BCD 2017

T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 115 METAL, WOOD & GLASS 3.3 LEAD FIXING FOR HISTORIC IRONWORK DAVID FIELD and ADRIAN LEGGE T  HE USE of lead as a means of securing iron into masonry dates back to pre- medieval times. The 12th-century ferramenta of the south oculus window of Canterbury Cathedral, which is almost 4.5m in diameter, is secured with a mixture of lead sheet/wool and lead putty. This technique was also used extensively during the Georgian and Victorian periods for securing architectural and cast iron components such as gates, railings and structural cramps Lead makes an excellent medium for securing iron as it can be caulked down to provide a strong friction-fit and, with proper preparation, it forms an almost watertight joint. (The term ‘caulk’ is used to describe both the process and the material used to seal a gap and make it watertight. According to the OED, it derives from Old Northern French cauquer or cauchier , ‘to tread, press with force’.) The lead in the joint should extend above the surface of the stonework and slope down and away from the ironwork so as to carry away any moisture. Care must be taken to avoid over-caulking because the lead can put strain on the stonework, especially near edges, which can cause surface spalling or even deep cracks and faults. Un-braced fixing points can fail when loads are applied to the structure. The movement of the iron in the stonework will tend to compress the lead creating loose joints, increasing the possibility of water ingress through capillary action and increasing the risk that the stonework will split due to frost or rust jacking. As iron in the joint rusts it can expand up to seven times the volume of the original iron, generating huge expansion strains, often causing failure of the surrounding stonework and, in severe cases, affecting the integrity of the building. Localised corrosion in ironwork often occurs close to fixing points, usually due to a breakdown of surface coatings allied to the tendency of these areas to remain damp thereby creating an electrolytic environment. The iron fixings in the stonework may not be easily accessible and this can make maintenance and repair difficult. It is even more essential then that these areas are regularly inspected so that maintenance, painting, and repair can take place before the situation becomes critical. The use of resins for fixing can be an acceptable alternative for historic ironwork. They adhere to both the iron and the stone, are easy to use, strong and create a water tight seal. There is no risk of strain on the stonework from the fixing process as there is no caulking required. It should be recognised that this is relatively recent technology (certainly when compared to lead fixing) and there may be difficulties associated with this system which have yet to come to light. Also, it should be acknowledged that part of the ethos of conservation is to retain not only the design and materials but also the skills and processes involved in the original work. REMOVING LEAD-FIXED IRONWORK There is no easy way to remove ironwork from masonry and the process is likely to cause some damage to the surrounding stone. If possible, remedial work should therefore be carried out in situ using the National Heritage Ironwork Group (NHIG) principles of minimum intervention (see Further Information). Where removal cannot be avoided, the first step is careful inspection to see if the ironwork can be deconstructed by removing screws, bolts, pins or wedges, and to check the condition of the surrounding stone. If the masonry is already so badly damaged that it must be replaced, for example as a result of rust expansion, cutting the masonry is the easiest option. Otherwise the next step is to remove the lead from the joint. This is best done by chain drilling holes around the bar with a slightly smaller diameter than the width of lead. Using a slow speed drill and moving the bar as the lead is removed will compress the lead, which can then be drilled again. The use of a small, thin chisel may be useful to act as a gouge, but it is inevitably a slow, tedious process and may not be a viable option in areas which are difficult to access. Once the lead has been removed, attaching a clamp to the iron bar and hammering metal wedges between it and the stone will gradually lift it up. A piece of wood can be used to protect the masonry. Removal can be complicated because the holes are often dovetailed at the base Poured lead still contained by the clay bund, and two caulking tools (Photo: A Legge)