Building Conservation Philosophy for masonry repair

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On one level it could be assumed that as this is a Norman Cathedral, following an ... 11: Recarving of voussiors to door arch, St Magnus Cathdral, Orkney.

Building conservation philosophy for masonry repair: Part 2 - ‘Principles’ Alan M Forster, School of the Built Environment, Heriot-Watt University, Edinburgh (email [email protected])

Abstract The techniques available for the repair of historic masonry structures are extremely wide ranging. The advantages and disadvantages of each type of repair can be evaluated in terms of cost, time and quality as with modern projects. It is however, important to realise that when repairs to historic buildings are selected they must conform to building conservation philosophy, or an ethical and principle based evaluation. This paper (part 2 of 2) establishes what is meant by principles in this context and wherever possible applies practical examples to illustrate these concepts. Design/methodology/approach Evaluative literature review of the principles encapsulated within building conservation philosophy utilising them to stimulate discussion on practical repair interventions. Findings It has been shown that the principles of building conservation philosophy must be considered prior to making decisions relating to masonry repair. These repairs have varying degrees of defensibility, and will ultimately lead to good or bad conservation approaches. This paper briefly discusses the principles, highlighting some of the issues that may be initially confusing to the practitioner. Originality/value The evaluation of building conservation philosophy for masonry repair, and more specifically the ‘principles’ have been little studied. The importance of this cannot however be over stated, as far from being an esoteric concept it affects every practical repair. This work brings together the study of the philosophical and practical, enabling practitioners to better understand the ramifications of building conservation philosophy for their projects. It must however be emphasised that as with any aspect of philosophy, there is not necessarily a right or wrong answer, only higher levels of defence for the selected repairs. Keywords Building Conservation Philosophy, Masonry Repair, Principles Paper type View point / conceptual paper


Introduction This paper is the 2nd part of an investigation into the philosophy of masonry repair. The 1st paper (Forster 2010) dealt ostensibly with the building conservation ‘ethics’ relating to masonry repair for historic buildings, whilst this part will evaluate building conservation ‘principles’. The ‘principles’ are specific criteria upon which conservation works should be based (Bell 1997:27-33), whilst ethics, form the broader issues or key concepts to be considered. For this reason this paper has a greater technical focus than part 1. The majority of conservation principles were established over 100 years ago by William Morris (SPAB: 1877) and his then newly founded Society for the Protection of Ancient Buildings. Generally, ethics and principles should be considered in conjunction, however, for the purpose of this work they have been separated. The principles include (Bell 1997; 2733): Principles (Specific criteria upon which conservation works should be based)  Minimal (Least) intervention (or conservative repair)  Legibility (Honesty and distinguishability)  Materials and techniques (Like for like materials)  Reversibility  Documentation (Meticulous recording and documentation)  Sustainability Repairs selected, based upon the ethical concepts and a combination of the principles should be defensible, and should in theory lead to naturally ‘good’, well founded conservation interventions. However, it could be argued that some of the principles conflict, creating tensions, potentially confusing the technical intervention decision. It is also important to understand that there are no absolutes in conservative repair only greater levels of defence for selected repairs, and that the first rule of conservation is that there should be ‘no dogmatic rules’ (Powys: 1995:3). Powys emphasises that ‘no fixed rule can be set up to be followed invariably. Each case must be considered on its own merits’. As mentioned in the first paper, Burman (1995:4) highlights that ‘conservation philosophy can be seen either positively or negatively. It can generate much discussion, or it can invigorate and inform our decision taking about the care and repair of historic buildings. There is constant need for observation and debate, in a constructive and harmonious spirit. There are no ‘goodies’ and ‘baddies’, but there are skilful and unskilful solutions to the repair of old buildings’. Each of the principles will be discussed individually, however, in situation where clarification can be gained from comparing and contrasting the principles this will be undertaken. Minimal (Least) intervention (conservative repair) Building conservation is arguably one of the rare fields where ‘less can be more’, meaning to do nothing can be best. The concept of least intervention is extremely important as the more fabric that is removed the less of the original building will remain, with a corresponding reduction in the cultural significance of the structure. 2

Minimal, or least intervention has been defined as, ‘as much as is necessary’ (Brereton, 1995: 7) and, ‘as little as possible’ (Feilden, 2003: 235). The decision to replace deteriorated or defective masonry must be based upon need. A badly eroded façade may not necessarily require intervention. Conversely, if the masonry has eroded to such a degree that the masonry is loosening and beginning to collapse or structural integrity is in question then intervention will be required. This decision should be undertaken with the input from a suitably qualified conservation accredited structural engineer. See Plate 1.

Plate No. 1: Eroded masonry façade at Doune Castle An extremely good example of over zealous masonry repair can be seen at the ‘Keep’ at Newcastle-Upon-Tyne (plate 2). The decision to reface large sections of the tower, were probably driven by cost considerations, but the intervention has detracted from the integrity and authenticity of the building. The unnecessary works have led to more historic fabric being removed than was necessary.

