The Maintenance and Repair of Architectural Cast Iron
Historical Overview by MargotGayle>
Parts of this story: What is Cast Iron?~~ Maintenance and Repair ~~ Deterioration~~ Condition Assessment ~~ Cleaning and Paint Removal~~ Painting and Coating Systems~~ Caulking, Patching,and Mechanical Repairs ~~ Duplication and Replacement~~ Dismantling and Assembly ~~ Substitute Materials ~~ Maintenance ~~ Summary ~~ Reading List
The preservation of cast-iron architectural elements, including entirefacades, has gained increasing attention in recent years as commercialdistricts are recognized for their historic significance and revitalized.This Brief provides general guidance on approaches to the preservationand restoration of historic cast iron.
Cast iron played a preeminent role in the industrial development ofour country during the 19th century. Cast-iron machinery filled America'sfactories and made possible the growth of railroad transportation. Castiron was used extensively in our cities for water systems and street lighting.As an architectural metal, it made possible bold new advances in architecturaldesigns and building technology, while providing a richness in ornamentation.
This age-old metal, an iron alloy with a high carbon content, had beentoo costly to make in large quantities until the mid-18th century, whennew 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 werefirst employed in the 1820s in theaters and churches to support balconies.
By the mid-1820's, one-story iron storefronts were being advertised inNew York City. Daniel Badger, the Boston foundryman who later moved toNew York, asserted that in 1842 he fabricated and installed the first rollingiron shutters for iron storefronts, which provided protection against theftand external fire. In the years ahead, and into the 1920s, the practicalcast-iron storefront would become a favorite in towns and cities from coastto coast. Not only did it help support the load of the upper floors, butit provided large show windows for the display of wares and allowed naturallight to flood the interiors of the shops. Most importantly, cast-iron storefrontswere inexpensive to assemble, requiring little onsite labor.
A tireless advocate for the use of cast iron in buildings was an inventiveNew Yorker, the self-taught architect/engineer James Bogardus. From 1840on, 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 urbanconflagrations. He also stressed that the foundry casting processes, bywhich cast iron was made into building elements, were thoroughly compatiblewith the new concepts of prefabrication, mass production, and use of identicalinterchangeable parts.
In 1849 Bogardus created something uniquely American when he erectedthe first structure with self-supporting, multi-storied exterior walls ofiron. Known as the Edgar Laing Stores, this corner row of small four-storywarehouses that looked like one building was constructed in lower Manhrattanin only two months. Its rear, side, and interior bearing walls were ofbrick; the floor framing consisted of timber joists and girders. One ofthe cast-iron walls was load-bearing, supporting the wood floor joists. Theinnovation was its two street facades of self-supporting cast iron, consistingof 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 sandmold in a foundry, machined smooth, tested for fit, and finally trundledon horse-drawn drays to the building site. There they were hoisted intoposition, then bolted together and fastened to the conventional structureof timber and brick with iron spikes and straps
The second iron-front building erected was a quantum leap beyond theLaing Stores in size and complexity. Begun in April 1850 by Bogardus, witharchitect Robert Hatfield, the five-story Sun newspaper building in Baltimorewas both cast-iron-fronted and cast-iron-framed. In Philadelphia, several iron-frontswere begun in 1850: The Inquirer Building, the Brock Stores, and the PennMutuai Building (all three have been demolished). The St. Charles Hotelof 1851 at 60 N. Third Street is the oldest iron-front in America. Framingwith cast-iron columns and wrought-iron beams and trusses was visible ona vast scale in the New York Crystal Palace of 1853.
In the second half of the 19th century, the United States was in anera of tremendous economic and territorial growth. The use of iron in commercialand public buildings spread rapidly, and hundreds of iron-fronted buildingswere erected in cities across the country from 1849 to beyond the turnof 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 IronHistoric District alone has 139 iron-fronted buildings. Regrettably,a large proportion of iron-fronts nationwide have been demolished in downtownredevelopment projects, especially since World War II.
