The Building Conservation Directory 2023

86 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 2 3 | C E L E B R AT I N G 3 0 Y E A R S C AT H E D R A L C O M M U N I C AT I O N S identify failures and detachment of the masonry face where there are visible spalls, fractures or other defects. These failures might not always be caused by hidden cramps, but their cause should always be investigated, and they are often the first signs that corroding cramps are an issue for a building. Often the masonry affected by iron cramps can be in a fair condition with only minor issues, but sometimes cramp corrosion can be the singular area of greatest concern. At its worst, rust-jacking can open course joints, causing distortion throughout and threatening the stability of the structure. MITIGATION MEASURES AND REPAIRS Options for treatment include minor repairs and repointing to slow the rate of moisture ingress, but generally there is little by way of mitigation against corroding cramps, bar removal. Maintenance – As always, a little maintenance can help prevent extensive deterioration. Major sources of water include blocked gutters and downpipes, damaged flashings and open mortar joints. Regular checks of joints are required, including those of parapet copings, cornices and others facing the sky in particular. Repointing works may be classed as a form of minimal intervention, reducing moisture ingress by closing joints. However, such an intervention would not ultimately address any deformation or instability of a structure, nor would it prevent further damage to the stone or corrosion to the cramps. Indent repairs – The most common approach for isolated and discrete defects on an ashlar block, such as a small spall or cracking from a corroding ferrous cramp, would be to repair the area using a stone indent, whereby a new stone is cut and shaped to fit the damaged area. The ferrous fixings should be removed in the least invasive manner available during the stone repair works and replaced with a stainless steel equivalent. Where possible, blocks should be repaired or replaced to avoid interrupting the original courses. Indents overlapping previous joints and courses are a big no no! Dismantling and rebuilding – Another option is to dismantle areas of masonry where feasible, remove all ferrous cramps, and rebuild using new stainless-steel cramps. This approach is most often used for memorials and small masonry structures. For large areas this can be labour intensive and is certainly against best conservation practice of minimal intervention, but it can be a viable approach where it is deemed necessary and can be afforded. Cathodic protection – Although widely accepted, cutting out or dismantling to remove fixings is not always considered appropriate. One non-destructive method which can be explored and developed for use on a structure is cathodic protection. This prevents corrosion by converting all of the anodic sites on the metal surface to cathodic sites by applying an electrical charge to the cramp. Cathodic protection may seem an extremely complex approach, but it is a proven one which avoids having to replace the masonry. As early as the 1990s English Heritage applied cathodic protection to corroding ferrous cramps of the Inigo Jones Gateway in the grounds of Chiswick House. Impulse radar was employed for the survey, identifying not only the location of metal fixings but also giving the team an indication of their condition and effect on surrounding masonry. The successful application of cathodic protection requires a multi-disciplinary approach, combining expertise in metals and masonry conservation practice with a good understanding of materials science and modern technology. The expense of this approach places it beyond the means of most property owners, and where the cramps are embedded in lead there are added complications in terms of effectiveness and potential preferential corrosion. Where a heritage asset is of such value or significance that the levels of structural intervention must be kept to the absolute minimum, the use of this keyhole surgery style approach may provide the solution to the thorny problems associated with rusting ferrous metal in historic constructions, be they stone, brick, terracotta or concrete. THE IMPORTANCE OF CRAFTSMANSHIP Working in the built environment invariably involves making informed judgments and compromises to achieve the best overall outcomes. Although the repair and conservation of built heritage is often undertaken by people who do not specialise in conservation exclusively, a good understanding of the philosophies The healthy growth of plants like buddleia is a sure sign of excessive moisture ingress which will inevitably lead to frost damage, salt movement and the corrosion of any embedded metalwork. Good maintenance is essential. Where cramps are suspected, their presence can be confirmed by non-destructive technology such as hand-held metal detectors.