Grey cast iron, normally produced from pig iron, is a ferrous carbon alloy which is heated until liquefied and then poured into a mould to solidify. Grey cast iron, the most commonly used in historic architecture since it was the only type produced on an industrial scale until the 1940’s, has a specific microstructure determined by its graphite content. This structure gives the iron its grey colour when it fractures.
It has a higher carbon content (anywhere from 2 – 4%) than wrought iron or steel. This increased proportion of carbon means cast iron tends to be brittle and have less tensile strength and shock resistance than wrought iron.
Cast iron’s strong points lie in high compressive (load bearing) strength, a relatively low melting point, ease of casting and machining and a high resistance to corrosion.
Cast iron is produced from pig iron, an intermediate product from smelting iron ore with a particularly high carbon content (around 4 – 5%). Very brittle as a result, pig iron had no direct application, asides from producing wrought and cast iron.
The pig iron is smelted down in a furnace and combined with varying proportions of steel, carbon and sometimes scrap iron whilst phosphorus and sulphur are burnt out of the composition. Additional carbon and silicon are then added to desired levels and the final molten iron is fed into a pattern (casting) to solidify.
Pattern making & Casting
Patternmaking is a skilled trade and casting quality depends primarily on the original design and condition of the pattern. Casting patterns are usually made from wood, metal, wax and plaster of paris. In metalworking, mahogany wood was most commonly used before being transferred to a sand casting.
Made in inverse to the final casting, their design is particularly intricate, having to allow for shrinkage by including extra voids (risers)
Casting processes have been in use since antiquity, particularly in sculpture and weaponry, where complex shapes were required. In metal casting, green sand casting is one of the most popular, traditional and simplest techniques for creating castings. It is still employed in cast iron production to this day.
Green sand casting uses sand mixed with varying proportions of water, silica, graphite, clay, sludge and anthracite. The varying quantities effect surface finish, mould-ability and degassing of the molten metal.
The two part inverse pattern is placed in the mold and wet sand is packed around it. The sand mold contains two parts, the drag (bottom), and the cope (top). The parting line between the cope and drag allows for the mold to be opened and the pattern to be removed once the impression has been made.
A complex gating system delivers molten metal to the cavities left by the pattern. Once cooled, the casting process is complete and the component can be finished, having ridges, etc removed prior to use.
Cast iron production dates back as far as 500BC in China, where it was being used to produce statues and pagodas, but didn’t become popular in Europe until the invention of the blast furnace in the Middle Ages allowing iron to be poured and cast. It was then utilised in warfare and in the 1500’s cast iron was predominantly used to produce cannon balls (previously made from bronze).
As Abraham Darby pioneered new iron refining processes and coke replaced charcoal at his Coalbrookdale furnaces in the early 1700’s, cast iron became more popular being used mainly for the production of pots and kettles.
The invention of the reverbatory furnace and steam power innovations occurring in the late 1700’s led to the mass production of cast iron - a cheaper less labour intensive alternative to wrought iron. Embellishments to buildings became affordable, fashionable and a typical feature of 19th century architecture.
The industry peaked in the late 1800’s and its use became widespread in structural and ornamental work. Cast iron’s flexibility as a material allowed for the development of particularly ornate patterns and designs. - Often utilised in combination with wrought iron to take advantage of their combined compressive and tensile strengths.
Towards the end of the 19th century cast iron was ensued by steel. The invention of the Bessemer converter, standardisation of steel production and a series of infamous failures of structural cast iron influenced the rapid onset of cheap steel and its use in construction.
Cast iron was the first significant structural metal – primarily due to its impressive compressive strength. Its first use of this kind, was in the construction of textile mills in the late 1700’s.
The air in the mills was filled with textile fibres creating a highly flammable atmosphere, which when combined with a wooden framed structure, resulted in a disturbing number catching fire and burning to the ground.
Cast iron, considered a non-combustible material, seemed a reasonable solution to the problem and a combination of cast iron columns and wrought iron beams were used to create the framework of the mill buildings.
Ditherington Flax Mill in Shrewsbury, constructed in 1796, is recognised as the first cast iron framed building in the world – coined the grandfather of skyscrapers, despite only being five stories high.
Following the mills, many other warehouses and factory buildings were constructed from cast iron during the Industrial Revolution. The strength of cast iron columns limited the need for solid walls, contributed to fire proofing and allowed for larger and more numerous windows, while the versatility of the material enabled the mass production of lavish structures and building ornamentation
Some of the most remarkable iron-framed buildings of the 1800’s were constructed from cast iron - ideal for the support of slim glazing bars used in conservatories, glass houses and exhibition buildings.