In addition to these exterior uses, many public buildings display magnificentexposed interior ironwork, at once ornamental and structural.Remarkable examples have survived across the country, including the PeabodyLibrary in Baltimore; the Old Executive Office Building in Washington,D.C.; the Bradbury Building in Los Angeles; the former Louisiana StateCapitol; the former City Hall in Richmond; Tweed Courthouse in New York;and the state capitols of California, Georgia, Michigan, Tennessee, andTexas. And it is iron, of course, that forms the great dome of the UnitedStates Capitol, completed during the Civil War. Ornamental cast iron wasa popular material in the landscape as well, appearing as fences, fountainswith statuary, lampposts, furniture, urns, gazebos, gates, and enclosuresfor cemetery plots. With such widespread demand, many Americanfoundries that had been casting machine parts, bank safes, iron pipe, orcookstoves added architectural iron departments. These calledfor patternmakers with sophisticated design capabilities, as well as knowledgeof metal shrinkage and other technical aspects of casting. Major companiesincluded the Hayward Bartlett Co. in Baltimore; James L. Jackson, CornellBrothers, J. L. Mott, and Daniel D. Badger's Architectural Iron Works inManhattan; Hecla Ironworks in Brooklyn; Wood & Perot of Philadelphia;Leeds & Co., the Shakspeare (sic) Foundry, and Miltenberger in NewOrleans; Winslow Brothers in Chicago; and James McKinney in Albany, N.Y.
Cast iron was the metal of choice throughout the second half of the19th century. Not only was it a fire-resistant material in a period of majorurban fires, but also large facades could be produced with cast iron atless cost than comparable stone fronts, and iron buildings could be erectedwith speed and efficiency. The largest standing example of framing withcast-iron columns and wrought-iron beams is Chicago's sixteen-story ManhattanBuilding, the world's tallest skyscraper when built in 1890 by WilliamLeBaron Jenney. By this time, however, steel was becoming available nationally,and was structurally more versatile and cost-competitive. Its increaseduse is one reason why building with cast iron diminished around the turnof the century after having been so eagerly adopted only fifty years before.Nonetheless, cast iron continued to be used in substantial quantities formany other structural and ornamental purposes well into the 20th century:storefronts; marquees; bays and large window frames for steel-framed, masonry-cladbuildings; 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 possibleby the use of metals. Of these, cast iron was the pioneer, although itsperiod of intensive use lasted but a half century. Now the surviving legacyof cast-iron architecture, much of which continues to be threatened, meritsrenewed 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% andusually 3.0 to 3.7%) that makes it more resistant to corrosion than eitherwrought iron or steel. In addition to carbon, cast iron contains varyingamounts of silicon, sulfur, manganese, and phosphorus.
While molten, cast iron is easily poured into molds, making it possibleto create nearly unlimited decorative and structural forms. Unlike wroughtiron 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 resistantto buckling than other forms of iron, it can withstand great compressionloads. Cast iron is relatively weak in tension, however and fails undertensile loading with little prior warning.
The characteristics of various types of cast iron are determined bytheir composition and the techniques used in melting, casting, and heattreatment. Metallurgical constituents of cast iron that affect its brittleness,toughness, and strength include ferrite, cementite, pearlite, and graphitecarbon. Cast iron with flakes of carbon is called gray cast iron. The "grayfracture" associated with cast iron was probably named for the gray,grainy appearance of its broken edge caused by the presence of flakes offree graphite, which account for the brittleness of cast iron. This brittlenessis the important distinguishing characteristic between cast iron and mildsteel.
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 physicalassociation, that is, it is not alloyed. This gives wrought iron its characteristiclaminated (layered) or fibrous structure.
Wrought iron can be distinguished from cast iron in several ways. Wrought-ironelements generally are simpler in form and less uniform in appearance thancast-iron elements, and contain evidence of rolling or hand working. Castiron often contains mold lines, flashing, casting flaws, and air holes.Cast-iron elements are very uniform in appearance and are frequently usedrepetitively. Cast-iron elements are often bolted or screwed together, whereaswrought-iron pieces are either riveted or forge-molded (heat welded) together.
Mild steel is now used to fabricate new hand-worked metal work and torepair old wrought-iron elements. Mild steel is an alloy of iron and isnot more than 2% carbon, which is strong but easily worked in block oringot form. Mild steel is not as resistant to corrosion as either wroughtiron 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 dangerousand should be carried out only by experienced and qualified workmen usingprotective equipment suitable to the task. In all but the most simple repairs,it is best to involve a preservation architect or building conservatorto assess the condition of the iron and prepare contract documents forits treatment.
As with any preservation project, the work must be preceded by a reviewof local building codes and environmental protection regulations to determinewhether any conflicts exist with the proposed treatments. If there areconflicts, particularly with cleaning techniques or painting materials,then waivers or variances need to be negotiated, or alternative treatmentsor materials adopted.
