Preservation by Prevention: Iron Fences and Railings

Hammering a glowing bar of iron

Hammering a glowing bar of iron

One of my favorite features about the building that houses the Trust’s Washington office is the historic iron fence around the front yard. Iron fencing comes in two types, cast-iron and wrought-iron. Wrought iron is an older technology than cast iron in the west. Though the Chinese had developed the technology to cast iron as early as the 3rd century BCE, cast iron was not found in Europe until the Medieval period. Both types of metal are used in historic American fencing, often in combination, with cast-iron posts supporting wrought-iron panels. Iron fences were used in urban areas to separate small front yards from the sidewalk. They were also commonly used in cemeteries to surround family burial plots. Iron railings were equally ubiquitous in certain cities, found as hand rails along stoop sides and as balustrades along porch and balcony edges.

Wrought Iron

Wrought iron is almost pure iron, with a carbon content of less than 1 percent. Blacksmiths form wrought iron by a repeated process of applying heat and subsequently hammering an iron bar. This process aligns the iron crystals along the length of the piece and results in a metal with a higher tensile strength – resistance to pulling forces – than cast iron. Because they are formed by hand, wrought-iron pieces are often simply designed and slightly irregular. Evidence of tooling can distinguish wrought iron as well. Wrought-iron pieces are typically held together with welds or rivets.

Cast Iron

Cast iron is less pure than wrought iron, with a carbon content of at least 1.7 percent, but ranging as high as 4 percent. This makes a much harder metal than wrought iron, but it also makes cast iron extremely brittle. Unlike wrought iron, cast iron is very weak in tension and can break without warning when subjected to tensile stress. But, it is extremely strong in compression – when subjected to a crushing force – and was commonly used to manufacture support columns and other load-bearing elements. Thin, cast-iron columns made their first appearance in the U.S. during the 1820s as supports for church balconies and the like. Cast-iron storefronts and facades came later in the 19th century, spreading quickly across the country, until cast iron was supplanted by steel around the turn of the 20th century.
Cast-iron architectural elements are formed by pouring molten iron at a temperature of 2700 degrees Fahrenheit into a sand mold. The cast iron is allowed to cool and harden, after which the mold is removed. Sand molds are formed by pressing a pattern – often carved wood or plaster – into wet sand that has been packed into a form. When the pattern is removed, it leaves a void that will be filled with molten iron. For three-dimensional pieces, two molds are joined to form a single unit whose void is filled by pouring molten iron into tunnels that empty into it. Marks from these tunnels are machined off after the piece is cast. Elements with a single molded side are formed in open-backed molds. Cast pieces are bolted together to form repetitive patterns, such as fencing, or, in the case of large elements like columns, to form complete three-dimensional shapes. Cast iron is distinguished by its mold marks and repetitive patterns.

Maintenance and Repair

Cast-iron fencing with signs of oxidation, photo by Adam Ward

Cast-iron fencing with signs of oxidation, photo by Adam Ward

Iron oxidizes, or rusts, quickly when exposed to air and moisture. When pollutants are present, the amount of moisture in the air required to begin oxidation is lowered, meaning that proper care of ironwork in urban environments is especially critical.

To protect ironwork from the elements and prevent oxidation the iron should be painted with a good primer and top coat. After it is exposed to the air, bare ironwork will begin to oxidize anywhere from minutes to hours. Historically, a red-colored, lead-based paint known as red lead was used as a primer with oil-based top coats. Because lead paints are now outlawed, alkyd rust-inhibitive primers and alkyd finish coats are commonly used instead. Latex and water-based paints are not suitable as primers because they will cause oxidation of the underlying iron. There is some danger in using water-based paints as top coats, as well, because they can cause oxidation if there is a flaw in the primer coat, which can allow direct contact between the latex and the metal. Rust converters, which turn any iron oxide on the metal into stable iron tannate or iron phosphate, may be used under the primer coat if oxidation cannot be fully removed prior to painting.
Minor oxidation can be removed from ironwork on site, although significant amounts of rust are often most easily removed off-site by dipping the pieces in dilute phosphoric or sulfuric acid. To remove loose paint and oxidation on-site, use a fine wire brush and hand scrapers. Heavy wire brushes or very aggressive scraping can gouge and damage details, so this must be done with extreme care. If the entire fence is made of cast iron, rather than a mix of cast and wrought irons, low-pressure sand blasting (not to exceed 100 pounds per square inch) may be the most effective method of removing deteriorated paint and oxidation on-site. In addition, alkaline compounds like methylene chloride or potassium hydroxide can be effective paint removal alternatives to abrasive blasting.
If your historic ironwork has structural problems, the first step is to check all connections. Cast-iron elements will be bolted together, and you may simply need to tighten the bolts. If bolts are missing or are rusted, or the bolt holes are rusted, more work will be necessary to ensure tight connections. Missing hardware should be replaced with iron or stainless-steel bolts of the same size and style. Rusted hardware can be drilled out using a bit smaller than the bolt’s diameter. When necessary, the bolt hole can be re-tapped and fitted with new hardware. Wrought-iron pieces that are out-of-shape can be carefully bent into place, sometimes with the assistance of heat. Breaks in wrought iron must be welded, a process usually best left to a professional. Similarly, repairs to broken pieces of cast iron are best left to a professional as they can easily be damaged.
Joints between elements of cast iron should be closed to ensure a watertight seal. This prevents water from entering the composition, which can either cause rust from the inside out or freeze and crack the cast iron. Caulk and fillers are used for this purpose. Fillers should have iron particles in them to ensure that they thermally expand and contract in a manner similar to the ironwork.
Iron fencing and railings are significant components of historic landscapes and buildings. Building owners should periodically inspect and complete minor repairs to keep these features in good condition. However, if major repairs are needed, it is highly recommended to consult with a historic ironwork specialist. Many of the advanced techniques for restoring ironwork are damaging, either to the ironwork or to your health, if not conducted properly.

Sources

Church, Jason. Basics in Iron Fencing Care: Participant Guide. National Center for Preservation Technology and Training. (Accessed September 15, 2009).
Church, Jason. “Iron Fence Repair Video (2007-03) Transcript.” National Center for Preservation Technology and Training. (Accessed September 15, 2009).
Gayle, Carol and Margot Gayle. “The Emergence of Cast-Iron Architecture in the United States: Defining the Role of James Bogardus.” APT Bulletin 29, no. 2 (1998): 5-12.  (Accessed September 14, 2009).
Hawkes, Pamela W. “Paints for Architectural Cast Iron.” APT Bulletin 11, no. 1 (1979): 17-36. (Accessed September 14, 2009).
Howell, J. Scott.  “Architectural Cast Iron: Design and Restoration.” APT Bulletin 19, no. 3 (1987): 51-55. (Accessed September 14, 2009).
Howell, J. Scott and Jim Lauder.  “Letter to Editor: Re: Reproducing Cast Iron Balustrades.” APT Bulletin12, no. 3 (1980):  5-7. (Accessed September 14, 2009).
New York Landmarks Conservancy. Historic Building Facades: The Manual for Maintenance and Rehabilitation. New York: John Wiley & Sons, 1997.
Waite, John G. “Preservation Brief 27: The Maintenance and Repair of Architectural Cast Iron.” National Park Service. (accessed September 14, 2009).
Young, Robert A. Historic Preservation Technology. Hoboken, New Jersey: John Wiley & Sons, 2008.

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