Context 170 - December 2021

30 C O N T E X T 1 7 0 : D E C E M B E R 2 0 2 1 Many original components made of iron, includ- ing radials, ogees, straps, bolts, deck plates and deck end plates, required repair and/or strength- ening. The stone abutments were also examined and repaired.The conservation approach agreed that new additions required for strengthening would use modern materials, notably stainless steel. Replacement (replicated) items would be made of a historically accurate material, such as ‘pure iron’. This depended on engineering advice regarding structural suitability, bimetallic coupling properties and the degree of revers- ibility. Many new bespoke ironwork parts were made, each new piece being unique. Grit blasting Corrosion of packing pieces between the main beams and the deck plates laid laterally across had caused the deck to lift in places. This and all other corrosion was removed by grit blasting. As we felt that it was important to leave as much in situ as possible, many repairs involved adding steel braces, individually designed and bolted into position. In this way no original iron mem- ber was removed, even if cracked. Elsewhere new pieces were manufactured, with wooden plates and components being carefully cut to size to act as models for the iron replacements. The cast-iron elements include five main arches rising from base plates and supported on the abutments by verticals rising up to the deck, and within the span by middle and upper ribs bound together with braces, straps, diagonal stays, ogees and circles. The spear-headed bal- ustrade sat on the overhanging deck plate ends, supported by swan-neck braces. The bridge’s joints use carpentry techniques, the joints often passing through oversize slots in the ironwork and fixed in position by cast-iron wedges and lead packing, or by bolted blind dovetails, or pinned by mortice and tenon. Every tenon was even pinned with a ‘ternail’, a technique used in timber framing. Each of the five main frames of the bridge was individually examined and repairs specified. The deck was stripped back to the original deck plates. The deck plate wedges, which provided lateral restraint, were replaced with ‘pure iron’ replacements by Barr and Grosvenor. A protec- tive membrane was laid across the deck plates to prevent future water damage. Prior to resurfac- ing, the cast-iron water pipes were replaced by the water company with flexible plastic pipes. This was an improvement as the cast-iron pipes had caused condensation within the structure. The new deck was installed using lightweight cement bound aggregate. This was topped with a binder course of hot-rolled asphalt with The self-supporting scaffold incorporated a public walkway, allowing people to watch work in progress. The Iron Bridge repaired, strengthened and repainted