Deterioration
Common problems encountered today with cast-iron construction includebadly rusted or missing elements, impact damage, structural failures, brokenjoints, damage to connections, and loss of anchorage in masonry.
Oxidation, or rusting, occurs rapidly when cast iron is exposed to moistureand air. The minimum relative humidity necessary to promote rusting is65%, but this figure can be lower in the presence of corrosive agents,such as sea water, salt air, acids, acid precipitation, soils, and somesulfur compounds present in the atmosphere, which act as catalysts in theoxidation process. Rusting is accelerated in situations where architecturaldetails provide pockets or crevices to trap and hold liquid corrosive agents.Furthermore, once a rust film forms, its porous surface acts as a reservoirfor liquids, which in turn causes further corrosion. If this process isnot 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 twodissimilar metals react together in the presence of an electrolyte, suchas water containing salts or hydrogen ions. The severity of thegalvanic corrosion is based on the difference in potential between thetwo metals, their relative surface areas, and time. If the more noble metal(higher position in electrochemical series) is much larger in area thanthe baser, or less noble, metal, the deterioration of the baser metal willbe more rapid and severe. If the more noble metal is much smaller in areathan the baser metal, the deterioration of the baser metal will be muchless significant. Cast iron will be attacked and corroded when it is adjacentto more noble metals such as lead or copper.
Graphitization of cast iron, a less common problem, occurs in the presenceof acid precipitation or seawater. As the iron corrodes, the porous graphite(soft carbon) corrosion residue is impregnated with insoluble corrosionproducts. As a result, the cast-iron element retains its appearance andshape but is weaker structurally. Graphitization occurs where cast ironis left unpainted for long periods or where caulked joints have failedand acidic rainwater has corroded pieces from the backside. Testing andidentification of graphitization is accomplished by scraping through thesurface with a knife to reveal the crumbling of the iron beneath. Whereextensive graphitization occurs, usually the only solution is replacementof the damaged element.
Castings may also be fractured or flawed as a result of imperfectionsin the original manufacturing process, such as air holes, cracks, and cinders,or cold shuts (caused by the "freezing" of the surface of themolten 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 chillingduring the casting process.
Condition Assessment
Before establishing the appropriate treatment for cast-iron elementsin a building or structure, an evaluation should be made of the property'shistorical and architectural significance and alterations, along with itspresent condition. If the work involves more than routine maintenance,a qualified professional should be engaged to develop a historic structurereport which sets forth the historical development of the property, documentsits existing condition, identifies problems of repair, and provides a detailedlisting of recommended work items with priorities. Through this processthe significance and condition of the cast iron can be evaluated and appropriatetreatments proposed. For fences, or for single components of a buildingsuch as a facade, a similar but less extensive analytical procedure shouldbe followed.
The nature and extent of the problems with the cast-iron elements mustbe well understood before proceeding with work. If the problems are minor,such as surface corrosion, flaking paint, and failed caulking, the propertyowner may be able to undertake the repairs by working directly with a knowledgeablecontractor. If there are major problems or extensive damage to the castiron, it is best to secure the services of an architect or conservatorwho specializes in the conservation of historic buildings. Depending onthe scope of work, contract documents can range from outline specificationsto complete working drawings with annotated photographs and specifications.
To thoroughly assess the condition of the ironwork, a close physicalinspection must be undertaken of every section of the iron constructionincluding bolts, fasteners, and brackets. Typically, scaffoldingor a mechanical lift is employed for close inspection of a cast-iron facadeor other large structures. Removal of select areas of paint may be theonly 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 originallydesigned, or the stress patterns have been redistributed. Areas that areabnormally stressed must be examined to ascertain whether they have suffereddamage or have been displaced. Damage to a primary structuralmember is obviously critical to identify and evaluate; attention shouldnot be given only to decorative features.
The condition of the building, structure, or object; diagnosis of itsproblems; and recommendations for its repair should be recorded by drawings,photographs, and written descriptions, to aid those who will be responsiblefor its conservation in the future.
Whether minor or major work is required, the retention and repair ofhistoric ironwork is the recommended preservation approach over replacement.All repairs and restoration work should be reversible, when possible, sothat modifications or treatments that may turn out to be harmful to thelong-term preservation of the iron can be corrected with the least amountof damage to the historic ironwork.
