Article by: Peter Clement, Chairman and Managing Director of Clement Windows Group Limited.
The first metal windows were made from wrought iron by medieval blacksmiths. These simple frames were glazed with either stained glass or clear leaded lights, and were mostly used for ecclesiastical buildings and major country houses whose owners were among the few people who could afford them. At this time, leaded lights were also installed direct to masonry or wood, and secured with copper wires to vertically or horizontally fixed metal bars known as 'ferramenta' or 'saddle bars'. Minimal engineering skills were required to make windows with fixed lights. However, casement windows (a window with at least one light which can be swung open), demanded considerable dexterity and craftsmanship in order to produce the fittings required, including the gudgeon plates on which they hinged, decorative handles, handle plates and stays. These designs might not meet today's standards for draught and weather protection, but they were often beautiful examples of 'the blacksmith's art'.
The earliest window glass in general use was variously known as 'muff glass', 'broadsheet' or 'cylinder glass'. It was made by blowing a cylindrical vessel which was then opened up at each end and split from end to end to form a sheet. In the late 17th Century this method of production was largely superseded by 'crown glass', also known as spun glass. This produced much clearer glass, involved manipulating and spinning the semi-molten glass to form a disc from which small panes could be cut. Both glass types have a distinctive beauty when light reflects off them.
The earliest glass was extremely expensive and was only available in relatively small panes without severe distortion, typically 6" x 6" maximum. As a result almost all windows of the Tudor and Jacobean periods were made up of leaded light panels, often with diamond shaped panes called 'quarries'. The quarries were joined together to form the window light using 'H'-section strips of lead, called 'cames', which were soldered together to make up one large glazed area. (Copper was used in place of lead, particularly during the Arts and Crafts Movement in the late 19th Century). Where flat sections of wrought iron were used to make up a frame, the leaded light was fixed to it with wire secured with lead solder. In later windows a copper rivet was used instead of the wire, but otherwise the design was much the same. In both instances it would have been usual to weatherproof the light at its junction with the frame with putty (a mix of pounded whiting and linseed oil) angled to shed water.
With the rise of Palladian architecture in the early 18th Century, wrought iron windows fell out of favour as timber sashes were considered more suitable for the new, elegant style of domestic architecture. Nevertheless, wrought iron windows with leaded lights continued to be used in churches and in more humble domestic buildings, and in the 19th Century their use was made fashionable by a succession of historical revivals including the Gothic Revival and the Arts & Crafts Movement.
It was the advent of a more accurate metal casting method in the mid 18th Century that provided a metal alternative to timber for the construction of sash windows. For the first time metal windows were manufactured in a controlled production environment: they had come out of the forge and into the factory. These first 'cast metal windows' functioned in an identical fashion to softwood sash windows with two vertically sliding panels of glazing, one set behind the other and counterbalanced with weights for ease of operation. The casting process enabled the window manufacturer to produce a stylised product and offer a choice of designs. Detailing which had only previously been seen on wood could now be included in the timber 'pattern' from which the metal window was cast. The result was a metal sash with glazing rebates and decorative 'T'-section glazing bars showing rounded edges and ovolo mouldings - features not seen before on metal windows.
By the middle of the next century more sophisticated casting methods allowed further expansion of the ranges available and, not surprisingly, demand from architects and builders increased considerably. James Gibbs chose cast copper sash windows in 1750 for what is now Warrington Town Hall. Similarly, John Carr installed iron sashes in the side elevation of Flitcroft's Palladian style Wentworth Woodhouse in Yorkshire. Inverary Castle was supplied with cast iron sashes in 1783 made by the Carron Company but apparently not without some problems, and as the next century dawned, a comprehensive selection of cast iron casement windows were offered in the Coalbrookdale catalogue of 1801.
During the reign of George III, cast metal sashes had acquired universal appeal and an impressive portfolio of uses, amongst which were general housing, factories, utility and Government buildings. Indeed, the first 'fireproof mill' - William Strutt's Derby Cotton Mill of 1792 - had iron windows. As well as an inherent resistance to fire, iron was increasingly demanded for its security characteristics, desirable for those infamous institutions of the time, the workhouse and lunatic asylum. In 1796 the Retreat at York featured cast iron windows, one of the earliest homes for the mentally disabled to do so. Fifty years later cast iron windows were a standard component for asylums. As a patent granted in 1848 put it: "cast iron sash windows appear to possess advantages for lunatic asylums, workhouses and schools, since when open the sash bars present a guard against patients escaping or children falling, yet offer no obstruction to free ventilation." As the Regency period approached, metal windows became common in homes and in 1805 were used in the royal residence of Kew Palace. In 1833 London's Encyclopaedia of Cottage, Farm and Villa Architecture reported: "Windows of cast iron are very fit for cottages and are now made of different forms and very cheap."
