Iron ore was traditionally smelted in a blast furnace, originally using charcoal then later coke and coal, although peat was used on a limited basis in some areas. Limestone was added as a flux to reduce the temperature at which the ore melted, and to assist the removal of impurities in the form of slag. The resulting iron was run from the base of the furnace or ‘tapped’ and run into open indentations in the ground known as pig beds for their fanciful resemblance to suckling pigs. The pig iron was manageable by hand and could be re-melted in a small cupola furnace* to make castings.
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The castings were originally made in open sand moulds in the ground (sometimes directly from the blast furnace) but, as the industry developed, specialist moulding boxes and moulding sands were introduced. The last remaining architectural iron founders in operation today largely use the same processes as in the 18th and 19th centuries.
The moulder prepared the ‘green sand’, so called because it was used in its raw or ‘green’ state. High quality moulding sand was highly prized, and was largely recycled in the foundry, actually becoming better with use. It contained clay particles among the quartz grains, which were hydrophilic, making it slightly sticky when damp. Fine coal dust was also added throughout the sand, which burned out as the molten metal came into contact with it, helping to take the gases away from the casting and preventing the formation of gas bubbles in the finished product.
A pattern*, usually made of wood, but sometimes of cast iron, lead or plaster, was placed on a board with a box around it, or used as a ‘loose’ pattern (not on a board but resembling the finished casting). The facing sand was finished in plumbago* which was ‘rammed’ or pressed up against the pattern, followed by successive layers of rammed sand. The pattern would then be carefully removed and the process repeated in the other half of the moulding box. When complete, the two sides of the box would be brought together and the molten cast iron taken from the cupola furnace and poured into the mould through pre-formed gates* and risers*. Once cooled, the box would be opened and the casting removed. The excess metal left by the gates and risers would be removed and the casting cleaned up.
Most architectural cast ironwork uses grey iron* for manufacture. Cast irons have varying degrees of ductility*, but all are fairly brittle. Impact resistance is minimal, although the material is excellent in compression and therefore ideal for columns.
Decorative ironwork was largely undertaken in wrought iron until the latter half of the 18th century, when cast iron became increasingly and the demand for mass-production. The evolution of architectural cast ironwork in the 19th century has its stylistic roots firmly embedded in earlier wrought iron forms, and the significant wrought ironwork of smiths such as Jean Tijou in the 17th century had a lasting influence into the 20th century.
During the first decades of the 18th century, cast iron was increasingly used in component form within wrought iron assemblies, as is the case at Chirk Castle near Llangollen, where the entrance gates and piers by the Roberts brothers erected in utilised cast iron in the gate pier bases and pediments alone. However, its use as a standalone medium in railings in particular became increasingly important. The earliest significant example of this is generally considered to be the cast iron railings installed around St Paul’s Cathedral in . Cast at Lamberhurst in Kent, the castings weighed 200 tons.
While Sir Christopher Wren disapproved of the use of cast iron to bound his building, the Arts and Crafts architect William Lethaby later praised them highly: ‘I do not see how the railings could have been better. They are heavy and rather blunt as befits the situation and the material of which they are made’. Some of these railings were removed and sold at auction in , but much of the work still survives, and it is a testament to both the design and the material that they remain in excellent condition. As Lethaby suggested, the design and execution of the ironwork at St Paul’s reflects an advanced understanding and appreciation of the material. The construction is massive, perhaps too much so, but the ironwork is assembled and jointed like carpentry; the joints are tight and neat throughout. James Gibbs used cast iron railings of a similar design for the Senate House in Cambridge in around , but unlike those at St Paul’s, here he utilised wrought iron bars inserted between the railings.
Isaac Ware’s architectural treatise A Complete Body of Architecture published in contains several plates illustrating ironwork in the form of gates and railings. Ware makes comment on the architectural use of cast iron in a decorative context: 'Cast iron is very serviceable to the builder and a vast expense is saved in many cases by using it; in rails and balusters it makes a rich and massy appearance when it has cost very little and when wrought iron, much less substantial, would cost a vast sum'. Indeed, the comparatively low production costs of cast iron compared to the labour intensive costs of wrought iron manufacture are pivotal to the rise of the material in a decorative function.