Cleaning and Paint Removal
When there is extensive failure of the protective coating and/or whenheavy corrosion exists, the rust and most or all of the paint must be removedto prepare the surfaces for new protective coatings. The techniques availablerange from physical processes, such as wire brushing and grit blasting,to flame cleaning and chemical methods. The selection of an appropriatetechnique depends upon how much paint failure and corrosion has occurred,the fineness of the surface detailing, and the type of new protective coatingto be applied. Local environmental regulations may restrict the optionsfor cleaning and paint removal methods, as well as the disposal of materials.
Many of these techniques are potentially dangerous and should be carriedout only by experienced and qualified workers using proper eye protection,protective clothing, and other workplace safety conditions. Before selectinga process, test panels should be prepared on the iron to be cleaned todetermine the relative effectiveness of various techniques. The cleaningprocess will most likely expose additional coating defects, cracks, andcorrosion that have not been obvious before.
There are a number of techniques that can be used to remove paint andcorrosion from cast iron:
Hand scraping, chipping, and wire brushing are the most common and leastexpensive methods of removing paint and light rust from cast iron. However, they do not remove all corrosion or paint as effectivelyas other methods. Experienced craftsmen should carry out the work to reducethe 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 buildupor substantial corrosion. Grit blasting is fast, thorough, and economical,and it allows the iron to be cleaned in place. The aggregate can be ironslag or sand; copper slag should not be used on iron because of the potentialfor electrolytic reactions. Some sharpness in the aggregate is beneficialin that it gives the metal surface a "tooth" that will resultin better paint adhesion. The use of a very sharp or hard aggregate and/orexcessively high pressure (over 100 pounds per square inch) is unnecessaryand should be avoided. Adjacent materials, such as brick, stone, wood,and glass, must be protected to prevent damage. Some local building codesand environmental authorities prohibit or limit dry sandblasting becauseof the problem of airborne dust.
Wet sandblasting is more problematic than dry sandblasting for cleaningcast iron because the water will cause instantaneous surface rusting andwill penetrate deep into open joints. Therefore, it is generally not consideredan effective technique. Wet sandblasting reduces the amount of airbornedust when removing a heavy paint buildup, but disposal of effluent containinglead or other toxic substances is restricted by environmental regulationsin most areas.
Flame cleaning of rust from metal with a special multi-flame head oxyacetylenetorch requires specially skilled operators, and is expensive and potentiallydangerous. However, it can be very effective on lightly to moderately corrodediron. Wire brushing is usually necessary to finish the surface after flamecleaning.
Chemical rust removal, by acid pickling, is an effective method of removingrust from iron elements that can be easily removed and taken to a shopfor submerging in vats of dilute phosphoric or sulfuric acid. This methoddoes not damage the surface of iron, providing that the iron is neutralizedto pH level 7 after cleaning. Other chemical rust removal agents includeammonium citrate, oxalic acid, or hydrochloric acid-based products.
Chemical paint removal using alkaline compounds, such as methylene chlorideor potassium hydroxide, can be an effective alternative to abrasive blastingfor removal of heavy paint buildup. These agents are often availableas slow-acting gels or pastes. Because they can cause burns, protectiveclothing and eye protection must be worn. Chemicals applied to a non-watertightfacade can seep through crevices and holes, resulting in damage to thebuilding's interior finishes and corrosion to the backside of the ironcomponents. If not thoroughly neutralized, residual traces of cleaningcompounds on the surface of the iron can cause paint failures in the future. For these reasons, field application of alkaline paint removersand acidic cleaners is not generally recommended.
Following any of these methods of cleaning and paint removal, the newlycleaned iron should be painted immediately with a corrosion-inhibiting primerbefore new rust begins to form. This time period may vary from minutesto hours depending on environmental conditions. If priming is delayed,any surface rust that has developed should be removed with a clean wirebrush just before priming, because the rust prevents good bonding betweenthe primer and the cast-iron surface and prevents the primer from completelyfilling the pores of the metal.
Painting and Coating Systems
The most common and effective way to preserve architectural cast ironis to maintain a protective coating of paint on the metal. Paint can alsobe decorative, where historically appropriate.