Plate No. 2: Newcastle-Upon-Tyne, Keep


Clearly all masonry components will eventually require intervention. It is the author’s view that prioritisation for selective replacement of badly deteriorated cornices, string courses and hood moulds would be easy to substantiate as they are weathering details, offering protection to the underlying substrate. That being said, these components could be repaired using less intrusive methods that would enable the retention of a greater amount of the original fabric. The durability of these alternatives (such as plastic repair) are poorer than replacing natural stone, and it is obvious that a ‘trade off’ situation is present, namely, retention of fabric against longevity of repair. The repair options available to those attempting to repair historic masonry, range from overzealous (leading to potential cost savings, but, indefensible conservation) to the puritan (potentially philosophically good conservation, but costly). An example of this situation could be the use of pinning and dowelling techniques that would enable the retention of a higher degree of historic fabric, rather than replacing masonry. It must however, be emphasised that good conservation need not be expensive and a puritan philosophical approach can in many cases be the least costly option. It is also well recognised that regular maintenance can be the most effective method to reduce decay, and as William Morris, stated ‘stave off decay by daily care’ (SPAB, 2008). When confronted with decaying masonry substrates, the following options may be considered:  Do nothing  De-scale masonry  Replace with natural stone  Indent with natural stone  Plastic repair in lime mortar  Plastic repair in an alternative materials (OPC and Lithomex)  Pinning / dowelling & flaunching  Consolidation and open wall head treatment  Rebuilding Minimal (Least) intervention: No intervention & Descaling In situations in which the masonry appears aesthetically poor but does not require intervention doing nothing can be a defensible position. If concerns are however raised about falling masonry, de-scaling could be an appropriate action. Either of these approaches may create an unhappy aesthetic outcome. Descaling is the process of removing loose, delaminating and friable sections of stone from the building. However, the amount of stone that could be removed without replacement could eventually lead to structural concerns. In most cases, descaling should be the superficial removal of surface masonry only.


Plate No. 3: Delaminating argillaceous (clay rich) sandstone, Doune Castle That being said, the cultural significance that is placed upon a building would greatly influence the decision to remove delaminating stone or to consolidate (see plate 2). It is clear that cost would greatly increase if pinning, grouting and flaunching techniques were to be employed. This issue goes to the heart of philosophical driven interventions as one would be unlikely to use such crude conservation techniques such as descaling on monuments of international importance. Minimal (Least) intervention: Doweling and flaunching of natural stone Retention of delaminating masonry that may have been built face bedded (on ‘cant’) can be achieved by pinning the layers back together with the use of roughened nylon, or stainless steel dowels (often threaded bar), in conjunction with modified lime grouts. An example of the use of this technique can be seen in figure 1, in which a series of small holes are drilled through the surface of the unbound masonry. Grout is injected into the holes, into which the dowels can be inserted. The holes are then capped with a pigmented lime mortar to disguise the repair. Stainless steel threaded dowel (or roughened nylon) surrounded by a lime grout

Delaminating stone

Figure No.1: Dowelling techniques for delaminating stone


The lime based grout used as a consolidant can be modified with the addition of casein to increase its workability properties. The lime grouts set to form a calcium caseinate matrix rather than a pure calcium carbonate (calcite) or hydraulic lime [a combination of calcite and calcium silicate hydrates (C-S-H)] (Forster, 2002). Clearly, this type of intervention is useful for ensuring the survival of deteriorated masonry and can be considered as being a least intervention approach when compared with descaling or replacement stone. Minimal (Least) intervention: Replacement stone Replacement of deteriorating natural stone with a suitably matched stone type (determined by the British Geological Survey BGS) will lead to a repair that will have a longer life expectancy than alternative techniques. However, this may only be achieved by ‘cutting back’ sound masonry to a point at which sufficient bearing can be achieved to accommodate the new stone. Alternatively, stainless steel cramps can be used to retain thinner replacement stone. This technique could be argued to be better in terms of retaining the highest degree of original masonry, (as you are not cutting back masonry to the same degree) but could be criticised for introducing a non traditional repair process (i.e. cramping). Additionally, the longevity of the repair will be shorter than replacing thicker sections of stone. Building in a stone without cramps requires a minimum of 100mm of stone to be cut back from the surface, to enable safe bearing of the stone on the lower masonry units. It can be seen that a ‘trade off’ situation may arise where cost out-balances loss of historic fabric. An alternative approach is to utilise lime based plastic repair techniques.

Plate No. 4: Built in replacement natural stone without cramps The depth to which a stone needs to be built back into the substrate is of key importance. If a thin piece of stone is built into a wall without fixing restraint (say 40mm) it is possible that upon erosion of the mortar joint, rotation leading to falling masonry could occur, with potentially fatal consequences. Figure 2 diagrammatically represents this hypothetical situation.


40mm deep replacement stone

Eroded mortar joints leading to lowering of position of replacement stone

Potential for rotation of replacement stone and falling masonry due to increased clearance of stone upper bed and adjacent stones lower bed

Figure No.2: Insufficiently thicker replacement stone, leading to potential rotation Plate 4 illustrates the use of replacement natural stone built in a minimum of 100mm and substrate preparation for lime based plastic repairs. Minimal (Least) intervention: Indenting natural stone Indenting stone to ashlar or to carved and sculptural components is a traditional, highly skilled, repair technique. This is based upon the removal of localised damaged stone, replacing it with newly dressed and / or carved sections. Plate 5 illustrates a good example of a natural sandstone indent. These repairs are clearly least intervention in nature and enable the continuity of the aesthetics and integrity to be achieved. They are also initially honest as the intervention can be clearly read. However, overtime the blurring of the old and new fabric may occur as patination develops.