Early castings, which were relatively plain and easier to mould and cast, were replaced by increasingly ornamental and stylised designs as the potential of cast iron was realised at Coalbrookdale and then at Carron, through increasingly fine design and pattern work. This would also have required a development in pattern-making and moulding skills, as exemplified by the work of the Haworth brothers at Carron, brought from London to Scotland by the architects Robert and James Adam.
The rise of the Adam brothers was inextricably linked to the rise of the Carron company and of the Scottish ironfounding trade. Early examples of their work used wrought iron often in conjunction with other metals such as copper and brass, but the influence of cast iron gradually appears in their work towards the end of the 18th century. Details such as decorative cast iron finials, usually to classical motifs, were introduced and used in conjunction with wrought iron railings.
A Scottish example of this can be found at the tomb of James Bruce in Larbert Old Church, where remnants of the original railings enclosing the burial site survive. Erected in around , these railings were delicately forged in wrought iron, intersected by a more substantial wrought iron newel post, and topped with a decorative cast iron urn. They are now badly corroding and delaminating, nevertheless, they provide an excellent illustration of the transition period between wrought and cast iron, as the Adam brothers started to use the mass production benefits of cast iron in the finial detail, while retaining delicacy and craftsmanship in the forged bars and cope rails.
The monument to James Bruce was a large cast iron obelisk, which is itself an early Scottish example of the potential of cast iron as a decorative medium for a striking feature. Unfortunately, it has been moved from its original location adjacent to the graveyard where it sat on a masonry plinth where the family remains lie. It can now be found in the church car park, a less than dignified location for such an important monument.
The Adam family became inextricably linked with Carron in the early s when they became shareholders, with John Adam the most prominent in the affairs of the company. The elegance of their designs was carried over to railings and other architectural work.
The execution of the iron bridge at Coalbrookdale by Abraham Darby and the Coalbrookdale Company in was a significant datum in the use of cast iron as a construction material and as a decorative medium. However, the decorative work that Coalbrookdale was later to become famous for is not particularly evident on the iron bridge. The central panel was cast in an open mould (there is evidence of porosity in the panel and generally across the bridge components). The iron bridge is important less as a decorative expression in architectural ironwork, than as a pivotal statement in the versatility and use of the material.
Significant advances in the technology of smelting and working iron were intertwined with the rise of the material for decorative purposes, its potential so clearly demonstrated by the Adam brothers and Carron in Scotland. The technological advances and natural resources realised in Scotland in the late 17th and early 18th centuries were to create strong foundations for the significant architectural ironfounding industry which was to follow. World-famous names like Walter MacFarlane & Co, Lion Foundry, McDowall Steven & Co and the Sun Foundry of George Smith were established within a 30 mile radius of Glasgow. Coalbrookdale excelled in quality but never matched the range and output of those north of the border.
British firms pre-fabricated cast iron palaces, fountains, bandstands, railway stations and bridges, shipping them to the far reaches of the globe. Specifiers in India or Brazil could order from a stock pattern book, selecting a weathervane or rainwater gutter, a clock tower or urinal. The national output peaked around , although it had really started to accelerate with the drive to improve sanitary facilities starting in the s. While catalogues showed high quality ornamental work, most firms relied on the manufacture of sanitary ware to build their business.
The taste for ornamental cast ironwork shifted in the Edwardian period, with most firms responding with more subtle art nouveau stylings. The advent of both wars impacted heavily on the industry, with a loss of customers and skilled labour as foundries shifted towards war work.
The post-war housing boom provided a temporary respite in the manufacture of sanitary ware and baths in particular, but this too was short lived as pressure from overseas imports started to bite. A handful of firms embraced the shift to using cast iron as a constructional medium in building facades, and wonderful examples remain largely unknown in our towns and cities. Selfridges (Oxford Street, London) and Unilever House (Victoria Embankment, London) used large amounts of cast iron, and Burtons the tailors used cast iron extensively in its shop fronts in the post-war period.
After a series of often acrimonious amalgamations and takeovers alongside a general decline saw the sad demise of a once formidable industry. Some smaller firms did survive and, ironically, saw a resurgence of conservation and restoration work in the s re-instating ironwork removed for the war effort or repairing the profusion of Victorian cast ironwork in our public parks. The resurgence was principally driven by the Heritage Lottery Fund Urban Parks Programme. The handful of surviving firms remains under increasing pressure on various fronts. To retain them we need to use them, and to understand and appreciate the high quality that ornamental cast ironwork can achieve.