Before removing paint from historic architectural cast iron, a microscopicanalysis of samples of the historic paint sequencing is recommended. Calledpaint seriation analysis, this process must be carried out by an experiencedarchitectural conservator. The analysis will identify the historic paintcolors, and other conditions, such as whether the paint was matte or gloss,whether sand was added to the paint for texture, and whether the buildingwas polychromed or marbleized. Traditionally, many cast-iron elements werepainted 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 protectivecoatings. All loose, flaking, and deteriorated paint must be removed fromthe 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 ironif it is compatible with the proposed coatings. The retention of old paintalso preserves the historic paint sequence of the building and avoids thehazards of removal and disposal of old lead paint.
It is advisable to consult manufacturer's specifications or technicalrepresentatives 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 absolutelydry before painting. Unless the paint selected is specifically designedfor exceptional conditions, painting should not take place when the temperatureis expected to fall below 50 degrees Fahrenheit within 24 hours or whenthe relative humidity is above 80 per cent; paint should not be appliedwhen there is fog, mist, or rain in the air. Poorly prepared surfaces willcause the failure of even the best paints, while even moderately pricedpaints can be effective if applied over well-prepared surfaces.
Selection of Paints and Coatings
The types of paints available for protecting iron have changed dramaticallyin recent years due to federal, state, and local regulations that prohibitor restrict the manufacture and use of products containing toxic substancessuch as lead and zinc chromate, as well as volatile organic compounds andsubstances (VOC or VOS). Availability of paint types varies from stateto state, and manufacturers continue to change product formulations tocomply with new regulations.
Traditionally, red lead has been used as an anticorrosive pigment forpriming iron. Red lead has a strong affinity for linseed oil and formslead soaps, which become a tough and elastic film impervious to water thatis highly effective as a protective coating for iron. At least two slow-dryinglinseed oil-based finish coats have traditionally been used over a red leadprimer, and this combination is effective on old or partially deterioratedsurfaces. 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-containinglinseed oil paints. They dry faster than oil paint, with a thinner film,but they do not protect the metal as long. Alkyd rust-inhibitive primerscontain pigments such as iron oxide, zinc oxide, and zinc phosphate. Theseprimers are suitable for previously painted surfaces cleaned by hand tools.At least two coats of primer should be applied, followed by alkyd enamelfinish coats.
Latex and other water-based paints are not recommended for use as primerson cast iron because they cause immediate oxidation if applied on baremetal. Vinyl acrylic latex or acrylic latex paints may be used as finishcoats over alkyd rust-inhibitive primers, but if the primer coats are imperfectlyapplied 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-lastingprotection. These coatings typically require highly clean surfaces andspecial application conditions which can be difficult to achieve in thefield on large buildings. These coatings are used most effectivelyon elements which have been removed to a shop, or newly cast iron.
One particularly effective system has been first to coat commerciallyblast-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 primerson clean metal or applied to previously-painted surfaces in sound condition.Epoxies are particularly susceptible to degradation under ultraviolet radiationand must be protected by finish coats which are more resistant. There havebeen problems with epoxy paints which have been shop-applied to iron wherethe coatings have been nicked prior to installation. Field touching-up ofepoxy paints is very difficult, if not impossible. This is a concern sinceiron exposed by imperfections in the base coat will be more likely to rustand more frequent maintenance will be required.
A key factor to take into account in selection of coatings is the varietyof conditions on existing and new materials on a particular building orstructure. One primer may be needed for surfaces with existing paint; anotherfor newly cast, chemically stripped, or blast-cleaned cast iron; and a thirdfor flashings or substitute materials; all three followed by compatiblefinish coats.
Application Methods
Brushing is the traditional and most effective technique for applyingpaint to cast iron. It provides good contact between the paint and theiron, as well as the effective filling of pits, cracks, and other blemishesin the metal. The use of spray guns to apply paint is economical, but doesnot always produce adequate and uniform coverage. For best results, airlesssprayers should be used by skilled operators. To fully cover fine detailingand reach recesses, spraying of the primer coat, used in conjunction withbrushing, 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 appearanceof spray-applied and roller-applied finish coats is not historically appropriateand should be avoided on areas such as storefronts which are viewed closeat hand.
