Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
Margot Gayle, David Look, John Waite. Metals in America’s Historic Buildings. Washington,DC: National Park Service, .
L. William Zahner. Architectural Metals. New York: John Wiley & Sons, .
Also do a web search, review periodicals oriented to the building trades including among others: Traditional Building, Period Home; check local industrial directories; contact local preservation architects, general contractors, and metal subcontractors.
NOTE: USUALLY EVERY DIFFERENT ELEMENT TO BE RE-CASTED REQUIRES EITHER ONE PATTERN OR ONE PROTOTYPE FROM WHICH SAND MOLDS CAN BE TAKEN.
Whilst every effort has been made to make this guide as informative and detailed as possible for owners of ornamental and architectural heritage ironwork this is not a definitive resource. This guide aims to provide an outline of the material, offer practical guidance on maintenance and highlight signs to look out for which may indicate signs of deterioration in cast iron. If you are uncertain about anything concerning your architectural cast ironwork, it is always advisable to get in touch with Dorothea Restorations first, who are more than happy to discuss best practice and a potential plan of action for cast iron in need of specialised care.
Cast iron production innovations in the 18th century completely changed the face of British architecture and design. As coke replaced charcoal in the blast furnace and steam power developed in the late ’s, mass production of cast iron followed. Embellishments to buildings became affordable, fashionable and a typical feature of 19th-century architecture.
The industry peaked in the late ’s and the innovation and creative use of cast iron at the time led to its use in a range of designs, from kitchen products and ornamental architecture through to complete structures such as bandstands.
Traditional decorative architectural castings of this period were made using grey cast iron, derived from iron ore. Cast Iron constitutes around a 4% carbon content (higher than wrought iron which is <1% carbon) giving it a crystalline structure and making it brittle by nature. Where wrought iron is good in tension, cast iron is weak and excels in compressive strength (find out more here).
Unlike wrought iron, cast iron is neither ductile nor malleable and cannot be forged. Traditionally, reusable patterns were created from wood and used to press a shape into green sand. Molten iron was then poured into the void and left to cool.
The type of metal will influence both its maintenance and restoration methods. Therefore, it is important to be able to establish which metal type you are working with (cast, wrought ironwork or neither).
Since cast iron is formed by pouring its molten form into a mould, usually made of two halves, identification of cast iron is most easily done by finding a mould line. Cast objects are generally of larger sections than those produced in wrought iron by a blacksmith.
A mould can be used multiple times for particular casting designs. As such, cast ironwork tends to have identical elements with little or no variation and designs tend to have repeating sections.
Furthermore, cast iron can often be identified by the fixing methods used. Flanges, lap joints, mortise and tenon along with concealed bolts are common methods of joining castings together.
Castings may also have a manufacturer's name cast into them, which with a little research may aid identification but also start to build the historical context.
Before discussing caring for cast iron, there are some important safety notes that need attention.
Cast iron can be coated with lead-based paints; often with a 75% lead content. Lead is a very strong poison and there are serious health risks associated with it. Lead presence needs to be tested and then safety procedures must be followed when removing or disturbing lead-based paints in any way.
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Balconies, verandas, canopies, window guards, etc are potentially very dangerous. Both from working at heights perspective, and the risk of them falling on somebody below if there is a serious structural issue. This kind of maintenance works MUST be carried out by an expert.
Regular Inspections
The most important thing you can do to look after architectural cast iron is to have it periodically inspected by a qualified professional. You’ll be able to form a conditional assessment and ascertain areas where your cast iron may require intervention, if at all.
Annual inspections should check for:
Keeping cast iron clean should be part of your routine care plan. It is very easy to over-clean cast iron, causing damage to decorative elements or destroying protective coatings. That said, insufficient cleaning will reduce the longevity of the ironwork and predispose it to corrosion and deterioration.
The best approach should really be decided on an individual case basis. Historical significance, your budget and performance requirements will of course vary. Larger scale projects such as bare metal restoration and serious corrosion should only be undertaken by a professional.
Minor patch repairs can normally be undertaken by the owner but it is essential to follow some basic procedures to ensure the paintwork doesn’t fail.
