BCD 2017

90 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 Common defects include loss or damage or poor previous repairs. Many such items can be restored accurately, avoiding conjecture, by referring to catalogues or similar items. Where objects have cracked, these are often found to have been repaired quite crudely and may just need some attention paying to the materials and type of repair. However, more serious cracking is predominantly caused by rust expansion on ferrous metal armatures, and sometimes this can be serious, requiring careful dismantling and rebuilding of the object. A good quality metal detector may be useful in determining the distribution of ferrous metal armatures. Removal of a ferrous armature is easiest where the extent of the cracking allows the object to be easily dismantled into sections. In other cases it may be necessary to leave the more embedded items in the object, possibly with the injection of a rust inhibitor to the remaining sections and suitable crack filling. Any exposed metalwork such as cut ends should be treated with rust inhibitors and suitable paints, and where further water ingress is likely to be a critical factor, the injection of a highly flexible epoxy resin (such as Rotafix CB10T) into cracks may be required to seal them. (Other treatments for embedded ferrous metal work are also possible in exceptional circumstances such as realkalization and cathodic protection.) The other main reason for cracking is mortar shrinkage. This can continue for years after casting and is often a sign that the mortar used was binder rich or had a very high water content (w/c ratio). This cracking tends to be finer and shorter than the larger and longer cracking associated with ferrous metal armatures. It may result in surface crazing and small items may become loose or drop out. Surface consolidation with mortars, grouts or even gauged lime shelter coats which are carefully rubbed into the cracks may be helpful. Occasionally, cracking can be the cause of micro decays in the mortar, such as locally high sulphate contents in stone aggregates leading to expansive cracking. This type of micro decay is rare and should be identified by petrography during mortar analysis. However, this information can be critical to determining a correct conservation approach to the object (such as improved weather shedding). A common cause of cracking in artificial rockwork is established tree growth as this causes disruption to the background masonry. Maintenance of tree growth and the control of root development (using geotextile barriers for example) has to be considered in any landscape scheme involving Pulhamite. Calcite leaching seems to be more of an issue in more recent reproduction cement castings than in 19th-century objects, but probably can occur in both. The deposition of calcium either from ground water or from mortar leaching is more common in some Pulhamite water features or underground features, where modern tufa materials or minor stalactites are now being formed by calcium deposition. Structural repairs, minor re-attachments and armatures for repairs are usually carried out with stainless steel rods, helical pins or wire, all of which should be of 316 grade stainless steel (or A4 for threaded rods). These can be set into predrilled and cleaned holes which have been filled with resin. These should be either specially designed and tested polyester resins or suitable epoxy resins, and special consideration should be given where resins are to be used in permanently saturated areas. Mortar repairs, rebuilding lost sections and recasting should be carried out using closely matching binders and aggregates. These can range from weaker natural hydraulic limes (used by William Ranger for example) which are largely now unavailable, to more hydraulic limes which are more easily matched from the wide range of modern NHLs and ‘natural’ cements (see www.Rocare.eu ). Vicat, which manufactures Prompt Natural Cement (PNC), has carried out extensive testing on PNC/NHL blends and PNC-only mortars and this information has been produced in English. Generally, such mortars provide a good colour and technical match for early cement mortars and can also be used as grouts. Casting does require special attention to the water content and aggregate selection and will often require the addition of fibre to prevent shrinkage cracking in the longer term. Pigments are frequently found to have been used in historic work and they are often essential for colour matching in conservation work. We highly recommend that only high quality manufactured ferrous oxide pigments are used (such as those made for use in the cement industry by companies like Bayer) especially where there is a chance of continual or frequent saturation. We have identified failures in repair mortars due to the high sulphate content of earth pigments (some with about 20% sulphate content) causing Thaumasite reactions, and preventing strong hydraulic limes from setting in persistently damp conditions. The high quality pigments Cracks in the leg of an Austin eagle caused by a rusting metal armature: the armature was removed and replaced with a stainless steel one, and the foot rebuilt in a matching mortar (Photos: Simon Swann)