BCD 2019

113 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 Matching glazing bar sections for repair. Available sections may need to be modified to exactly match original section sizes. Leaded lights being rebuilt by a specialist restoration company (Recclesia). The glass is being cleaned and retained in its original location by using a rubbing of the original panel. New cast iron windows manufactured to replace existing units (Photo by © Dominic Grosvenor, Barr and Grosvenor). Once in the workshop the windows can be fully dismantled for treatment. Where paint layers are to be removed, sampling should be undertaken to record evidence of earlier treatments and colour schemes, whether for replication later or simply for record purposes. The treatment of the frames to remove paint or corrosion will require a methodology specific to the particular material type and to minimise the risks from lead-based paints. Iron frames may be flame stripped or air abrasive cleaned, whereas bronze and aluminium frames require less aggressive cleaning techniques. Corroded or missing frame sections will require repair with new sections to match the existing. Some limited sections are commercially available, although most of the missing ones will require bespoke new sections made to match the original details. These are then fixed mechanically or by welding or brazing to the original frame to reinstate full section profiles. Trials will always be required when welding repair techniques are used to ensure that full penetration welds can be made to the original material without stressing or distorting it. Following repair of the frames the glazing can be reinstated using salvaged glass that was removed prior to the refurbishment, and new glass that matches the original material where possible in terms of movement and colouring. For frames that are putty glazed an appropriate metal glazing putty should be used rather than a traditional linseed oil putty, as unlike timber frames the oils cannot be drawn out of the putty. Metal or timber glazing beads should be refurbished and refitted where in good condition, or new elements manufactured where they are missing or damaged through use. Where leaded lights have been removed it is likely that they also require refurbishment. Where a full rebuild is required, the original glass quarries should be salvaged and carefully cleaned before reassembling to match the original appearance, and any leading repairs should be made using lead ‘cames’ of the same section as the original. Missing ironmongery should be replaced to match the original and adjusted to ensure that the completed opening lights can be closed to seal against the frame. This can include the fitting of wedge blocks to support the opening light in its closed position, therefore preventing long-term distortion of the frame. Protection It is important to choose a suitable protection system that will prevent the metalwork from corroding and tarnishing. Ferrous frames will require full protection after treatment with a suitable paint coating system. These can be based on traditional oil paint (albeit lead-free) or modern, longer life epoxy or alkyd paint systems. The frames can also be zinc flame sprayed to provide additional protection, which is a similar process to galvanising. However, it is less prone to distortion so is more suitable for complex frame sections. Bronze frames can be chemically repatinated or coated with tinted lacquers to achieve a traditional aged appearance before being coated with waxes and buffed to a high shine. Aluminium frames once treated can be coated in clear lacquers to retain their silver appearance. Improvements Original windows can be improved to provide greater thermal or acoustic insulation by replacing glass panes with modern glazing systems. Slim double-glazed units can be produced in thicknesses as little as 10mm, providing a significant thermal improvement over single-glazing, and the new Pilkington Spacia glazing system can supply vacuum sealed units which are only 6mm thick while still achieving comparable thermal performance to that of a 27mm double-glazed unit. However, where new modern glass systems cannot be utilised then secondary glazing systems fitted to match the original window fenestration is often a more viable option and avoids heat-loss through the metal frame itself. Draught sealing is also a popular improvement. However, again this can be limiting if the frames were not originally designed for seals. The application of self- adhesive draught seals can be effective after frames have been refurbished and gap variances improved. Security The use of laminate or toughened glass may be required where the windows need to conform to Part N of the building regulations which will also provide enhanced security over a standard glazed unit. Window locks or restrictors can also be fitted as part of the refurbishment. The choice of locks should be carefully considered to be in keeping with the overall window aesthetic and any stays should restrict the window without stressing the units unnecessarily. Replacement In extreme cases it may be necessary to consider replacement of the window units rather than undertaking repairs. Where this is necessary the replacement should be sympathetic to the original design intent, using similar materials while conforming to modern standards for thermal performance and security. Often the decision to replace windows is taken without a full assessment of the windows themselves which can lead to poor quality imitations and the loss of important heritage features. Replacement should always be the last resort and not taken as the easy option. A successful refurbishment of traditional windows starts with a detailed condition survey and the development of a suitable repair specification that can be taken on by an experienced contractor with good knowledge of the window system and applicable repair techniques. Repairs can never achieve the performance requirements of modern window systems, however this should never be used as a reason in itself to replace traditional hand crafted and historically significant items. Acknowledgements: With many thanks to Peter Meehan for his assistance and advice in identifying suitable photographic images. RICHARD BAISTER is an Icon accredited conservator and partner in Heritage Project Management (www. heritageprojectmanagement.com ) which provides specialist heritage services to main contractors and local authorities in support of large heritage refurbishment projects.