Plate No. 5: Indent to sandstone carved enrichment (Frauenchirk, Dresden) When approaching the recarving of a sculptural carved enrichment, good reproduction can be achieved using a ‘point’ machine that can minimise the degree of conjecture. Fixing of these indents can take various forms with dowelling, and grouting or bonding using epoxy resins being commonly adopted. The indent shown in plate 6 highlights that even the smallest sections of stone can be reattached and carved in situ. In this case the fingers of a statue in Dresden, have been reattached


Plate No. 6: Indent to hand of sculpture (Dresden) prior to being carved in-situ A well executed indent should be almost seamless with the adjacent stone. The poorly executed indent shown in plate 7 is clearly of a different geological composition to the surrounding host masonry and the joint width is unnecessarily large. In addition, the existing masonry has been cut out using a stihl saw and evidence of grinder marks can be seen in the right hand lower corner.

Plate No. 7: Poorly executed indent to ashlar sandstone As previously mentioned, various techniques can be used to ‘fix’ indents, ranging from lime grouts to epoxy resins. The defensibility of the use of lime is probably greater than the use of resin, however, a case for resin bonded indents could be made if it is sparingly used (BS7913, 1998). Over-hanging indents for repairs to cornices and string courses must be well designed, executed and utilise dowelling techniques. If this is not achieved the likelihood of falling masonry is increased. Alarmingly, 1275 incidents of falling materials and debris from buildings were reported by 25 local authorities, over a 2 year period, and of these 80% involved buildings greater than 100 years old (Soanne, 2008). Masonry accounted for 40% of the reports, with the greatest number of issues relating to external walls.


Minimal (Least) intervention: Plastic repair The use of plastic repairs may have advantages in terms of retaining the highest quantity of original fabric. Ashurst and Ashurst (1988:36) establish criteria to aid the decision making process for the utilisation of plastic repairs, amongst which the following questions are to be posed ‘will the use of mortar enable more original material to be retained than if stone was used? And will the use of mortar avoid disturbing critically fragile areas?’ Disadvantages of these forms of repair include that they will not be as durable, or as aesthetically acceptable as replacing natural stone. The use of Ordinary Portland Cement based plastic repairs, are clearly indefensible as they are technically incompatible with the masonry substrate due to their tendency to entrap moisture and being relatively inflexible. These factors can effect the bond between the plastic repair and the masonry substrate (Forster, 2002) increasing the probability of laminar debonding (sheet failure) (Meek, 1996). Exacerbating this situation, a quantity of the masonry substrate is often still bound to the rear of the plastic repair, increasing the loss of historic masonry fabric. When undertaking plastic repair (see plate 8) various techniques can be utilised to ensure bond to the substrate. Suction bonds are the generally accepted method for repair for non-overhanging interventions. These require the drawing of fine particles from the fresh mortar into the pore structure of the substrate, that upon setting and hardening develop a physically bond. The term suction bond is derived from the suction forces that draw the liquid water from the repair mortar into the substrate. The strength of these forces are related to the permeability, the pore size and pore size distribution of the host masonry. It must be emphasised that suction bonds are difficult to achieve in relatively impermeable masonry substrate types due to the inability of the binder to be drawn into the pores.

Plate No. 8: Execution of plastic repair to ashlar façade, Edinburgh In situations where the efficacy of suction bonds are potentially limited, armatures can be included. Traditional armature methods include: - Slate / tile housed in recessed pockets (adhered in epoxy resin) - Slate / tile housed in recessed pockets (adhered in lime grouts) - Ceramic armatures - Non ferrous wire frames


The efficacy of armatures used is a function of substrate quality, the number of fixings, the spacing between them, the stability and strength of the wire frame work (if applicable), and obviously the weight of the repair. Arguments for the use of ceramic armatures and tile pockets can be made on the basis that they have a greater compatibility with the substrate in terms of thermal and moisture movement. This is opposed to stainless steel that will have a greater coefficient of linear expansion than the substrate. The use of lime based plastic repairs are philosophically and technically more defensible than those previously mentioned, as lime was the traditional binder for initial construction. Although all plastic repairs are limited in design life when compared with replacement stone they are more often than not, cheaper to execute. The major advantages of utilising a lime mortar for plastic repair is that they have a higher permeability than OPC and are therefore more ‘breatheable’(Forster, 2002 & Hughes, 1986) and they have better flexural response (Allen et al, 2003). They are therefore more compatible with the lime and masonry built substrate. Colour matching of plastic repairs to the substrate can be achieved by various mechanisms. Traditionally, the colour of the lime mortar was derived from the aggregate and more specifically, the ‘fines’ (Gibbons, 2003). It is generally difficult to ascertain a true colour match with the surrounding stone, and hues will vary subtly. This could be argued to be an honest approach due to the differentiation of the old and new. The counter argument could be made that this debases the integrity of the building as a whole and a better match would go some way to rectify this situation. Minimal (Least) intervention: Profiled plastic repair Profiled repairs can be a suitable type of intervention that can lead to the conservation and retention of carved enrichments and ‘straight through’ work, such as string courses and cornices. This type of repair is executed by building up thin layers of lime mortar around an armature system, with the final profile being established with the use of a ‘horse’. It is the author’s view that plastic repairs are in most cases less suitable than replacing natural stone for weathering details. This is due to the exposure levels and increased incidence of surface water run off that they will encounter. Premature failure could be correlated with corresponding likelihood of falling masonry. In situations where the profiled repair overhangs, (for example, cornices) the likelihood of falling masonry may be significantly increased due to the dead load of the material. It is clear that even if a case was made to undertake such repairs on philosophical grounds, the increased risk of falling masonry, and the implications for health and safety legislation must be paramount in the designer’s mind. Minimal (Least) intervention: Sculptural stone replacement When replicating sculpted and carved stone it is clear that a great deal of evidence is required to avoid conjecture. If the sculpture is in good condition and removed off site, a ‘point machine’ can be used to produce a suitable likeness. This situation does not occur frequently and in reality sculptural pieces that have been externally located are eroded to greater or lesser degrees. The replication of these