Caulking,Patching, and Mechanical Repairs
Most architectural cast iron is made of many small castings assembledby bolts or screws. Joints between pieces were caulked to preventwater from seeping in and causing rusting from the inside out. Historically,the seams were often caulked with white lead paste and sometimes backedwith cotton or hemp rope; even the bolt and screw heads were caulked toprotect them from the elements and to hide them from view. Although oldcaulking is sometimes found in good condition, it is typically crumbledfrom weathering, cracked from the structural settlement, or destroyed bymechanical cleaning. It is essential to replace deteriorated caulking toprevent water penetration. For good adhesion and performance, an architectural-gradepolyurethane sealant or traditional white lead paste is preferred.
Water that penetrates the hollow parts of a cast-iron architectural elementcauses rust that may streak down over other architectural elements. Thewater may freeze, causing the ice to crack the cast iron. Cracks reducethe strength of the total castiron assembly and provide another point ofentry for water. Thus, it is important that cracks be made weathertightby using caulks or fillers, depending on the width of the crack.
Filler compounds containing iron particles in an epoxy resin bindercan be used to patch superficial, nonstructural cracks and small defectsin cast iron. The thermal expansion rate of epoxy resin alone is differentfrom that of iron, requiring the addition of iron particles to ensure compatibilityand to control shrinkage. Although the repaired piece of metal does nothave the same strength as a homogeneous piece of iron, epoxy-repaired membersdo have some strength. Polyester-based putties, such as those used on autobodies, are also acceptable fillers for small holes.
In rare instances, major cracks can be repaired by brazing or weldingwith special nickel-alloy welding rods. Brazing or welding of cast ironis very difficult to carry out in the field and should be undertaken onlyby very experienced welders.
In some cases, mechanical repairs can be made to cast iron using ironbars and screws or bolts. In extreme cases, deteriorated cast iron canbe 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 sectionwith a new casting rather than to splice or reinforce it. Cast-iron structuralelements that have failed must either be reinforced with iron and steelor replaced entirely.
A wobbly cast-iron balustrade or railing can often be fixed by tighteningall bolts and screws. Screws with stripped threads and seriously rustedbolts must be replaced. To compensate for corroded metal around the boltor screw holes, new stainless steel bolts or screws with a larger diameterneed to be used. In extreme cases, new holes may need to be tapped.
The internal voids of balusters, newel posts, statuary, and other elementsshould not be filled with concrete; it is an inappropriate treatment thatcauses further problems. As the concrete cures, it shrinks, leavinga space between the concrete and cast iron. Water penetrating this spacedoes not evaporate quickly, thus promoting further rusting. The corrosionof the iron is further accelerated by the alkaline nature of concrete.Where cast-iron elements have been previously filled with concrete, theyneed to be taken apart, the concrete and rust removed, and the interiorsurfaces primed and painted before the elements are reassembled.
Duplication and Replacement
The replacement of cast-iron components is often the only practical solutionwhen such features are missing, severely corroded, or damaged beyond repair,or where repairs would be only marginally useful in extending the functionallife of an iron element.
Sometimes it is possible to replace small, decorative, nonstructuralelements using intact sections of the original as a casting pattern. Forlarge sections, new patterns of wood or plastic made slightly larger insize than the original will need to be made in order to compensate forthe shrinkage of the iron during casting (cast iron shrinks approximately1/8 inch per foot as it cools from a liquid into a solid). Occasionally,a matching replacement can be obtained from the existing catalogs of ironfoundries. Small elements can be custom cast in iron at small local foundries,often at a cost comparable to substitute materials. Large elements andcomplex patterns will usually require the skills and facilities of a largerfirm that specializes in replication.
The Casting Process
Architectural elements were traditionally cast in sand molds. The qualityof the special sands used by foundries is extremely important; unlike mostsands they must be moist. Foundries have their own formulas for sand andits admixtures, such as clay, which makes the sand cohesive even when themold is turned upside down.
A two-part mold (with a top and a bottom, or cope and drag) is used formaking a casting with relief on both sides, whereas an open-top mold producesa flat surface on one side. For hollow elements, a third patternand mold are required for the void. Many hollow castings are made of twoor more parts that are later bolted, screwed, or welded together, becauseof the difficulty of supporting an interior core between the top and bottomsand 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 imprintof the pattern in the small mold. Molten iron, heated to a temperatureof approximately 2700 degrees Fahrenheit, is poured into the mold and thenallowed to cool. The molds are then stripped from the casting;the tunnels to the mold (sprues) and risers that allowed release of airare cut off; and ragged edges (called "burrs") on the castingare ground smooth.