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This set of guidelines provides general information on the characteristics and common uses of cast iron and identifies typical problems associated with the material. See also: “Checklist for Inspecting Cast Iron Failures”.
Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. It is primarily composed of iron (Fe), carbon (C) and silicon (Si), but may also contain traces of sulphur (S), manganese (Mn) and phosphorus (P). It has a relatively high carbon content of 2% to 5%. It is hard, brittle, nonmalleable (i.e. it cannot be bent, stretched or hammered into shape) and more fusible than steel. Its structure is crystalline and it fractures under excessive tensile loading with little prior distortion. Cast iron is, however, very good in compression. The composition of cast iron and the method of manufacture are critical in determining its characteristics.
The most common traditional form is grey cast iron. Common or grey cast iron is easily cast but it cannot be forged or worked mechanically, either hot or cold.
In grey cast iron, the carbon content is in the form of flakes distributed throughout the metal. In white cast iron, the carbon content is combined chemically as carbide of iron. White cast iron has superior tensile strength and malleability. It is also known as ‘malleable’ or ‘spheroidal graphite’ iron.
Cast iron is still manufactured by much the same process as it was produced historically. Iron ore is heated in a blast furnace with coke and limestone. This process “deoxidizes” the ore and drives off impurities, producing molten iron. The molten iron is poured into molds of the desired shape and allowed to cool and crystallize.
Upon manufacture, cast iron develops a protective film or scale on the surface which makes it initially more resistant to corrosion than wrought iron or mild steel. Finishing may include bituminous coatings, waxes, paints, galvanizing and plating. In addition, there are a variety of treatments that can reduce rusting and corrosion caused by environmental factors. Factory preservative treatments are typically barrier coatings intended to prevent the castings from oxidizing (rusting) in the presence of humidity and oxygen in the air.
Margot Gayle, David W. Look, John G. Waite. Metals in America’s Historic Buildings. Washington, DC: National Park Service, . (USGPO -332-360)
L. William Zahner. Architectural Metals: A Guide to Selection, Specification, and Performance. New York City: John Wiley & Sons, .
Cast iron is used in a wide variety of structural and decorative applications, because it is relatively inexpensive, durable and easily cast into a variety of shapes. Most of the typical uses include:
Historic markers and plaques
Hardware: hinges, latches
Columns, balusters
Stairs
Structural connectors in buildings and monuments
Decorative features
Fences
Tools and utensils
Ordnance
Stoves and firebacks
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Piping
The basic cast iron material in all of these applications may appear to be the same, or very similar. However, the component size, composition, use, condition, relationship to adjacent materials, exposure and other factors may dictate that different treatments be used to correct similar problems. Any material in question should be evaluated as a part of a larger system and treatment plans should be based upon consideration of all relevant factors.
Cast iron is extremely strong and durable when used appropriately and protected from adverse exposure. It is much stronger in compression than in tension, therefore it is commonly found in columns, but not in structural beams. It is, however, highly susceptible to corrosion (rusting) when exposed to moisture and it has several typical problems which usually can be identified by visual inspection. The following sections will identify and discuss the most common problems encountered with cast iron. For general guidance on inspecting for cast iron failures, see -01-G.
The typical deterioration or corrosion process for cast iron is a one-step straight line process of oxidation (or rusting) which begins on exposure to air and moisture and will continue (unless interrupted) until the metal is gone. This process is described in the following section.
Rusting, or oxidation, is the most frequent and easily recognizable form of cast iron deterioration. Cast iron is highly susceptible to rusting when the humidity is higher than 65%. Iron (Fe) combines with oxygen (O) in the presence of water vapor (H2O) to become rust (Fe2O3). This process can take place at significantly different rates depending on the material composition, protective treatments applied and severity of exposure. If rusting occurs at a rapid rate, it can result in severe damage or total loss of a component in a short time; therefore, the presence of any rust on a cast iron artifact should alert the observer to the presence of a serious problem. Rusting can occur when the humidity is as low as 58% in the presence of certain pollutants, especially sulfur dioxide, ammonia sulfates or even the presence of body oils from touching. Reducing the humidity to 30% or below has been found to be effective in preventing rusting, however this is not a practical solution for outdoor cast iron.