objects are clearly prone to be a conjectural interpretation, with the newly carved piece being potentially misguiding and dishonest. A decision to remove a deteriorating externally located sculpture must not be taken lightly. That being said, when the piece has lost significant quantities of carved detail then the decision to take down and or re-carve may be necessary. Alternatively the commissioning of a contemporary piece could be considered as opposed to reproduction. Hill (1995) believes that the philosophy of repair for carved and sculpted pieces raises complex issues that do not require consideration for dimensional stone. That being said, the author believes that philosophically the same problems occur in both forms of repair as there is potential for conjecture in each. It is the author’s view that if insufficient documentary evidence exists to guide the replacement of a sculpture then a contemporary piece should be commissioned, that does not detract or compete with the building. This approach would be honest, support masonry craft skill, and if well executed, form an integral part of the building’s history. This type of modern intervention would undoubtedly be the product of talented and creative minds and potentially result in a product that may add to the ‘listing description’ of a building (Maguire, 1997). Minimal (Least) intervention: Reuse of existing pinnings When undertaking repointing to rubble masonry it is important, wherever possible to retain the original pinnings stones (also known as Galleting) for later reuse. The location of the pinning stones should be noted prior to raking out in an attempt to ensure the correct sequence for reinstating (Gibbons, 2003). This approach conforms to the principle of least intervention and will ensure that the masonry construction style is not compromised. Additionally, it increases the speed of the repair as the contractor does not need to source, sort or cut new pinning stones.

Plate No. 9: Rubble masonry with high quality pinning work The omission of pinning stones in lime pointing works will lead to failure of the mortar due to excessive shrinkage (Gibbons, 2003). From a philosophical perspective the use of pinnings are essential to enable the integrity of the fabric not to be compromised. See plate 9.


Legibility (Honesty and distinguishability) and Honest Repair Article 12 of the Venice Charter (1964) states that 'Replacements of missing parts (of fabric) must integrate harmoniously with the whole, but at the same time must be distinguishable from the original so that restoration does not falsify the artistic or historic evidence'. Legibility, or honest repair could be considered as a clear solution to the principle of avoidance of conjecture. It can utilise different materials and / or construction techniques to highlight new repairs to the host masonry. This approach can obviously create tension as it may visually detract from the integrity of the structure and introduce different performance characteristics into the building. Earl (2006:108) points out that 'Aggresively visible repairs can distract attention from the very qualites that mark out a building for preservation. A little discretion may be no bad thing'. The degree to which any repair is clearly distinguishable from original work, is often subjective. For example, when assessing the replacement of well matched natural stone, it is evident that the identification of new work from existing fabric may be difficult to distinguish especially post weathering. A geologist, may upon close examination of the masonry be capable of determining new from old, but to the lay person the fabric may appear seamless. This poses the question, is the distinguishability of repairs meant only for expert interpretation, or is it meant to be read by the lay person? The use of a stone type that is clearly different from the host would be readable, but would create technical performance deficiencies. These problems are generally associated with incompatible physical characteristics of the selected stone, leading to accelerated decay of the original fabric (Wilson, 2005). A philosophically, defensible intervention could be to utilise an alternative masonry type that is clearly distinguishable from the historic fabric. One such example of this can be seen in plate no. 10 in which the decayed stone has been replaced with red brick. Hill (1995:16) is critical of this approach believing that ‘making repairs in brick today for philosophical reasons serves simply to advertise that philosophy’. Obviously, brick, tile and stone slip repairs are very honest, but are criticised for being aggressively obtrusive and detracting from the integrity of the building as a whole. Additionally, technical arguments are often used to discredit the use of these types of repairs in so much as they have the potential to alter the performance of the wall, and more specifically, the moisture handling and load bearing characteristics. In theory if the historic fabric is composed of masonry that has a relatively high permeability function then the newly inserted impermeable brick will prohibit moisture transfer, leading to the alteration of the breatheability characteristics of the wall as a whole.


Plate No. 10: Honest repair to Roman walls in Barcelona That being said, alternative specification with high permeability, low strength bricks could act in a sacrificial manner and therefore aid the ultimate performance and survival of the historic masonry, whilst fulfilling the principle of honesty and distinguishability. Honest repair: Rebuilding masonry Reconstruction and rebuilding of masonry must be based upon solid evidence. If significant sections of masonry have deteriorated to such a degree that rebuilding is necessary to stabilise a structure, evidence must exist to direct the replacement. Approaches to this situation can include reconstruction adopting a masonry style and material that are influenced by the intact surrounding materials or reconstruction using alternative material and / or masonry styles, thereby, enabling honest interpretation of different construction phases. The approach taken clearly has ramifications for the integrity and aesthetic outcome of the building. The author has seen several examples of masonry stabilisation and reconstruction undertaken in obtrusive red brick inserted into the stone substrate. This approach has clearly been driven by the concept of honesty and distinguishability. However, an alternative approach could have been to utilise natural stone slips or an alternative masonry bonding style with less detriment to the integrity of the structure. Honest repair: profiled masonry The replication and reconstruction of tracery and other moulded masonry must be based on evidence (i.e. sufficient existing profile to enable templating). If the deterioration has occurred to such a degree that this evidence is tenuous then it may be necessary to adopt an honest repair approach creating a new moulding detail or a contemporary design. In most situations moulding detail may be sufficiently evident to enable templates to be produced without conjecture. Clearly, templates would be taken from the most representative surviving sections of masonry, reducing the degree of conjecture. Selective replacement voussiors have been undertaken at St Magnus Cathedral, Orkney. It has been assumed that sufficient detail existed to enable restoration to be achieved with a minimum of conjecture. That being said, if the degree of erosion in the voussiors has resulted in no meaningful evidence of carved enrichment being left then the decision to recarve and replace may lead to philosophical problems. On one level it could be assumed that as this is a Norman Cathedral, following an


