly known as fiberglass, is often used
as a lightweight substitute for historic materials, including cast iron,
wood, and stone. In its most common form, fiberglass is a thin, rigid,
laminate shell formed by pouring a polyester resin into a mold and then
adding fiberglass for reinforcement. Like epoxies, fiberglass is non-corrosive,
but is susceptible to ultraviolet degradation. Because of its rather
flimsy nature, it cannot be used as a substitute for structural elements,
cannot be assembled like cast iron and usually requires a separate anchorage
system. It is unsuitable for locations where it is susceptible to damage
by impact (Fig. 20), and is also flammable.
Glass fiber-reinforced concrete, known as GFRC, is similar to fiberglass
except that a lightweight concrete is substituted for the resin. GFRC
elements are generally fabricated as thin shell panels by spraying concrete
into forms. Usually a separate framing and anchorage system is required.
GFRC elements are lightweight, inexpensive, and weather resistant. Because
GFRC has a low shrinkage coefficient, molds can be made directly from
historic elements. However, GFRC is very different physically and chemically
from iron. If used adjacent to iron, it causes corrosion of the iron
and will have a different moisture absorption rate. Also, it is not
possible to achieve the crisp detail that is characteristic of cast
iron.
Maintenance
A successful maintenance program is the key to the long-term preservation
of architectural cast iron. Regular inspections and accurate record-keeping
are essential. Biannual inspections, occurring ideally in the spring
and fall, include the identification of major problems, such as missing
elements and fractures, as well as minor items such as failed caulking,
damaged paint, and surface dirt.
Records should be kept in the form of a permanent maintenance log
which describes routine maintenance tasks and records the date a problem
is first noted, when it was corrected, and the treatment method. Painting
records are important for selecting compatible paints for touch-up and
subsequent repainting. The location of the work and the type, manufacturer,
and color of the paint should be noted in the log. The same information
also should be assembled and recorded for caulking.
Superficial dirt can be washed off well-painted and caulked cast iron
with low-pressure water. Non-ionic detergents may be used for the removal
of heavy or tenacious dirt or stains, after testing to determine that
they have no adverse effects on the painted surfaces. Thick grease deposits
and residue can be removed by hand scraping. Water and detergents or
non-caustic degreasing agents can be used to clean off the residue.
Before repainting, oil and grease must be removed so that new coatings
will adhere properly.
The primary purpose of the maintenance program is to control corrosion.
As soon as rusting is noted, it should be carefully removed and the
protective coating of the iron renewed in the affected area. Replacement
of deteriorated caulking, and repair or replacement of failed flashings
are also important preventive maintenance measures.
Summary
The successful conservation of cast-iron architectural elements and
objects is dependent upon an accurate diagnosis of their condition and
the problems affecting them, as well as the selection of appropriate
repair, cleaning, and painting procedures. Frequently, it is necessary
to undertake major repairs to individual elements and assemblies; in
some cases badly damaged or missing components must be replicated. The
long-term preservation of architectural cast iron is dependent upon
both the undertaking of timely, appropriate repairs and the commitment
to a regular schedule of maintenance.
Reading List
Ashurst, John, and Nicola Ashurst with Geoff Wallis and Dennis Toner.
Practical Building Conservation: English Heritage Technical Handbook:
Volume 4 Metals. Aldershot, Hants: Gower Technical Press, 1988.
Badger, Daniel D., with a new introduction by Margot Gayle. Badger's
Illustrated Catalogue of Castlron Architecture. New York: Dover Publications,
Inc., 1981; reprint of 1865 edition published by Baker & Godwin,
Printers, New York.
Gayle, Margot, and Edmund V. Gillon, Jr. Castlron Architecture in
New York: A Photogrnphic Survey. New York: Dover Publications Inc.,
1974.
Gayle, Margot, David W. Look, AIA, and John G. Waite. Metals in America's
Historic Buildings: Part I. A Historical Survey of Metals; Part II.
Deterioration and Methods of Preserving Metals. Washington, D.C.; Preservation
Assistance Division, National Park Service, U.S. Department of the Interior,
1980.
Hawkins, William John III. The Grand Era of Cast-Iron Architecture
in Portland. Portland, Oregon: Binford & Mort, 1976.
Howell, J. Scott. "Architectural Cast Iron: Design and Restoration,"
The Journal of the Association for Preservation Technology. Vol XIX,
Number 3 (1987), pp. 5155.
Park, Sharon C., AIA. Preservation Briefs 16: The Use of Substitute
Materials on Historic Building Exteriors. Washington D. C.: Preservation
Assistance Division, National Park Service, U. 5. Department of the
Interior, 1988.
Robertson, E. Graeme, and Joan Robertson. Cast-Iron Decoration: A
World Survey. New York: Watson-Guptill Publications, 1977.
Southworth, Susan and Michael. Ornamental Ironwork: An Illustrated
Guide to Its Design, History, and Use in American Architecture. Boston:
David R. Godine, 1978.
Waite, Diana S. Ornamental Ironwork: Two Centuries of Craftsmanship
in Albany and Troy, New York. Albany, NY: Mount Ida Press, 1990.
Acknowledgements
This Preservation Brief was developed by the New York Landmarks Conservancy's
Technical Preservation Services Center under a cooperative agreement
with the National Park Service's Preservation Assistance Division, and
with partial finding from the New York State Council on the Arts. The
following individuals are to be thanked for their technical assistance:
Robert Baird, Historical Arts & Casting; Willcox Dunn, Architect
and Cast-Iron Consultant; William Foulks, Mesick Cohen Waite Architects;
Elizabeth Frosch, New York City Landmarks Preservation Commission; William
Hawkins, III, FAIA, McMath Hawkins Dortignacq; J. Scott Howell, Robinson
Iron Company; Maurice Schicker, Facade Consultants International; Joel
Schwartz, Schwartz and Schwartz Metalworks; and Diana Waite, Mount
Ida Press. Kim Lovejoy was project coordinator and editor for the
Conservancy; Charles Fisher was project coordinator and editor for the
National Park Service.
Washington, D.C. October, 1991
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