BCD 2019

112 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 1 9 C AT H E D R A L COMMU N I C AT I ON S Wrought iron window to former mill showing jacking corrosion of the frame and opening light sections (All photos: Richard Baister unless otherwise stated) Early aluminium windows showing surface corrosion to the metalwork together with physical damage where services have been introduced to the building and degradation of the timber glazing beads Cast iron pavement lights removed for full assessment in the workshop to the reduced costs of iron production and working techniques developed during the Industrial Revolution. Windows would be cast at the foundry using sand moulds based on a master pattern, allowing cost effective replication of the units. However, cast iron being more brittle resulted in heavier window sections being required over wrought iron units. Cast iron is typically more corrosion resistant than wrought iron, however it also needed to be painted to retain its aesthetic appearance and prevent corrosion. Typical defects include fractures of the cast iron sections with possible loss of elements such as glazing bar sections, and distortion and porosity of the sections as a result of poor casting techniques or sections cooling at different rates. Bronze or architectural bronze (brass) was often used as an alternative to wrought iron on more significant buildings where the material could be patinated and expressed as part of the overall building design. The material is very corrosion resistant but its high cost of production resulted in it mainly being used on more prestigious projects. As a material it has good corrosion resistance, however it needed to be patinated and waxed to provide an acceptable aesthetic finish. Typical defects include surface corrosion of the brass where protective wax coatings have been lost with the formation of green copper sulphate and carbonate deposits. Aluminium windows, although commonplace today, were not commercially available in the UK until the 1930s due to the very high cost to produce the metal from its base ores. Windows were made either by casting, by mechanically joining bar and plate material or, more commonly, by using extruded profiles to form close fitting window sections. Aluminium corrodes readily to form a layer of surface oxides which then act to protect the main body of the material. This can change the appearance of the aluminium, and surface coatings (passivating) are often applied to retain its natural appearance. Typical defects include salt or bimetallic corrosion, and brittleness of sections through work hardening. Copper and zinc although not commonplace have also been used for window fabrication. These are typically bespoke and need to be carefully considered in determining an appropriate repair specification. Pavement lights became very popular during the 19th century and were typically formed of cast iron or bronze with cast glass blocks. Being situated at ground level, they are exposed to very harsh conditions with a high risk of impact damage to the glass blocks or the grilles themselves. Although original paint coatings may have been applied to the cast iron, they are often not maintained through the life of the pavement light. Typical defects include physical damage due to excessive loading, and damage to glass blocks. THE ASSESSMENT PROCESS Essential in any assessment process is understanding the heritage significance of the windows themselves. This may be largely based on the originality of the windows to the building, but may also include other factors such as the construction materials or any unusual features or aesthetics that have been incorporated. Where windows are deemed to be of greater heritage significance, increased efforts should be made to retain and repair them. For listed buildings the repair or replacement of windows may be seen as a material change, requiring prior approval from the local authority (listed building consent), and specialist advice from an accredited heritage professional should always be sought. The majority of condition assessments are carried out on site and it is necessary to fully inspect and record the condition of the window. This can be difficult if the windows have been poorly maintained in the past and defects are covered with many layers of paint coatings. Wherever possible the assessment should be carried out in situ as removal can put the window at risk of further damage. However, in some cases it may be necessary to carefully remove sample windows for inspection in the workshop where the true condition and difficulties associated with the refurbishment can be established. Each window should be examined both internally and externally to identify and record: • original construction material – either through inspection or sample analysis • physical size of the window including section profiles and glazing thicknesses • visible defects on the window including any wear in operation and missing ironmongery • the fixing method and the condition of fixings • paint coatings and other finishes, including sampling to identify lead- based paint systems where necessary. A photographic record should be made of the internal and external elevations of the window, with specific defects recorded on dedicated photo sheets. From the condition assessment a treatment recommendation can be prepared which details the works required to undertake repairs. The extent of the repairs will vary upon the findings and a number of options from minimum intervention through to full replacement can be considered. These may include: Stabilisation Where minimal repairs are required it may be possible to undertake treatments that will stabilise the condition of the windows and prevent further degradation without the need for their removal, which could result in damage to the surrounding masonry. This may include ensuring that paint coatings are reinstated or that mechanisms are lubricated and made functional again. They would then not require more invasive works at this point in time. Repairs Minor repairs can be carried out in situ, however more extensive repairs require the removal of the window to a workshop. Removal should be carried out by companies with experience of the particular window system, with all units individually tagged to record their original location. The window may need to be partly deglazed to access fixings or to assist with the disassembly, in which case all removed components should be tagged together. Temporary windows may be required while the existing are being refurbished and usually these should be weathertight to prevent unnecessary damage to the building interior.