architectural tradition, with common decorative forms, then it could be extrapolated that ‘dog tooth’ enrichment would have been most likely adopted. However, if in reality the masons deviated from the norm and created a transitional or freeform detail, would it still be the correct approach, or would this be conjectural and dishonest?

Plate No. 11: Recarving of voussiors to door arch, St Magnus Cathdral, Orkney As mentioned in Paper 1, this problem confronted those entrusted with the repair of York Minster’s, West door. The decision to utilise a contemporary design, rather than attempting to recreate the existing, avoided this situation. Honest repair: Tile repairs SPAB were pioneers in the development and adoption of honest repair techniques with tile repairs for masonry substrates being one solution. These repairs utilise tiles or natural stone slips that are built in a coursed manner. They are clearly distinguishable to the lay person and could therefore be considered philosophically defensible.

Plate No. 12: Honest repair in the form of plain tiles As previously mentioned criticism could, however, be made that they detract from the integrity and beauty of the structure. It is the author’s view that in certain masonry types these repairs can work aesthetically well, and it must be emphasised that a wide range of options are also available when specifying honest repair methods. For example, the use of stone slips in natural stone substrates are clearly honest, whilst being less obtrusive than the clay plain tile option. These repairs do not detract from the host masonry, and may therefore be considered as a more sensitive intervention.


The use of plain tile and stone slip repairs have been criticised for being labour intensive, and therefore cost ineffective especially when compared to replacing natural stone. There may be a certain amount of validity in this view, however, it must be emphasised that if cost were the basis for our repair strategies then many indefensible interventions would be undertaken. Honest repair: replacement stone The utilisation of tile repairs are clearly a response to concerns of deviation from the principle of honesty and distinguishability. This raises the question, is all replacement stone ultimately dishonest? What motivates some practitioners to select obtrusive repair over less distinguishable types of materials? These philosophically driven interventions are clearly in the fore of the minds of the professionals who choose to utilise tile repairs, in favour of replacement stone. Hill (1995:16) discusses replacement stone, believing ‘with some it has become an article of faith that no stone should ever be replaced on the grounds that the original fabric is sacrosanct. Also, that to put in new stone is an act of deception on the public, if not the present, then of the future’. Hill (1995:16) continues ‘if it is essential for any part of a building to be replaced, then the replacement should be in a foreign material such as tile or brick. Thus, the reasoning runs, there is no deception so far as the public is concerned, for replacements are obviously such. One may be permitted to ask what is being achieved by the insertion of tile, brick, or other foreign materials into an ancient building. Structural stability, yes, but, at the expense of the visual satisfaction that the public is entitled to expect. To carry this view to its logical conclusion, the collapse of a tracery window calls for the insertion of a single sheet of glass in a timber frame’. It is important to realise that honest repair is being discussed in a polarised manner by Hill (1995), and as we have seen there are various methods to achieve honesty, without complete compromise of the building’s integrity. It is clear that the main argument for the replacement of natural stone is that it ensures the integrity of the building and this appears to be especially important in ‘living’ buildings such as neo-classical ashlar built structures, whilst the use of honest repairs upon ‘dead’ buildings appear acceptable to Hill (1995). Hill (1995:17) takes a negative stance towards honest repair to masonry, highlighting that ‘English Heritage, the guiding guardian of historic sites and buildings, indulges in the academically suspect practice of ‘improving’ the appearance of archaeological sites by inserting stone where none existed at the time of excavation, so why should a historic building in current use not be repaired in the original material in order to preserve the original design? The overriding consideration must surely be for the integrity of the original design, It seems extraordinarily arrogant to say that ours is the last generation that will, instead, see only weathered and barely recognisable features in place of the chrisp outlines that were intended, set in a façade patched with brick and tile’. Hill (1995:18) clearly favours replacing fabric in natural stone highlighting that ‘there is nothing sacrosanct about a piece of worked stone, merely on the grounds that it once had a face or some detail on it that has long disappeared or become unsound’. This argument could be somewhat dangerous as it could be used to substantiate unnecessary replacement of fabric, on conjectural grounds.


