Wrought Iron and Steel Windows

Eleni Makri


  The distinctive black vitrolite facade of the Daily Express building, London  
  The Daily Express Building in Fleet Street, London (Ellis and Clarke, 1932) (Photo: Carole King)   

Historic iron window frames were produced in wrought iron, cast iron or mild steel. This article concentrates on wrought iron and mild steel windows and their conservation.

Wrought iron is the purest form of iron used in construction, containing between one and four per cent impurities and less than one per cent carbon. It is fibrous and malleable and can be welded. Mild steel is an iron-carbon alloy containing up to about two per cent carbon and has qualities similar to wrought iron.

Left unprotected, iron corrodes back to its original state (iron oxide). Wrought iron is the most resistant to this process and mild steel the most susceptible.

Wrought iron is either charcoal iron or its successor after 1784, puddled iron. Large scale production in the UK came to an end in 1973 with the closure of Thomas Walmsley’s Atlas Forge in Bolton, Lancashire. Shortly afterwards the forge was rebuilt at Blists Hill Open Air Museum in Ironbridge where it resumed production for a few days a year.

Mild steel was a new iron-carbon alloy first produced in 1855 by Henry Bessemer in his Bessemer converter in an effort to reduce the production cost of wrought iron. Continuous advances in its production led to it replacing wrought iron and the establishment of the modern steel industry.


Wrought iron fenestration evolved from medieval window construction and from ecclesiastical stained-glass window construction in particular.

The 12-light Armada window of Sutton House, Hackney, London, dating from the early 16th century is a rare complete survival of early domestic fenestration. Typically for an early domestic window, all 12 lights are fixed.

The wrought iron opening casement appeared in the late 16th to early 17th century, initially as a single element which was less than the full height of the opening and set in predominantly fixed fenestration. In the next century both the number and size of opening casements increased and they now occupied the full height of the opening.

Changes in construction methods and materials (from timber-frame to masonry, brick or stone) saw the introduction in the early 17th century of the classical four-light cross-window. An example with wrought iron fenestration is a 1707 cross-window with three fixed lights and an opening wrought iron casement at a farmhouse in Pilning, Gloucestershire.

The late 17th century former manor house in Freckenham, Suffolk, retains original wrought iron casement windows with rectangular leaded lights. Some of the leaded lights contain crown glass, which became available in the late 17th century. While the counter-balanced timber sash window gradually became the fashionable standard for the houses of the wealthy following its use at Chatsworth in 1676, the wrought iron casement remained in use throughout the 18th century. In the 19th century wrought iron casements were sometimes used in attics and service rooms. In the late 19th and early 20th century, the wrought iron window became popular again through the Gothic Revival and in the work of Arts and Crafts architects such as Edwin Lutyens. However, in the 1850s wrought iron production was considered expensive and labour intensive and this gave rise to the production of metal windows (both sashes and casements) in cast iron.

By the late 19th century the development of the Bessemer process had enabled the production of sections from hot-rolled steel which were considerably cheaper than those made from wrought iron. At first, steel windows sought to replicate earlier wrought iron fenestration, as is evident from the early mild steel casements of Henry Hope & Sons Ltd and WF Crittall in the Brooking Collection at Cranleigh in Surrey. Among these, the 1891 casement by Hope is a new metal casement window in its own right. The 1909-10 example is encased in a thick timber frame and comprises a fixed light and an opening light very much in the tradition of the historic cross-casement.

  Early casement window with leaded panes and elaborate metal latch terminating in spiral form Black-painted iron window in heavily descayed timber frame Factory-made window
  Inward opening casement, mid 17th century
(Brooking Collection)
17th-century wrought iron casement window with oak frame from vernacular farm building near Winchester, Hampshire (Brooking Collection) Early factory-made window constructed from wrought iron sections and incorporating rubber draught-proofing (Brooking Collection)

Following the introduction of standard window sections, often referred to as the ‘universal suite’, in 1918-20 by the newly formed Steel Windows Association (1918-23), the use of steel windows flourished. They were suitable for a range of architectural applications in the inter-war years and beyond, both in Britain and internationally. By 1954, Crittall was the biggest of the three main suppliers of steel windows, accounting for 40 per cent of production. The other two were Henry Hope & Sons and Williams & Williams, who jointly accounted for 25 per cent of production. This may explain why Crittall became synonymous with 20th century steel windows.

In revivalist examples of the inter-war period, such as at The Fox public house at Bix, Oxfordshire, universal suite steel sections were combined with timber frames and leaded glazing to produce strip and oriel neo-Tudor windows for a neo-Tudor architectural idiom. The fenestration of The Railway Tavern in Crouch End, London was constructed in a similar fashion. The possibilities for revivalist expression can be seen in the ground floor fenestration of Elizabeth House in Highgate, London (Richardson and Gill, 1930), and in particular in the Crittall French doors and combined fixed side lights, which all contribute to the building’s neo-Georgian idiom.