The castings are shop-primed to prevent rust, and laid out and preassembledat the foundry to ensure proper alignment and fit. When parts do not fit,the pieces are machined to remove irregularities caused by burrs, or arerejected 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.
Dismantling and Assembly ofArchitectural Components
It is sometimes necessary to dismantle all or part of a cast-iron structureduring restoration, if repairs cannot be successfully carried out in place.Dismantling should be done only under the direction of a preservation architector architectural conservator who is experienced with historic cast iron.Extreme care must be taken since cast iron is very brittle, especiallyin cold weather.
Dismantling should follow the reverse order of construction and re-erectionshould occur, as much as possible, in the exact order of original assembly.Each piece should be numbered and keyed to record drawings. When work mustbe carried out in cord weather, care needs to be taken to avoid fracturingthe iron elements by uneven heating of the members.
Both new castings and reused pieces should be painted with a shop-appliedprime coat on all surfaces. All of the components should be laid out andpreassembled to make sure that the alignment and fit are proper. Many ofthe original bolts, nuts, and screws may have to be replaced with similarfasteners of stainless steel.
After assembly at the site, joints that were historically caulked shouldbe filled with an architectural-grade polyurethane sealant or the traditionalwhite lead paste. White lead has the advantage of longevity, although itsuse is restricted in many areas.
FLASHINGS
In some instances, it may be necessary to design and install flashingsto protect areas vulnerable to water penetration. Flashings need to bedesigned and fabricated carefully so that they are effective, as well asunobtrusive in appearance. The most durable material for flashing ironis terne-coated stainless steel. Other compatible materials areterne-coated steel and galvanized steel; however, these require more frequentmaintenance and are less durable. Copper and lead-coated copper are notrecommended for use as flashings in contact with cast iron because of galvaniccorrosion problems. Galvanic problems can also occur with the use of aluminumif certain types of electrolytes are present.
Substitute Materials
In recent years, a number of metallic and non-metallic materials havebeen used as substitutes for cast iron, although they were not used historicallywith cast iron. The most common have been aluminum, epoxies, reinforcedpolyester (fiberglass), and glass fiber-reinforced concrete (GFRC). Factorsto consider in using substitute materials are addressed in PreservationBriefs 16, which emphasizes that "every means of repairing deterioratinghistoric materials or replacing them with identical materials should beexamined 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 lighterin weight, more resistant to corrosion, and less brittle than cast iron.However, because it is dissimilar from iron, its placement in contact withor near cast iron may result in galvanic corrosion, and thus should beavoided. Special care must be taken in the application of paint coatings,particularly in the field. It is often difficult to achieve a durable coatingafter the original finish has failed. Because aluminum is weaker than iron,careful analysis is required whenever aluminum is being considered as areplacement material for structural cast-iron elements.
Epoxies are two-part, thermo-setting, resinous materials which can bemolded into virtually any form. When molded, the epoxy is usually mixedwith fillers such as sand, glass balloons, or stone chips. Since it isnot a metal, galvanic corrosion does not occur. When mixed with sand orstone, it is often termed epoxy concrete or polymer concrete, a misnomerbecause no cementitious materials are included. Epoxies are particularlyeffective for replicating small, ornamental sections of cast iron. Sinceit is not a metal, galvanic action does not occur. Epoxy elements musthave a protective coating to shield them from ultraviolet degradation.They are also flammable and cannot be used as substitutes for structuralcast-iron elements.
Reinforced polyester, commonly known as fiberglass, is often used asa lightweight substitute for historic materials, including cast iron, wood,and stone. In its most common form, fiberglass is a thin, rigid, laminateshell formed by pouring a polyester resin into a mold and then adding fiberglassfor reinforcement. Like epoxies, fiberglass is non-corrosive, but is susceptibleto ultraviolet degradation. Because of its rather flimsy nature, it cannotbe used as a substitute for structural elements, cannot be assembled likecast iron and usually requires a separate anchorage system. It is unsuitablefor locations where it is susceptible to damage by impact, andis also flammable.
Glass fiber-reinforced concrete, known as GFRC, is similar to fiberglassexcept that a lightweight concrete is substituted for the resin. GFRC elementsare generally fabricated as thin shell panels by spraying concrete intoforms. Usually a separate framing and anchorage system is required. GFRCelements are lightweight, inexpensive, and weather resistant. Because GFRChas a low shrinkage coefficient, molds can be made directly from historicelements. However, GFRC is very different physically and chemically fromiron. If used adjacent to iron, it causes corrosion of the iron and willhave a different moisture absorption rate. Also, it is not possible toachieve the crisp detail that is characteristic of cast iron.