Cast Iron has a good resistance to corrosion. However without proper care cast iron will still come under threat.
The main ways in which cast iron is likely to deteriorate are:
The biggest threat to cast iron is exposure to water and oxygen, together causing oxidisation (rust). Cast iron is at its most susceptible to rusting when exposed to high humidity or where the detailing of a casting forms areas likely to trap water.
Sacrificial or galvanic corrosion pertains to the deterioration of one metal to another when in direct contact in the presence of water. One metal will be nobler and the other baser (in this case cast iron).
The base metal will sacrifice material to the nobler causing deterioration. Copper, nickel and brass are examples of metals that cast iron will sacrifice to. Zinc, on the other hand, is a heavier metal than iron, meaning it will corrode instead of iron – hence its use in paint primer as a protective coating.
Due to its composition, cast iron is particularly brittle. On impact, where wrought iron will bend, cast iron will shatter. Keep in mind however, that what might appear to be a fracture on first inspection might not be. Sometimes, original casting defects can appear to look like breaks but are purely cosmetic.
Sudden thermal shock causes strain in a material by unequal expansion, either in the same material by high thermal gradients, or in interfaces between materials with different coefficients of thermal expansion.
In short, when subjected to high heats, such as fire damage, cast iron’s strength and stiffness will decline.
Now we understand the ways in which cast iron deteriorates, it’s easy to start to see external factors that can contribute to and accelerate its decay.
Corrosion isn’t always immediately visible but there are certain warning signs which might indicate your cast iron requires attention
Dependent on its use, cast ironwork will deteriorate in a range of ways. The most common problems are outlined below with a brief indication of likely repair techniques.
Missing or broken cast iron finials, collars and husks
Since these embellishments act like water traps, they are particularly susceptible to corrosion over time.
Normally, these can be replaced by matching from stock of architectural patterns, which Dorothea holds a selection of. Alternatively, a new pattern can be made to fit with the existing design.
Railing Bar Fracture
Brittle and vulnerable to fracture, impact damage is quite common with cast iron railing bars. Where the fractured pieces have survived, it may be possible to repair them through pinning (dependent on section thickness) or an alternative cold repair technique. Failing this, a replica casting can be inserted.
Thin sections of cast iron e.g. frieze panels
Corrosion to already thin sections of cast iron can lead to significant loss of surface material, and eventually perforation. Dependent on whether this thinning compromises strength or structural integrity re-profiling with epoxy based fillers may be sufficient. In the worst case scenario, re casting is likely to be necessary.
Original casting flaws
Flaws from the casting process, appearing as holes or folds in the cast iron, can be commonplace, forming water traps and inevitably accelerating the corrosion process. Regular cleaning, filling and repainting to help prevent oxidisation of the cast iron are likely courses of action.
Cast Iron, whilst not impossible, can be particularly challenging to restore. Where repair works are required, this should be done with the intention of retaining as much of the original cast metal as possible.
Restoration will not necessarily mean restoring back to original condition but stabilising and preserving the cast ironwork dependent on its age, historical significance and character.
The use of inappropriate techniques, can damage your cast iron further and lead to further long term problems, so works should always be undertaken by a recognised expert.
As a general rule we do not recommended the welding of cast iron.
The sudden input of heat can cause thermal shock to the cast iron and create more cracks and more problems. In some cases items can be welded by using a process of pre and post heating of the iron but this is best done under controlled conditions in a workshop environment.
Cold repair techniques such as reinforcing the cast iron with plates, straps, metal stitching, threaded studs and/or bonding using epoxy resin are all particularly suitable for cast iron repairs. For more details have a look at our techinical cast iron resources here.
Where it is necessary for new sections or components of cast ironwork to be replaced, replication can often be achieved. Matching castings can be obtained from particular foundries who hold certain patterns. Where this is not possible, what remains of the system can be used as reference for a new pattern to be made.
Once repaired, protection of the cast iron is imperative to ensure the corrosion process has been stabilised. Zinc rich primer paints are often used to act as a protective layer.
Click below for guides on other architectural metalwork features:
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