In 1856 Sir Henry Bessemer pioneered a new production process for hot rolled steel, which had a dramatic effect on industrial growth and steel mills using his new techniques sprang up in the Midlands and North of England. As a world power, great Britain rapidly emerged as a mass-producer of steel, and it was from this point that the third stage of metal window development began.
'Crittall' were the largest and best placed manufacturer to take full advantage of the new opportunities, and the company played a leading role in revolutionising the worldwide use of the metal casement. Indeed, its name eventually became a generic term for steel windows. After the First World War the country demanded 'homes fit for heroes'. These houses, as with those in the construction book that followed the next War, almost invariably included steel windows which were inexpensive and readily available in a wide selection of suites, styles and standard sizes. Their use in all forms of architecture became prolific, in keeping with the new fashions and demands for low-cost, light, airy and well ventilated buildings. Subsequently steel window manufacturers became large and numerous. Millions of steel windows were fitted at home and abroad in commercial buildings, housing estates, Bauhaus-inspired creations and Henrietta Barnett's inspired vision of cottage-styled homes with tree-lined avenues, a utopian ideal typified by the Hampstead Garden Suburb. Huge numbers of steel windows were sold up until the 1970's (benefiting from mandatory hot dip galvanising in 1955) but thereafter sales have fallen dramatically and aluminium is now the dominant force in metal fenestration.
Metal windows which appear to be well beyond repair can be restored either in situ or in the workshop if the correct principles are followed - very rarely is this not possible. Common problems include distortion, excessive build up of paint, failed hinges and fittings, and rust. Surface rust always looks much worse than it really is, as it occupies up to seven times the volume of unoxidised metal. The starting point for any repair of alteration work is to appoint an experienced person, who can advise on any planning consents that are necessary. This is particularly relevant where a building is listed or in a conservation area. The second 'golden rule' is to record and photograph every item before any work commences. A window that is to be removed for repair should be securely labelled to identify it, and a note must be made of its type and its position in the building, to ensure that it can be returned to its original location later. Where leaded lights are concerned, a useful tip is to take a 'rubbing' of any leaded lights which are to be dismantled to record the diamond or rectangular pattern of glass and lead cames exactly. When the initial survey is complete, the frames and glazing should be protected externally and internally with timber boarding or heavy gauge plastic sheeting - building sites are often left unsupervised and are vulnerable to vandalism and theft.
If the metal windows are steel and made after the mid 1950's then they are likely to have been galvanised, revealing a tough silvery finish beneath the paint surface.
Renovation is not complicated:
The opportunity may also be taken to enhance the window's performance by applying weather-stripping and security fittings. Once on-site servicing is complete, properly applied paint systems and regular maintenance will ensure these windows will give many more decades of excellent service.
In the case of older ungalvanised metal windows made of iron, cast iron or steel, the restoration process is more involved due to the higher probability of severe corrosion. If the situation demands it, on-site servicing and restoration can provide excellent results. It is simple and inexpensive with glass repairs completed at the same time. However, to achieve the very highest standard of restoration, it is best to remove rusting metal frames from site for refurbishment in the workshop. This must be properly planned and carefully carried out with expert guidance and supervision to avoid further damage, and a 'method statement' should be prepared, detailing precisely the work to be undertaken including such protective measures as on-site recording procedures, labelling and the protection of any historic glass. Once the glazing and any old putty have been removed from the metal frames, paint and rust should be cleared from the surface of the metal to enable its condition to be inspected.
Neither wire brushes nor grinders can remove all surface rust effectively. Suitable alternatives include acid pickling and the use of air-borne abrasives. For acid pickling, phosphoric acid is preferable as it forms a protective phosphate layer on the surface of the iron. Air abrasives range from the use of a fine grit such as copper slag to G24 grade 'chilled iron grit' (described as a 'small crushed steel ball bearing') which will leave a clean roughened surface, as a preparation for further treatment. Generally, pressure should be kept below 60-70 psi. Both cleaning methods depend on the skill of the operator to know when to stop cleaning to avoid damaging the metal work.
