History of metal
windows
By Peter
Clement
Peter Clement is Chairman and Managing Director of Clement Windows Group Limited,
a family owned company who have been manufacturing, fixing and glazing metal windows for
a hundred years. He is a past President of the Steel Window Association and regularly gives
presentations concerning 'metal windows and glazing', recently taking part in English Heritage's
Framing Opinions campaign.
To fully understand the fundamental issues concerning metal window repair and restoration,
it is important to have some knowledge of how metal windows evolved. The historical development
of metal windows comprises three very distinct stages, mirroring the advances made simultaneously
in the engineering industry.
The art of the Blacksmith
 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of production was largely superseded by 'crown glass', also known as spun glass.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.
From Smithy to Factory: The advent of cast
iron
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."
Steel Windows
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.
Conservation and repair work
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.
Galvanised Steel Frames
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:
1. remove excess paint from all moving parts and components including hinges, using either
a hand-held mini grinder or rasping file, taking care not to damage the galvanising
2. gently clean off any debris and loose paint with a wire brush
3. using a wrench, ease and adjust casements that may have become distorted
4. oil and lubricate hinges and all working parts
5. repair damaged hinges and fittings and replace those which have been lost or which have
been too badly damaged to repair, either from stock or by having copies made to match
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.
Ungalvanised Metal Frames
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 sever 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.¹
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 450ºC, 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.
Leaded Lights
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.
Refitting the restored window
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
PETER CLEMENT is Chairman and Managing Director of Clement Windows Group Limited,
a family owned company who have been manufacturing, fixing and glazing steel windows for
a hundred years. He is a past President of the Steel Window Association and regularly gives
presentations concerning 'metal windows and glazing', recently taking part in English Heritage's
Framing Opinions campaign.
Clement Windows Group Bio
The Clement Windows Group, formerly Clement Brothers Haslemere Ltd, was founded by Jack
Clement and his three brothers in 1928, making the company the oldest family run windows
business in the U.K. Their father had previously worked on the famous 'Crystal Palace' and
so the family heritage of working with steel and glass had already been firmly established.
The companies' long and colourful history has spanned over nearly a hundred years and has
been involved with the steel window and its components at every stage of development. In
1964 the company's Glass & Glazing department were selected by Pilkington to install the
very first 'glas to glas' double glazed units. 1977 saw the company becoming a founding member
of the 'Glass & Glazing Federation'.
There are now three distinct parts to the operation: 'Clements Steel Windows' manufacture,
supply and install operating nationwide. Recent prestigious installations include new steel
windows and doors at the London Ritz, Hampton Court Palace, Windsor Castle and Victoria Railway
Station.
For more information contact Clement Windows Group:
Tel: 01428 643 393
Email: info@clementwg.co.uk
Website: Visit www.clementwg.co.uk
for more info
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