It is clear that this is a contentious issue, and the author would like to emphasise that both methods of repair are suitable in the correct situation. It would be unhelpful to restrict the range of repair options available on the basis of dogma. Honest repair: Surface finishes (tooling) to natural stone The principle of honesty of repair also poses questions about finish techniques to natural stone. The majority of natural stone would have been finished with tooling effects derived from the masons’ chisels and force with which they are used. Chisels claws and punches all create different finishes and give valuable information relating to the construction and reduction techniques employed by the masons. In a situation, where natural stone has been used as the repair material then the surface finish to be applied should take guidance from the surrounding existing masonry. It is evident that very similar finish techniques applied to geologically similar replacement stone, could lead to indistinguisability. The use of variation in tooling finishes could be applied to differentiate the new from the old. However, the clear ability to distinguish the original from the repair may be diluted as the finishes weather back. The decision to identify new works through finishes may only therefore be satisfactory within shorter time frames. Plate 13 illustrates very poor quality surface finish to natural stone. The finish has been created with the use of an angle grinder and bears little relation to the tooled finish associated with the original masonry. It could however, be argued that it is honest and clearly distinguishable. This view does not however draw upon the need to support traditional skills.

Plate No. 13: Poor quality applied finishes to natural stone, bearing no relation to the original finish (or techniques used to apply them) There is no excuse for poor quality or ill conceived interventions that could be argued to be honest. This could be used to substantiate poor workmanship in almost any application. Good, well executed honest repairs require high standards of workmanship and design creativity. Honest repair: Refacing traditional mass masonry The complete replacement of a dressed masonry façade could be required if the structural integrity is compromised, however, this situation is not common. The decision to replace the complete façade is generally aesthetically driven. This approach is philosophically difficult to defend as it contravenes the principle of least 16

intervention, reversibility, respect for historic patina and eventually honestly and distinguishability. That being said, if one were to assess this approach applying Hill’s (1995) concept of this being a ‘living’ building then the ethical concept of integrity of structure would potentially override the aforementioned principles. Honest repair – direct dating A common method of denoting repairs is to directly date the work. This is achieved by carving the date of construction onto certain elements of the structure. This approach is helpful but obviously would not be applied to every stone. Additionally, it could be argued that it detracts from the aesthetics of the carved component and may not indicate the age of associated masonry works. Materials and Techniques (Like for like materials) A contentious issue in building conservation philosophy is that of the apparent conflict between the principles of honest repair, and like for like materials replacement. As previously discussed the selection of natural stone for the repair of a building is the correct approach if assessing the repair based upon like for like material replacement. However, if the decision making process for the project is skewed to honesty then a puritan approach may be to deviate from this stance and utilise clearly different repair materials and techniques. Clearly, repair options and finish effects to natural stone could go some way to avoid the philosophical problems previously mentioned, however, when we attempt to apply philosophical parameters to lime mortars it becomes somewhat difficult. The replication of historic mortars on one level is very easy to achieve as all mortars are composed of a binder, an aggregate and water (if hydraulic lime is used as opposed to non hydraulic putty limes). This oversimplification of the materials is however, unhelpful. That being said, mortar specification requires a great deal of consideration and should always be directed by analysis that is ideally multi phase in nature. The sample of existing mortar should determine the physical characteristics such as, the nature of the lime (whether, hydraulic or non hydraulic), the grading and type of the aggregate and the basic mix proportions. Mortars that attempt to replicate historic aged mortars can never be fully achieved, when assessed on a microstructure level. This is due to the fact that the physical composition of a mortar modifies over the life of material with dissolution and recrystalisation of the material altering the amorphous and crystalline nature of the binder, and pore structure (Forster, 2007). In addition, the complexity of historic mortars is considerably greater than their modern counterparts, with variability in the firing of the material leading to alteration in reactivity of the binder, yielding different hydrated forms of Calcium Silicate Hydrates (C-S-H), calcite and portlandite (Forster, 2004b) The incorporation of fuel waste, such as ash is also common, further complicating the situation. The aggregates within these historic mortars can also be more varied than those aggregates available for modern conservation works (Hughes et al, 2005).


Rather than attempting to replicate these materials it may be best to adopt a performance based specification approach. These types of specification utilise the performance characteristics of the existing mortar and host masonry to guide the repair material. These characteristics include, permeability, pore size and pore size distribution, compressive strength, flexural response, capillarity, and resistance to frost and salts. However, this does not help with the key issue of distinguishability and the question could be asked ‘is there sufficient ability to identify existing lime mortar from repair mortars?’ If not how do we reconcile the need for like for like materials, based upon technical performance, that may be considered dishonest. One potential answer could be to encourage different surface finish effects, but this still does not reconcile the problem of honesty once the finish has deteriorated. Reversibility The principle of reversibility is easy to achieve with certain intervention types, such as free standing secondary structural frames for alleviating loading etc. However, when we attempt to evaluate the reversibility of masonry repairs the situation becomes more complicated. BS 7913 (1998:3) defines reversibility as the ‘Concept of work to a building, part of a building or artefact being carried out in such a way that it can be reversed at some future time, without any significant damage having being done’. An example of a reversible repair is that of pointing that could be reversed by cutting out the relatively soft lime mortar, and the replacement of natural stone that could also be ‘cut out’. One of the main issues of irreversibility in this area is directed at the use of overly hard cement based mortars on weak backgrounds. This situation has the potential to damage the substrate if attempts are made to remove them (Gibbons, 2003) and they are therefore only reversible with consequences. Masonry interventions within the latter half of the 20th Century have seen the widespread use of epoxy resins. These repairs are irreversible and if used zealously their relative impermeability can cause moisture entrapment. The limited use of epoxy resins can have benefits for retaining nylon and stainless steel dowels. However, alternative approaches can generally be utilised with similar results. Well specified and executed consolidation techniques using lime grouts are considered philosophically more defensible than epoxy resins as they utilise traditional materials that are out of place in historic structures. That being said, BS 7913 (1998) indicate that in certain situations, the isolated use of epoxy resins can be useful. Earl (2006:172) cites the Burra Charter (1999, article 15) indicating that ‘Non reversible change should only be used as a last resort and should not prevent future conservation action.’ An example of an intervention of this nature is that of remedial works to mass masonry wall core voiding and subsequent grouting techniques. The treatment of wall core voiding is both technically and philosophically difficult to remedy. If the wall core is to be grouted, the hydraulicity of the grouting material will need to be relatively high. Therefore the hydraulic set will be required to be the primary set mechanism, as opposed to carbonation as a side reaction (Hughes & Swann, 1998) to ensure that the material sets sufficiently and creates the structural capability that is required from the intervention. This set will derive little from the carbonation set process as it would be difficult, if not impossible to achieve within the