Significantly, this is also the window of the Art Deco movement and its varied and widespread use can be seen in examples such as the Arnos Grove (1932-34) and Turnpike Lane (1932) Underground stations (both by Charles Holden), the Hoover Building (Wallace Gilbert and Partners, 1935) and many public swimming pools. These buildings demonstrate yet another trend associated with steel windows – the development of a new colour range for the 1930s.

The potential for architectural expression provided by the steel window is also seen in structures which used more advanced building technologies such as the Boots D10 Factory in Nottingham (Sir E Owen Williams, 1931) and the Daily Express offices in London (Ellis and Clarke, 1932). Both of these buildings are clad concrete frames in which the envelope of the building, including the fenestration, is a lightweight system independent of the structural frame.

The steel window also became standard in the domestic buildings of the inter-war period, which otherwise continued to be built in the existing tradition of terraces, semi-detached houses and mansion flats. Notably, it was also Frank Lloyd Wright’s window of choice at Fallingwater in the United States (1935-39), with the frames painted in Cherokee red.


The conservation of the wrought iron window may involve repairs to the sub-frame on which the casement hangs (if it is metal), repairs to the casement frame including the window furniture, or repairs to the leaded glazing.

When repairs are being considered, an informed decision should first be taken on whether surviving glazing (lead cames and glass quarries) can be adequately protected during the works or whether it should be removed until repairs are complete. If repairs are required to the casement, for example, it is unlikely that removal of the glazing can be avoided, whereas for repairs to the sub-frame alone, the casement can be removed and safely stored to be re-hung later. Any work involving leaded glazing, including its protection, is highly specialised and requires the right skills to avoid loss or damage.

  Casement window (frame only) Crittall window with leaded panes of textured glass Badly rusted metal frame with pierced panel below top-hung light
  Factory-made steel casement window in the Arts and Crafts tradition from a large house in East Sussex, c1895 (Brooking Collection) Early 20th-century Crittall window from the 1904 alterations to Horton Priory House (1861), Monks Heath, Kent (Brooking Collection) Small top-hung light with vent panel in lower light made by Henry Hope of Birmingham from the Midland Hotel,
Morecambe, c1932-33 (Brooking Collection)

If repairs are necessary, the surface of the wrought iron sections should be cleaned back to healthy metal, primed, repaired and finally painted in the following sequence of works:

Degreasing Oil or grease should be completely removed from the metal surface using scraping tools, washing with warm water and detergent, then rinsing off with warm water. Non-caustic degreasing agents such as white spirit followed by clean swabs can also be used.

Surface preparation Paint should be tested to determine whether it is leadbased. If so, appropriate precautions should be taken during its removal. It may also be important to establish and identify original paint layers, which can be done through carefully taken scrapes.

Removing paint and rust from wrought iron is best achieved by heating, which causes the metal to expand and breaks down the adhesion of the rust, allowing it to be removed with a wire brush. This method does not affect the mill scale – the outer surface of the iron formed in the forge. Evidence suggests that this is a protective surface and its removal may accelerate decay (grit blasting of wrought ironwork is therefore inappropriate). Heating can cause thin wrought iron sections (less than 2mm thick) to warp. Heat travelling horizontally is a fire hazard and if the flame is traversed too slowly, debris can become fused to the surface. All these problems can be avoided if the window is dismantled and repaired in the workshop.

Other means of surface preparation include acid pickling (off-site), preferably in warm diluted phosphoric acid which forms a protective layer of phosphates on the metal surface, followed by thorough rinsing. Cleaning with hand-held tools only removes around 30 per cent of rust but is useful for in situ work as a preparatory stage.

Priming Once cleaned, the material must be immediately protected from rust with an appropriate primer or rust inhibitor, which should delay the formation of rust for around 24 hours.

Assessment It is now possible to fully assess the extent of necessary repairs. This may involve cutting out the corroded section (often the bottom rail or associated jamb ends) and welding in place a replacement section of the same material and profile.

Repair Welding wrought ironwork is also best done in the forge because of the laminated nature of the material. In situ gas welding (brazing) or arc welding is possible, but care should be taken that the welds extend to the full depth of the material to ensure all the laminated elements of the original are connected, as surface welds have no strength. Gas welding (brazing) is preferable as arc welding requires the use of a non-corrodible iron alloy wire or rod (mild steel is unsuitable as it will corrode).

Wrought iron is available for restoration work, primarily through the recycling of old material. Sources of charcoal iron are rather limited, but there are large quantities of 19th century puddled iron available from dismantled structures, which can be re-forged. Repairs to both charcoal and puddled iron can be made using reclaimed puddled iron as the two materials are quite similar.

