BCD 2019

111 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 1 9 METAL, WOOD & GLASS 3.3 Bronze windows in a London theatre (Photo by Jonathan Taylor) METAL WINDOWS RICHARD BAISTER W INDOWS ARE an important part of any building and their architectural style often reflects its age and overall heritage significance. They are often entirely functional in design and perhaps unlike many other building features have been developed in line with the availability of new materials and working techniques. Whereas windows originally performed as a functional blocking of an opening, we now expect them to meet high standards of light transmission, thermal performance and user operability and security, standards for which earlier windows were not originally constructed to achieve. When looking at the refurbishment of historic windows it is important to recognise these performance limitations together with the perceived difficulties of repair and future maintenance. However, these difficulties should not be seen as justification for their replacement with new items but simply as a requirement to undertake a full assessment and develop appropriate repair and enhancement proposals that carefully balance the requirements of function, aesthetics and heritage. In this way it should be possible to retain these heritage significant items for many years to come. DESIGN AND CONSTRUCTION Window design has always followed the availability and manufacture of glass, with the earliest windows incorporating small units of glass (known as quarries) joined together with lead to form a single fixed leaded light that filled the opening. However, to operate as an opening window a hinged frame was required, and with the advent of larger panes of glass, frames of timber or metal became the norm. The metals most commonly used for window frames are outlined below. Wrought iron is fibrous in nature and contains up to four per cent impurities which are introduced during the manufacturing process. It was first used for metal window production in the 16th century as it provided a stable framework to support leaded lights allowing them to be opened and closed for ventilation. A basic opening light would have been originally formed from flat wrought iron bars, fire welded at the joints by an experienced blacksmith with a leaded light copper wired to the framework. Greater weathering could be provided by soldering flat bars together to form more complex sections, a method which was used until hot rolled sections became more generally available. Steel became available for window production after 1856 when Sir Henry Bessemer developed his new production processes that allowed large quantities of low impurity steel to be manufactured at a much lower cost than the very labour intensive wrought iron production process. The introduction of standard window sections known as the universal suite in 1918–20 by the Steel Window Association then accelerated its use in window manufacturing up until its heyday in the 1960s, where it was extensively used for both domestic and civic projects. Crittall, the largest manufacturer of steel windows in the UK, was responsible for approximately 40 per cent of all domestic production – hence the generic use of their name to describe steel windows from the period. The longevity of steel windows was also improved significantly in 1955 by the mandatory requirement for hot dip galvanising of frames. Wrought iron and steel are strong in both compression and tension, allowing smaller and lightweight section sizes to be used in window manufacture with typical sections only 6mm thick. As a material it requires painting to provide an acceptable level of corrosion protection. Typical defects include: corrosion jacking of the wrought iron where paint coatings have failed causing localised distortion of the opening light; shakes and flaws in the metal from its manufacture leading to small areas of delamination; and dissimilar metal corrosion. Cast iron became a popular material for window manufacture in the 19th century due