depth of the wall core. Once the grouting process has been undertaken it would be virtually impossible to reverse without the removal of large sections of external masonry. In addition, the performance of the hardened grout will tend to be significantly different to that of the original wall core material (Beckman, 1995), and it is therefore not too unrealistic to assume that it would exhibit alteration of permeability, flexural response and compressive strength charateristics. Wall core mortars are generally, not dissimilar to bedding and construction mortars, in that they tend to be lime rich (Forster, 2004) and may have been manufactured using hot lime techniques. The authenticity of grouting materials are clearly questionable, however, these remedial works do pose technical and philosophical quandries. Reversibility: Structural repair and stabilisation Deflecting, bowing and leaning masonry walls may require remedial restraint to ensure survival. These interventions, should not be assessed in isolation and require evaluation of additional factors that have caused the structural defect (for example, failure of foundations and or defective roof timbers leading to rotation of the masonry wall). An honest repair approach was taken for the structural stabilisation of the towers at Fyvie Castle. The use of visually imposing straps are clearly honest, technically suitable and reversible, however, claims could be made that the intervention detracts from the integrity of the building and could be utilised to substantiate an alternative solution. See plate 14.

Plate No. 14: Honest repair in the form of stainless steel tying straps at Fyvie Castle The use of tie rods and externally located pattress plates are traditional methods of restraining lateral forces imposed upon the structure (Beckman, 1995). These interventions are reversible and are clearly honest as they visually express the repair. Alternative approaches, such as the construction of secondary buttressing to the external face of the masonry may also be reversible, but may be considered as being overly visually intrusive, detracting from the integrity of the building. Reversibility: Surface stone consolidation Problems associated with irreversible interventions are highlighted when assessing the use of ‘brethane’ which is a alkoxy silane consolidant (Ashurst and Ashurst, 1988:96).


These interventions prevent the masonry from breathing and also lead to the migration and formation of damaging salts that crystallise in the masonry. Paraloid ‘B’has been used as a stone consolidant for many years and is supposedly reversible with the use of a solvent to the treated area. However, when paraloid enters the pore structure of the porous material it will diffuse into the body of the material. It is difficult to determine the degree to which this can be removed satisfactorily. Lime water has been used as a traditional stone consolidant for many years. A calcium hydroxide [Ca (OH)2] saturated solution is sprayed onto the friable masonry and the process is repeated several times. The calcium hydroxide, is converted to calcite via carbonation with a corresponding binding effect between the sand or lime grains. The degree to which this process is reversible is questionable, however, in calcareous based sandstone and limestones the principle binding matrix is composed of calcite. These types of sandstone and lime stones should in theory benefit from lime water consolidation techniques. That being said, this technique does create problems as it introduces a great deal of water into the host material and can cause additional problems (Quayle, 1996). Sandstones that are argillaceous, or silaceous in nature are bound in different ways and the introduction of calcite into these materials may not be suitable. Recording and documentation Recording is defined in the Stirling Charter (2000:7) as ‘the description, depiction and analysis of any feature or area using drawings, survey, photographs and any other suitable means as well as the preservation of documents, photographs and other material relating to the feature or area in any earlier condition or use’ BS 7913 (1998:10) discusses recording and documentation stating ‘Records of work done, and of the fabric before, during and after the work should be maintained, and properly deposited and stored’. One of the most common uses of recording and documentation for masonry is that of templating undertaken prior to the works commencing. This is the process of tracing the outline of the masonry onto sheets of dimensionally stable tracing paper in conjunction with rigorous labelling / numbering system and photographic records. This can enable reasonable replication of the fabric when rebuilding is required. An alternative approach for masonry dis-assemblage (‘taking down’) is to use a rudimentary numbering system with the numbers being painted (with non permanent paint or chalk) directly on to the individual stones. The numbering should be undertaken in a logical manner to aid reconstruction. Once the numbering process has been completed a framed grid is then placed over masonry and photographs are taken. This combination of numbers and grid, enables relatively accurate reconstruction to occur. When dealing with fragments of masonry, a system of labelling can be used to direct rebuilding. Sustainability Sustainability has two meanings in the context of building conservation philosophy, namely, a ‘green’ agenda and also the perpetuation of a building’s utility. The ability