While mild steel has been used for repairs to wrought iron, this is no longer considered good practice, partly because of the much greater susceptibility of steel to corrosion. It is also well-established that repairs are best carried out using original material and techniques. This is both possible and appropriate in the case of wrought iron fenestration.

Painting Because of wrought iron’s natural corrosion-resistance it is sufficient to protect it with primer and paint. When choosing a protective paint system it is best to consult a reputable manufacturer who can advise on the compatibility of new and old paints and on the best system for the given conditions. Upgrading protection should be achieved by increasing the number of coats rather than using a more expensive system. Two coats of primer and four coats of airdrying paint should be adequate.

Dismantling and re-housing Dismantling and removing the sub-frame to a workshop can disturb the surrounding masonry, although historically fixings were usually filled (or ‘caulked’) with molten lead. This can be removed by heating the lead until it melts. If dismantling is necessary, the frame should be removed with minimum disturbance to the housing and surrounding fabric.

Re-housing into masonry requires abrasive cleaning to completely remove all corrosion to fixing lugs prior to painting with epoxy paints and fixing or caulking with lead or lead wool packing. Severely corroded lugs can be replaced with new wrought iron sections.


  Neighbouring terraced houses, one retaining original metal windows which follow the curve of the bay, the other with modern uPVC replacement windows  
  1930s terrace, Muswell Hill, London  

Steel window conservation began with the listing of inter-war buildings in the 1970s and 80s. The resultant demand has given a tremendous boost to the steel window industry, which has witnessed the emergence of new manufacturers and suppliers in recent years who have been instrumental in establishing and implementing good conservation practice.

After the introduction of the ‘universal suites’ in 1918-20, most steel windows were made of sections that had the same technical specifications, regardless of manufacturer. However, these original interwar sections are no longer in production so splicing repairs, while technically possible, are largely reliant on the availability of salvaged windows of the period. Another option is bespoke fabrication but this is expensive. The adjustment of currently produced sections is sometimes possible.

Produced before the introduction of galvanising in the 1950s, inter-war steel windows are prone to rust and their condition depends on how well they have been maintained. Rust is iron oxide formed by the reaction of iron with water and oxygen. Left unchecked, rust forming where paintwork has failed will spread beneath the paint by electrochemical reaction as the areas underneath the paint have less access to oxygen and are therefore differently charged.

The repair of inter-war steel windows may involve some or all of the following works:

In situ repairs
Degreasing and removal of debris and dust This should be carried out as part of the regular maintenance of steel windows and in preparation for any repairs. Degreasing can be carried out using the same methods as for wrought iron windows (see above). The removal of debris and dust can be assisted by careful brushing and vacuum cleaning.

Preparation All hardware (except hinges) may have to be removed and glazing masked or temporarily removed, depending on condition.

  Tudor vernacular style inn with casement windows with leaded panes  
  Inter-war steel windows were used to imitate Tudor glazing at The Fox public house, Bix, Oxfordshire, 1936  

Surface preparation In situ stripping back of paint and rust to healthy metal may be carried out using a variety of tools such as needle guns, disc sanders, hand scrapers, wire brushes and sandpaper. Grit blasting is a more rigorous alternative but extra care should be taken as the blasting medium can collect in crevices where it holds moisture and can cause rust or distortion through build up. Health and safety considerations relevant to the removal of lead-based paint should be observed carefully. Paint samples should be taken to assist with the identification of the original colour scheme. The extent of stripping will depend on the condition of the window, sometimes it is enough to rub down, prime exposed metal and repaint.

Priming Stripped metal should be primed with a rust inhibitor immediately to avoid the re-formation of rust on exposed surfaces.

Realignment Once stripped of paint and/or cleaned of rust, window frames and casements can be realigned, adjusted and eased so that all operable windows are returned to good working order. Loss of alignment may be the result of rust behind the frames causing metal expansion and/or paint and debris build-up, both of which can cause distortion. Poor maintenance and userpressure on uncooperative windows may be contributory factors.

Assessment Condition and the extent of necessary repairs are assessed at this stage and a decision made on whether to carry out in situ repairs or to remove the window for repairs at a workshop or to replace it. A detailed schedule of window repairs may include all three possibilities as the condition of individual windows in a building may vary.

Metal repair Even when metal frames appear to be in very poor condition, often very little metal needs to be replaced. In situ repairs may involve the use of metal fillers. Piecing in of new metal is usually best carried out using brazing (rather than welding) because of its versatility and reduced fire risk. Replacing even small amounts of metal would require material from suitable reclaimed windows, or adjusting currently available steel window profiles.

