Whilst every effort has been made to make this guide as informative and detailed as possible for owners of ornamental and architectural cast iron this is not a definitive resource.
This guide aims to provide an outline to the material, offer practical guidance on maintenance and highlight signs to look out for which may indicate deterioration.
If you are uncertain about anything concerning your architectural cast iron, it is always advised to get in touch with Dorothea Restorations first, who are more than happy to discuss best practise 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 developments occurred in the late 1700’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 1800’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.
Identifying Cast Iron
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 section 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 manufacturers name cast into them, which with a little research may aid identification but also start to build the historical context.
Care and Maintenance
Before discussing caring for cast iron, there are some important safety notes that need attention.
SAFETY NOTE 1
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 anyway.
SAFETY NOTE 2
Balconies, verandas, canopies, window guards, etc are potentially very dangerous. Both from a working at heights perspective, and the risk of them falling on somebody below if there is a serious structural issue. These kind of maintenance works MUST be carried out by an expert.
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:
- Damage to paintwork
- Corrosion to components fixed into masonry
- Corrosion to underside surfaces of cast ironwork
- Corrosion to joints, fixings and other potential water accumulation points
- Signs of movement or instability in either the cast iron or supporting masonry
Keep Ironwork Clean
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.
Professional cleaning methods will depend predominantly on the level of corrosion, budget constraints, age and historical context or significance of your cast iron.
Accumulated dirt can be removed by hand using water, or water with a little detergent, and a cloth or bristle brush. It’s important not to use excessive amount of water, for instance, a high pressure power hose.
Cast iron should also be thoroughly dried after cleaning to avoid trapping water.
Very small bits of corrosion can be removed using a chisel or bronze wire brush and sandpaper before repainting. Take care not to damage the ironwork and don’t use mechanical tools.
Some levels of cleaning might not be suitable for certain historic cast iron, so if you are in any doubt as to cleaning methods contact a professional.
Painting Cast Ironwork
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.
- Remember safety note 1 – The existing paint is likely to be lead based and very hazardous to breathe in. There are some useful safety guidelines available here
- Surface preparation is key
- Clean, dry and well prepared surfaces mean the paint will adhere and protect your ironwork efficiently
- Care should be taken not to paint on heavily-rusted sections of cast iron as this can create air/moisture pockets which can increase corrosion rates.
- Check your paint is compatible with the existing paintwork and your ironwork. Most manufacturers will have a specified paint system you can use.
- When patching repairs, be careful to form a good overlap with the existing paintwork
- If painting over existing paint, roughen the surface by gently rubbing with sandpaper to ensure the paint has a good surface to adhere to
- Wait for optimum weather conditions for external works. Cold weather conditions (less than 5 degrees normally) can stop paint curing and a breeze can result in debris in your paintwork ruining the finish.
- Apply in several thin coats and always allow each coat to dry completely before applying another
Types of Decay, Deterioration and Corrosion
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:
- Galvanic or Sacrificial Corrosion
- Chemical Corrosion (rust)
- Mechanical or Thermal Shock
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.
Contributing Factors to Deterioration
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.
- Vegetation growth
- Accumulation of dirt and debris
- Flaking paint leaving ironwork exposed
- Inadequate paint layers and/or protective coating or inefficient preparatory methods applied
- Bad casting and/or design flaws – air pockets or interrupted pouring into moulds can lead to weak areas more inclined to fracture
- General Neglect
Signs of Cast Iron in Need of Intervention
Corrosion isn’t always immediately visible but there are certain warning signs which might indicate your cast iron requires attention
- An uneven surface to the ironwork termed “pitting”. This is often a sign of corrosion beneath paintwork
- Rusty coloured staining to supporting masonry
- Blistering to paintwork – another sign to corrosion beneath
- An oily residue on the paintwork surface
Typical Issues with Cast Iron
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.
Typical Restoration and Repair Techniques
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:
- Maintaining wrought ironwork
- Cast iron rainwater system care
- Maintaining metal casement windows
- Traditional leaded light maintenance
Case study: The British Museum Cast Iron Gates and Railings View All