Wrought iron may also be cleaned by heating with an oxyacetylene or oxypropane flame, as the resultant expansion of the metal breaks down the adhesion of the rust, allowing it to be wire-brushed off. This method cannot be used on cast iron that is brittle and liable to fracture in the heat. After cleaning it will be possible to identify those metal sections that are too badly corroded to provide adequate structural support. Frequently, this will be the bottom rail or 'cill' which is liable to deteriorate though prolonged contact with moisture, and is more vulnerable than the jamb and head. Steel and wrought iron sections may be repaired by cutting out the corroded section and welding in place a replacement section made from a similar material to the same profile. However, cast iron is almost impossible to weld due to its tendency to fracture when heated. An alternative, cold metal stitching, can be used.
Surface rust will appear as soon as cleaning is finished, so a protective coating should be applied as soon as possible. Of the various primers used historically, red lead was the most effective and one of the most commonly used, but it is highly toxic and waste material removed during cleaning should be treated accordingly. One of the best alternatives is a flame sprayed zinc primer to BS 5493, which is an excellent pre-paint treatment for ferrous metals. For steel windows, long term corrosion resistance can be achieved most effectively by hot dip galvanising to BS 729. This simple process takes only minutes to carry out. The frames are fully immersed in a bath of molten lead and zinc at temperatures reaching 450ÂºC, so that complete surface coverage is therefore achieved, including inner and outer surfaces, awkward corners and narrow gaps. However, be warned; hot dip galvanising is a harsh process and can result in historic windows turning into 'metal spaghetti'. Advice should therefore be taken from a Galvanisers Association member beforehand. Galvanised metal frames can be polyester powder coated to BS 6497 in a range of colours and finishes, matt, semi or high gloss. The applicators provide a warranty of 15 years but 20 years is not uncommon before re-painting is necessary.
It is important to remember that only specialist conservators should clean or work on stained glass, and some repairs to leaded lights, such as releading, may also benefit from their assistance. Before de-glazing frames with leaded lights, a useful recommendation is to cover both sides with a 'cling-film' type of plastic; this light sticks to the glass and will help prevent the lights from falling apart in the process. However, if a light does need remaking, all the original glass should be kept for reuse and labelled, not thrown away. Inevitably some work will be required following removal - the belief that a leaded light can be re-glazed without any repair work and not leak is a myth. Even if the leaded lights do not need re-leading, the perimeter lead will usually need to be replaced. In most instances the lead cement that holds the glass will also need repairing or replacing entirely.
Glass can be cleaned with a mixture of distilled water and methylated spirits, applied with a soft clean cloth, allowed to dry and then polished off with chamois leather. Modern detergents should not be used on lead or glass; commercial window cleaning materials will leave a discoloration on both surfaces. However, neutral soap can be used to remove organic growth. Where original glass has been lost or broken, new glass that will provide a near match to most originals is available from an increasing number of highly specialised, enthusiastic manufacturers and stockists, including crown glass, muff glass and others.
When re-fitting the restored metal windows it is important to use the correct materials: putty is traditional and preferable, but recently developed mastics and butyls may be acceptable if they will be painted. Silicone as an external perimeter seal is not normally appropriate although its qualities are commendable. In the past, metal windows or their wood surrounds which fitted direct to masonry, would have been bedded and pointed using mortar. A common practice was to mix lime and sand as a bedding compound similar to that used for re-pointing brickwork or masonry. Lime mortars used today should match the original as closely as possible, without the addition of Portland cement.
A well restored metal window will last as long as most new replacements, costs less than replacement, and produces highly satisfactory final results. Their appearance is graceful, elegant and pleasing to the eye, forming an intrinsic part of the building. These aesthetic virtues cannot be duplicated, nor can their historic interest and character. However, where it is not possible to repair an original, replicas can be manufactured using exact matching sections, original fittings and antique glass. Above all else, never replace original metal windows with crude substitutes in other materials.
Removing old paint layers can destroy the only evidence of the way a building was painted in the past. Consideration should be given to finding some way of leaving a sample behind, particularly in crevices, where early schemes are likely to survive.