  Brick facade with rectangular four-light steel windows under flat arches; single oval window also shown  
  Elizabeth House, Highgate, London, Richardson and Gill, 1930  

Hardware repair Operators, hinges and locks should be cleaned using a fine wire wheel. Operators may have small lubrication holes and these are sometimes painted over. Mechanisms may have seized and can be repaired by flushing out the gears, then freeing the works by oiling. Missing or broken hardware and hinges should be replaced, perhaps using matching parts from salvaged windows. Alternatively, readily available parts can be adapted although this may necessitate filling existing screw holes with steel epoxy or plug welds and tapping in new screw holes. If the hardware is a highly significant element of the historic window, reproductions can be made.

Re-glazing and weatherproofing Depending on the degree of distortion, de-glazing and re-glazing may be necessary. Cracked or broken glass and failing putty should be replaced. Re-placement putty should be appropriate to the use and be allowed to harden for approximately two weeks or longer before it is painted to match the colour of the fenestration. Silicone sealant is not aesthetically appropriate for conservation work. Weather-stripping using silicone beads can also be undertaken.

Painting Similar considerations to painting wrought iron apply (see above).

Workshop repairs
In cases of severe deterioration, the window can be removed to the workshop. As noted above in relation to wrought iron windows, dismantling and re-housing will involve disturbing the surrounding fabric, although it may still be possible to repair sub-frames in situ.

Once in the workshop, removal of flaking paint and corrosion can be carried out in a chemical bath of phosphoric acid. Unevenly distributed rust may have to be grit blasted. Test areas should always be carried out to determine the correct air pressure and size of grit, starting at a pressure of 40psi with a fine grit (usually copper slag) and not exceed 60-70psi. It is important that BS standards for abrasive cleaning should be carefully interpreted before applying to historic steel sections.

As with in situ work the stripped metal should be primed with a rust inhibitor immediately to avoid the re-formation of rust on exposed surfaces. Then realignment can be carried out as necessary using heat and pressure. Any perished metal sections may then be cut out and replacement matching metal sections welded in. Replacement metal can either be taken from matching salvaged windows or suitably adjusted, currently available sections. Matching replacements can be specially fabricated but this will be more expensive.

  French doors with leaded panes and overlights including arch detail  
  Neo-Georgian glazed metal doors at Elizabeth House  

Where appropriate, the repaired window can be powder-coated to the required colour over hot-dip galvanising, a zinc-coating process which improves rust and corrosion resistance reducing the requirement for regular maintenance.

Repair versus replacement
In situ repairs, where possible, are likely to be the most economical option. When replacement is inevitable, the appearance of the steel window can be matched reasonably closely with currently available sections, which include SMW or F sections (both introduced in 1920) and W20 sections (introduced in 1956). Severely deteriorated windows are more expensive to repair than to replace so it may be appropriate to reserve repair in the workshop for the most significant windows.

Replacement windows have the advantage of hot-dip galvanising and powder-coated finishes which make them nearly maintenance free, whereas inter-war windows require regular maintenance and repainting to prevent rusting. It is possible to treat inter-war windows that are in the workshop for repair by powder-coating over hot-dip galvanising. However, the cost of this may not be justified given the lower performance specification of inter-war windows when compared to contemporary upgrades, which include double glazing and jamb linings that can partially design out an inherent problem of cold bridging.

Pre-1918 sections are unique to a particular manufacturer and so are exceptionally important. Hot-dip galvanising and powder-coating are not suitable for these sections, which should be repaired and refinished as per the original window.

Finally, it should be noted that currently available steel sections provide an option which is both acceptable – in conservation terms and aesthetically – and financially realistic when it comes to the replacement of wrought iron and cast iron windows which are beyond repair.



This article is based on research carried out for the chapter on metal windows in Windows: History, Repair and Conservation (see Recommended Reading). The author wishes to thank Charles Brooking for allowing her to photograph the collection at Cranleigh and for his assistance with the illustration captions, and Chris Topp for providing useful research documents on wrought iron.

Recommended Reading

E Makri and R Harris, ‘Metal’, in Windows: History, Repair and Conservation, Michael Tutton and Elizabeth Hirst (eds), Donhead, Shaftesbury, 2007

SC Park, ‘The Repair and Thermal Upgrading of Historic Steel Windows’, Old House Journal, Active Interest Media, Washington DC, 1984



The Building Conservation Directory, 2012


ELENI MAKRI MArch BArch RIBA IHBC AABC is the founder and manager of Conservation PD (Planning + Design). Her previous roles include head of the heritage team at the Halpern Partnership (now Formation Architects) and local authority conservation officer.

Further information






Leaded lights

Metal windows

Secondary glazing

Steel windows

Timber windows

Window glass

Wrought ironwork

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