of a building to be in continuous use is essential for its survival, however, change must be sensitively managed. This view is discussed in Article 5 of the Venice Charter (1964) stating that 'The conservation of monuments is always facilitated by making use of them for some socially useful purpose’. Clearly alterations and additions to the masonry fabric, enabling this sensitive change to occur may be necessary. If these interventions are well designed, they should be readable, reversible and not detract from the integrity of the building. Summary This work does not profess to have definitive answers for masonry repair and should be seen as a mechanism to stimulate discussion between all parties involved in these processes. It is the author’s view that a great deal can be gained from better communication between craft and professional alike as both have valid contributions to make. Various techniques exist for the repair of historic buildings, some of which may be more defensible than others. That being said, they all have their place in the appropriate situation. The attitude and education of those specifying and undertaking the works may also vary considerably with consequent ramifications for the nature and sensitivity of the repairs selected. The selected repairs will not only depend on the technical issues but also the philosophical views held by the practitioner or overseeing statutory bodies. It is evident that the ethical concept of integrity (ostensibly discussed in Paper 1) of a structure is paramount in most practitioners minds. This potentially takes precedence over the principle of legibility or honest repair for those buildings that are still inhabited, or also known as ‘living’ buildings (Hill, 1995). For those buildings that are uninhabited, or in a ruinous state (also known as ‘dead’ buildings (Hill, 1995) the use of honest repair may achieve a higher degree of acceptance and be more widely adopted. The author is confident that if all parties involved in the repair of historic masonry structures undertake an evaluation of their repair strategies, set against the philosophical ‘principles’, then better, more considered conservation will be achieved. It is generally, those undertaking works who do not consider, or have no understanding of philosophy of repair who ultimately irreversibly damage our historic buildings. References Allen, G, Allen, J, Elton, N, Farey, N, Holmes, S, Livesey, P, Radonjic, M (2003) ‘Hydraulic Lime Mortar for Stone, Brick and Block Masonry’, Donhead , Dorset. Ashurst, J & Ashurst, N (1988), ‘Practical Building Conservation: stone masonry: English Heritage Technical Handbook Volume 1’, Gower Technical Press, Avon. Beckman, P (1995) ‘Structural Aspects of Building Conservation’ McGraw-Hill. Bell, D (1997) ‘Technical Advice Note 8: The Historic Scotland Guide to International Charters’ HMSO, Edinburgh.


Burman, P (1995) ‘A Question of Ethics’ (accessed, July 2009). Brereton, C. (1995) ‘The Repair of Historic Buildings-Advice on Principles and Methods’, London: English Heritage. BS 7913, (1998) ‘The principles of the conservation of historic buildings’ BSi. Burra Charter (1999) ‘International Charters for the Conservation and Restoration of Monuments and sites – ICOMOS’. Earl, J (2006) ‘Building Conservation Philosophy’ 3rd Ed, Donhead, Somerset. Feilden, B.M. (2003) Conservation of Historic Buildings, 3rd ed., Oxford: Architectural Press. Forster, A.M (2002) ‘An assessment of the relationship between the water vapour permeability and hydraulicity of lime based mortars with particular reference to building conservation materials science’, PhD Thesis, Heriot-Watt University. Forster A.M., (2004) ‘Hot lime mortars: a current perspective’. Journal of Architectural Conservation, Volume 3, Number 10, Nov. 2004, Pp 7-27. Forster A.M, (2004b) ‘How hydraulic lime binders work: hydraulicity for beginners and the hydraulic lime family’ Love your Building pub, Edinburgh. Forster A.M., (2007) ‘Binder loss in traditional mass masonry: a cause for concern?’ Structural Survey Vol 25, No. 2, 2007, Pp.148-170. Forster A.M., (2010) ‘Building conservation philosophy for masonry repair: Part 1 – Ethics,’ Structural Survey Vol 28, No. 2, Pp.tbc. Gibbons, P (2003) ‘The preparation and use of lime mortars’ Technical advice note 1, Historic Scotland. Hill, P (1995) ‘Conservation and the stonemason’, Journal of Architectural Conservation, Vol: 1 Part 2, pp. 7-20. Hughes J.J., Banfill P.F.G., Forster A.M., Livesey P., Nisbet S., Sagar J., Swift D.S., Taylor A., (2005) ‘Small-scale traditional lime binder and traditional mortar production for conservation of historic masonry buildings’, Proceedings of International Building Lime Symposium, Orlando, Florida, 2005, 1-13. Hughes, P. (1986) ‘The need for old buildings to ‘breathe’. (SPAB News Spring) Vol 18, pp1-3. Hughes, D & Swann, S, (1998) ‘Hydraulic limes – a preliminary investigation’ Journal of the building limes forum, Vol. 6, Privately published.


Maguire, R (1997) ‘Conservation and diverging philosophies’ Journal of Architectural Conservation Vol: 3: Part 1 pp. 7-18. Meek, T, (1996), 'Case studies of traditional lime harling: a discussion document', Kall Kwik publications. Powys, A.R (1995) ‘Repair of Ancient Buildings’ Society for the Protection of Ancient Buildings. Qualye, N.J.T (1996) ‘The case against limewater’ The Journal of the building Limes forum, Vol, 4 No.2 Privately Published. SPAB (1877) ‘Manifesto’ (accessed Septeber 2009). Soane, A (2008) ‘SCOSS: Confidential reporting on structural safety for Scottish buildings’ Scottish Building Standards Agency. The Society for The Protection of Ancient Buildings (SPAB) (2008) What is SPAB [Online]. Available from: [Accessed: 21/06/2009]. Stirling Charter (2000) ‘Historic Scotland’ HMSO. Venice Charter (1964) ‘International Charters for the Conservation and Restoration of Monuments and sites’ ICOMOS. Wilson, P (2005) ‘Building with Scottish stone’ Arcamedia.


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