Previous conservatories and glass houses had been fairly rudimentary in form, predominantly built form timber. The material properties of cast and wrought iron led to some of the most advanced and avant garde developments in vitreous architecture.
Cast and wrought iron allowed for the standardisation of components and the prefabrication of elements.Some of the most iconic and stunning examples include The Great Exhibition Centre at Crystal Palace and The Palm House at Kew Botanical Gardens.
Decorative and Ornamental Cast Ironwork
Since cast iron was so much cheaper to produce than wrought iron, its affordability led to an onset of decorative cast iron embellishments to structures throughout the 19th century.
Iron manufacturers in the 1800’s were producing extensive catalogues of elaborate designs and cast irons versatility led to use in a huge variety of situations; Balconies, railings, public benches, drinking fountains, public amenities, bandstands, fountains and statues.
The advent of steam power and mass production saw railways and canals spring up across Britain with great momentum in the 1800’s. 25,000 bridges were built between 1830 and 1855 and a mixture of cast and wrought iron were used widely in their construction. One of the most famous examples is Abraham Darby’s Iron Bridge in Coalbrookdale, the first arched cast iron bridge to be built in the world.
Cast Iron, whilst not impossible, can be difficult 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 normally mean stabilising its condition for future preservation of the cast ironwork dependent on its age, historical significance and character.
An effective way to remove oil, rust, loose paint and dirt warm/cold and high pressure water jetting are all techniques employed with cast ironwork. Detergents can also be added when necessary to help remove any build up of salts and oils.
Needle guns and descaling chisels can quickly and efficiently remove thick layers of paint and rust. Nominal dust is produced making it a suitable method when dealing with ironwork covered in hazardous lead based paints. Care does need to be taken however as there is a significant risk of surface damage.
Dependent on application, a variety of abrasive mediums are used in blast cleaning. Carried in a stream of high pressure air and propelled towards the ironwork. Whilst fast and effective it is also particularly aggressive. For cleaning cast iron water is often added to provide a cushioning effect particularly useful when dealing with fine and decorative cast ironwork.
This treatment involves heating the cast iron to soften the paint layer and loosen any rust. The sudden input of heat can cause thermal shock to cast iron and this method, whilst reasonably gentle, can lead to fracturing and requires an experienced professional.
Chemical Based Cleaning
Phosphoric acid based chemicals can be applied to the surface of cast iron to remove a build-up of corrosive material and old paint. Great care needs to be taken to avoid accumulation of chemicals in the micro structure of the cast iron.
Chemical baths involving a mix of acid and corrosion inhibitors can also be employed in some circumstance but require dismantling the ironwork into individual components.
Cold repair techniques such as reinforcing the cast iron with plates, straps, metal stitching or bonding using epoxy resin are all particularly suitable for cast iron repairs and the most suitable for in-situ restoration.
In some cases items can be welded or brazed by using a process of pre and post heating of the iron but this is best done under controlled conditions in a workshop environment.
Welding is generally not suitable for delicate cast iron, although sometimes appropriate in the case of heavier castings with less detailing. The casting requires preheating and post heating slowly and in a uniform manner to reduce the possibility of thermal shock and fracture.
Since brazing requires the introduction of heat this is also best done in a controlled environment by a specialist and requires careful pre and post heating. A copper alloy filler rod coated in flux is used to join pieces of cast iron together. These parts need to be cleaned prior to brazing to ensure a clean and strong joint.
A useful technique, for repairing fractures or attaching two decorative casting components together, though not suitable for thin sections. Pinning involves drilling a hole into each component and filling with a threaded pin bedded in epoxy to join them.
Plating or Bolt Repairs
Cast Iron often fractures with a clean break making bolts and plating a useful and relatively inexpensive form of repairing larger fractures. Zinc plated mild steel if normally bolted to the fracture in a flat section to join the cast iron parts together. Insulation to prevent galvanic corrosion between the repair plate and original cast iron is also required.
A traditional repair technique, metal stitching has been used for many years to repair larger pieces of cast iron - metal stitching repairs are not suitable for thin section cast iron (less than 8mm).
The process involves drilling a series of holes perpendicular to the fracture, then the areas between the holes are chiselled away and a steel “key” is inserted into the holes bridging the gap between the fractured pieces. Holes are then drilled along the crack and threaded to take special studs to further fill the crack before being painted to create a water tight seal
Two-pack epoxy adhesives are particularly suitable for non-structural repairs such as re profiling pitted cast iron and filling small gaps providing a relatively water resistant seal and helping to prevent future corrosion weak spots.
Click below for further guides on typical fabric specific conservation and restoration techniques employed:
With thanks to Geoff Wallis for the use of his material in the research and production of this article GW Conservation