Rusting is such a common problem that it is quite easily recognizable. Rust (Ferrous Oxide, Fe2O3, and Ferric Oxide, Fe3O4) is an orange colored surface coating, ranging in texture from scaly to powdery. It is loosely bound and the outer layers will usually come off when rubbed by hand or brushed against. It is not a deposit on the surface. Rust is the result of the combination of the iron (Fe) with oxygen (O) in the air, in the presence of moisture. The presence of rust means that some original iron material has been converted to iron oxide and irreversibly lost from the cast iron piece.
The probability of rust occurring is generally dependent upon two factors:
The degree of protection (usually a protective coating) provided to keep moisture from contact with the metal, and
The degree of moisture present in the air.
Protective coatings used on iron include bituminous coatings (such as tars), waxes, paints and sophisticated metallic coatings. Effective coatings, well maintained, provide the most reliable protection against rust and corrosion of cast iron, however, there are a wide variety of coatings available, and these can be confusing to users not thoroughly versed in the technical data for each type.
Humidity is the second factor affecting the rate of oxidation (rusting) of iron. It is generally accepted that rusting cannot begin unless the relative humidity is at or above 65% (this figure can be lower, however, in the presence of pollutants). Relative humidity is, however, not the only factor to be considered. Once rusting has started, at least two other phenomena may occur:
Some rust or ferrous oxide can become hydrated, i.e. it can contain moisture within its chemical structure, thereby exposing the iron to additional moisture, and
The porous rust may act as a reservoir for liquid water, keeping it in contact with the iron and perpetuating the rusting process.
Both of these conditions are microscopic in nature and invisible to casual inspection. Maintenance staff and trained personnel, however, should be aware of the processes, and the potential for the processes to damage the cast iron. The presence of visible rust is the symptom indicating that a problem exists. Appropriate action should be taken to prevent rusting, and where it does occur, to correct it with an appropriate treatment. See individual repair or preventive maintenance procedures for specific guidance as needed.
Many other factors can affect both corrosion and the rate of corrosion. Sea water, salt air, cements, plasters, ashes, sulphur, soils and acids can accelerate the corrosion of iron. Corrosion rates can also be accelerated where the detailing of the cast iron provides pockets which can collect and hold moisture and corrosive agents. Preventive maintenance plans should consider detailing, such as crevices and recessed areas, in establishing routine inspection techniques and frequency of inspection.
Cast iron contains carbon, in the form of graphite, in its molecular structure. It is composed of a crystalline structure as are all metals; i.e. it is a heterogeneous mass of crystals of its major elements (Iron, Manganese, Carbon, Sulphur and Silicon). One condition which can occur in the presence of acid rain and/or sea water is “graphitization.” The stable graphite crystals remain in place, but the less stable iron becomes converted to insoluble iron oxide (rust). The result is that the cast iron piece retains its shape and appearance but becomes weaker mechanically because of the loss of iron. Graphitization is not, however, a common problem. It generally will occur only after bare metal is left exposed for extended periods, or where failed joints allow the penetration of acidic rainwater to interior surfaces.
This corrosion process is galvanic, with the carbon present acting as the most noble (least corrosive) element and the iron acting as the least noble (most corrosive) element. The composition or microstructure of the iron affects the durability of the object because the rate of corrosion is dependent upon the amount and structure of the graphite present in the iron.
Barrier coatings are the most commonly used protective mechanisms for cast iron. Some type of coating (such as a wax, paint or metallic coating) should probably be considered an integral feature of cast iron in service. The absence of such a coating, or a failure in an existing coating should be corrected. Inspection should include a visual examination of all surfaces to determine if a coating exists, a fact which may be very apparent for opaque paints and coatings but substantially less apparent for clear lacquers, waxes or oils. Surfaces having the appearance of raw metal should be carefully examined for signs of rusting. Absence of a coating should be considered a major problem and corrective action should be undertaken. See individual repair or preventive maintenance procedures for specific guidance as needed.
Failure of a coating should also be identified and corrected. Coatings can wear away, crack, flake, blister, or peel away, indicating that the coating has failed and is no longer protecting the cast iron from moisture. Failed coatings can, in fact, trap moisture beneath the film and accelerate corrosion at certain points on the surface. Inspection of the surface should include a careful check for all of these types of coating failures. A record should be made of any coating failures observed so that corrective action may be taken.
Mechanical failures of cast iron are typically of two types and are relatively common problems.
