The Building Conservation Directory 2023

85 C AT H E D R A L C O M M U N I C AT I O N S 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 M A S O N R Y 3.2 IRON CRAMPS LEE BILSON C RAMPS WERE defined by architectural historian Carl Lounsbury as ‘iron staples used to hold two adjoining pieces of masonry together to prevent them from slipping.’ They were included in standard building practices in the UK from as early as the Tudor period, but they were in use long before this time and even the Romans’ use of iron elements drew on Greek precedents. In British built heritage, wrought iron cramps are commonly found from the 18th and 19th centuries when, with the rise in employment of masonry cladding, they were inserted to assist in lateral stability and to tie structural elements together. But they were also used by conservators well into the 20th century when repairing historic masonry structures. In simple terms, an iron cramp (sometimes called a dog cramp) is an iron bar with its two ends folded at right angles, which was let into adjoining masonry blocks by cutting a hole in each block connected by a shallow groove. The cramp was then set in place with a bed of mortar or lead. Masonry parapets often have cramps across the top of adjoining stones, ensuring the horizontal link between two blocks, as illustrated opposite. They were also used in masonry buildings to provide structural stability where desired by the builder, architect or engineer. You might spot them or signs of their use in steps, chimneys, cornices, coping stones, memorials and all manner of other masonry structures. DETERIORATION Wrought-iron retains a mineral component from the smelting process (slag) interspersed through the metal in fine layers which give it a degree of protection from corrosion. However this resistance gradually weakens over time so when embedded in a relatively porous masonry structure it eventually begins to corrode. This can be catastrophic for a building as the rust expands with such force that it can break apart huge pieces of masonry. This rust ‘jacking’ is a common problem where the upper surface is exposed to the elements, but one of the biggest blights is where embedded ironwork is exposed to naturally occurring moisture deep within the masonry structures, as this means that cramps can corrode even when hidden from any observer. INVESTIGATION Carrying out considered condition surveys of any historic building is the first step in any process of developing and outlining proposals for intervention. The aim should always be to gather as much information as one might need about the nature of the building, its structural makeup, the materials used and any previous interventions. Understanding the weathering and decay characteristics of any given building is key, but an open mind as to causes of failure is also important. In the case of cramps, it may be that the building in question generally does not fit the parameters or periods where embedded ironwork might be expected, but they could well have been introduced in previous repair phases. Typically, cramp damage is likely to be found in masonry structures wherever lateral restraint was required, such as to the top course of masonry. However, they were also used as ties to underlying material, for example, to tie a fine ashlar skin to a substrate of brick, as at Witley Court. They were often placed relatively near the stone face, and as such when corrosion inevitably occurs the resulting corrosion jacking leads to characteristic fractures and spalls in the face itself. This can be spotted across any given structure, most commonly at the upper corners of any affected blocks of masonry. Often there is a visible pattern of cracking and failure across a masonry facade which helps their identification. Where cramps are not as readily noticeable, the use of hand-held metal detectors or other non-destructive sensors can provide the surveyor with the locations of any concealed metalwork that has not yet become visible in surface decay. Tap testing, by gently tapping the stone with a small tool or even one’s hand, can also help A typical use of a metal cramp to connect blocks of masonry together in a parapet: a thin layer of mortar is all that protects it from the weather.