Maintenance
A successful maintenance program is the key to the long-term preservationof architectural cast iron. Regular inspections and accurate record-keepingare essential. Biannual inspections, occurring ideally in the spring andfall, include the identification of major problems, such as missing elementsand fractures, as well as minor items such as failed caulking, damagedpaint, and surface dirt.
Records should be kept in the form of a permanent maintenance log whichdescribes routine maintenance tasks and records the date a problem is firstnoted, when it was corrected, and the treatment method. Painting recordsare important for selecting compatible paints for touch-up and subsequentrepainting. The location of the work and the type, manufacturer, and colorof the paint should be noted in the log. The same information also shouldbe assembled and recorded for caulking.
Superficial dirt can be washed off well-painted and caulked cast ironwith low-pressure water. Non-ionic detergents may be used for the removalof heavy or tenacious dirt or stains, after testing to determine that theyhave no adverse effects on the painted surfaces. Thick grease depositsand residue can be removed by hand scraping. Water and detergents or non-causticdegreasing 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 protectivecoating of the iron renewed in the affected area. Replacement of deterioratedcaulking, and repair or replacement of failed flashings are also importantpreventive maintenance measures.
Summary
The successful conservation of cast-iron architectural elements and objectsis dependent upon an accurate diagnosis of their condition and the problemsaffecting them, as well as the selection of appropriate repair, cleaning,and painting procedures. Frequently, it is necessary to undertake majorrepairs to individual elements and assemblies; in some cases badly damagedor missing components must be replicated. The long-term preservation ofarchitectural 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: Volume4 Metals. Aldershot, Hants: Gower Technical Press, 1988.
Badger, Daniel D., with a new introduction by Margot Gayle. Badger'sIllustrated 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 NewYork: A Photogrnphic Survey. New York: Dover Publications Inc., 1974.
Gayle, Margot, David W. Look, AIA, and John G. Waite. Metals in America'sHistoric Buildings: Part I. A Historical Survey of Metals; Part II. Deteriorationand Methods of Preserving Metals. Washington, D.C.; Preservation AssistanceDivision, National Park Service, U.S. Department of the Interior, 1980.
Hawkins, William John III. The Grand Era of Cast-Iron Architecture inPortland. Portland, Oregon: Binford & Mort, 1976.
Howell, J. Scott. "Architectural Cast Iron: Design and Restoration,"The Journal of the Association for Preservation Technology. Vol XIX, Number3 (1987), pp. 5155.
Park, Sharon C., AIA. Preservation Briefs 16: The Use of SubstituteMaterials on Historic Building Exteriors. Washington D. C.: PreservationAssistance Division, National Park Service, U. 5. Department of the Interior,1988.
Robertson, E. Graeme, and Joan Robertson. Cast-Iron Decoration: A WorldSurvey. New York: Watson-Guptill Publications, 1977.
Southworth, Susan and Michael. Ornamental Ironwork: An Illustrated Guideto Its Design, History, and Use in American Architecture. Boston: DavidR. Godine, 1978.
Waite, Diana S. Ornamental Ironwork: Two Centuries of Craftsmanshipin Albany and Troy, New York. Albany, NY: Mount Ida Press, 1990.
Acknowledgements
This Preservation Brief was developed by the New York Landmarks Conservancy'sTechnical Preservation Services Center under a cooperative agreement withthe National Park Service's Preservation Assistance Division, and withpartial finding from the New York State Council on the Arts. The followingindividuals 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 YorkCity Landmarks Preservation Commission; William Hawkins, III, FAIA, McMathHawkins Dortignacq; J. Scott Howell, Robinson Iron Company; Maurice Schicker,Facade Consultants International; Joel Schwartz, Schwartz and SchwartzMetalworks; 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 ParkService.
Washington, D.C. October, 1991
This publication has been prepared pursuant to the NationalHistoric Preservation Act of 1966, as amended, wlhich directs the Secretaryof the Interior to develop and make available information concerning historicproperties. Technical Preservation Services (TPS), Heritage PreservationServices Division, National Park Service prepares standards, guidelines,and other educational materials on responsible historic preservation treatmentsto a broad public.