Cast iron may contain various imperfections due to the manufacturing process. These may occur due to air holes, interrupted pouring, uneven cooling (cold sheets), cracks and cinders. Where such imperfections occur, the piece may be weakened mechanically, sometimes severely. These manufacturing problems are not generally visible upon inspection; however, there are several non-destructive techniques of identifying these types of problems, such as the use of fluorescent fluids and ultraviolet lamps, or x-ray. These non- destructive techniques require specialized knowledge and equipment, and are not generally feasible for use by maintenance staff. They should be undertaken by specialists with experience.
Visible inspection may, however, enable detection of mechanical failures after the failure has occurred or begun to occur. Stress cracks in paint or metal may be symptomatic of this problem. Failures may begin as gradual separations which are visible upon inspection, and may be detected and corrected prior to a total, catastrophic failure of the piece. Linear cracks in paint film or metal should be investigated and/or monitored to determine if they are active. Non-destructive techniques may be used if symptoms exist, but the Regional Historic Preservation Officer (RHPO) should be consulted in the solicitation of professionals who are experienced in use of these techniques.
Larger cast iron pieces are generally systems composed of smaller castings, mechanically connected. This can even be the case for a simple baluster or historical marker. One of the most common failures that occurs with such systems is the failure of the connectors or joints. Loose, missing or broken screws, clamps or bolts may result in loose, failed or missing components. Visual inspection should include examination of cast iron pieces for sections which are loose and/or disoriented, and which have loose or missing screws or bolts. Further manipulation by hand, with probes, may indicate whether a casting is a discrete piece, mechanically attached, and whether or not it is in the early stages of working loose. It is especially important to detect connectors which are in danger of imminent failure if not corrected. Corrective action should be undertaken in either case, but the treatment plan should take into account the severity of the problem, consequences of failure and nature of the intervention required to correct the problem. See individual repair procedures for specific guidance as needed.
Another mechanical problem can be caused by inappropriate mechanical repairs to broken pieces. Some repairs may create openings that allow water penetration and “pockets” that collect water, both of which can cause problems. Castings which have been filled with concrete are also a potential problem since they may promote “crevice corrosion” due to entrapped water. Visual inspections should check for such conditions and where they exist, maintenance staff should plan to correct the problems and/or be vigilant for signs of deterioration.
Cast iron problems, especially corrosion problems, may be reduced or eliminated in cast iron that is an alloy of silicon, nickel, chromium and/or copper. For example, silicon is often present in cast iron to some degree, but it is not considered an alloy until the percentage exceeds the 3% upper range of non-alloy cast iron. Where silicon is present, a protective surface film develops during oxidation.
There are three main categories of cast iron alloys:
High silicon
High chromium
High nickel (frequently containing copper or chromium)
All of these alloys, plus copper alloys, have been tested and found to have increased corrosion resistance. The degree of increased resistance is dependent on many factors, primarily the alloying metal and the percentage of alloy relative to the carbon content of the cast iron. While a discussion of alloy durability and formulation is beyond the scope of this standard, users should be aware of the effect of alloying and consider the implications when ordering new cast iron replacement objects. Such consideration may involve experienced metallurgists, foundrymen, conservators, and historical architects.
The maintenance principles for cast iron are, in order of appearance:
Prevent rust and corrosion.
Paint and plug holes.
Maintain structural soundness.
keep it together with binding and bolts, welding, etc., and brace loose elements by resetting.
Recreate missing pieces using casting replacement parts (iron, aluminum, fiberglass, or epoxy), or wooden replacements, with appropriate composition and/or coatings to provide for color blending.
Cast iron requires continual maintenance. Check periodically for water collection spots and dry as necessary. Signs of corrosion are when rusty looking stain marks appear on the metal. If these areas are rubbed the metal surface is revealed as well as traces of perforation. Check for small chips in the coating surface and peeling of the coating surface.
Replace or repair as necessary if the damage is minimal missing and deteriorated pieces of metal prior to cleaning. If deteriorated condition is left unrepaired, perforation of the metal will occur and as a result structural failure.
Structural iron maintenance may require the services of a structural engineer when severe erosion or distortion occurs, to assist in the development of repair techniques when material loss is involved. For these repairs use only a professional iron worker. Before installation of new material verify the metal type and thickness. Prior to installation, remove all oil, dirt, and other debris from the surface. All surfaces shall be dry and free from frost.
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