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Building conservation philosophyfor masonry repair:

part 2 principlesAlan M. Forster

School of the Built Environment, Heriot-Watt University, Edinburgh, UK

Abstract

Purpose The techniques available for the repair of historic masonry structures are extremely wideranging. 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 tohistoric buildings are selected they must conform to building conservation philosophy, or an ethicaland principle based evaluation. The purpose of this paper (part 2 of 2) is to establish what is meant by

principles in this context and wherever possible apply practical examples to illustrate these concepts.Design/methodology/approach Evaluative literature review of the principles encapsulatedwithin building conservation philosophy utilising them to stimulate discussion on practical repairinterventions.

Findings It has been shown that the principles of building conservation philosophy must beconsidered prior to making decisions relating to masonry repair. These repairs have varying degreesof defensibility, and will ultimately lead to good or bad conservation approaches. This paper brieflydiscusses the principles, highlighting some of the issues that may be initially confusing to thepractitioner.

Originality/value The evaluation of building conservation philosophy for masonry repair, andmore specifically the principles have been little studied. The importance of this cannot however beover stated, as far from being an esoteric concept it affects every practical repair. This work bringstogether 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 emphasisedthat as with any aspect of philosophy, there is not necessarily a right or wrong answer, only higherlevels of defence for the selected repairs.

Keywords Building conservation, Regulation, Building specifications

Paper type Viewpoint

IntroductionThis paper is the second part of an investigation into the philosophy of masonry repair.The first paper (Forster, 2010) dealt ostensibly with the building conservation ethicsrelating to masonry repair for historic buildings, whilst this part will evaluate buildingconservation principles. The principles are specific criteria upon which

conservation works should be based (Bell, 1997, pp. 27-33), whilst ethics, form thebroader issues or key concepts to be considered. For this reason this paper has agreater technical focus than part 1. The majority of conservation principles wereestablished over 100 years ago by William Morris (SPAB, 1877) and his then newlyfounded Society for the Protection of Ancient Buildings.

Generally, ethics and principles should be considered in conjunction, however, forthe purpose of this work they have been separated. The principles include (Bell, 1997,pp. 27-33):

The current issue and full text archive of this journal is available at

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Buildingconservation

165

Structural SurveyVol. 28 No. 3, 2010

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0263-080XDOI 10.1108/02630801011058906


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Principles (specific criteria upon which conservation works should bebased)

. 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 principlesshould be defensible, and should in theory lead to naturally good, well foundedconservation interventions. However, it could be argued that some of the principlesconflict, creating tensions, potentially confusing the technical intervention decision. Itis 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 isthat there should be no dogmatic rules (Powys, 1995, p. 3). Powys emphasises thatno fixed rule can be set up to be followed invariably. Each case must be considered onits own merits.

As mentioned in the first paper, Burman (1995, p. 4) highlights that conservationphilosophy 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 historicbuildings. There is constant need for observation and debate, in a constructive andharmonious spirit. There are no goodies and baddies, but there are skilful andunskilful solutions to the repair of old buildings.

Each of the principles will be discussed individually, however, in situation whereclarification 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 extremelyimportant as the more fabric that is removed the less of the original building willremain, with a corresponding reduction in the cultural significance of the structure.

Minimal, or least intervention has been defined as, as much as is necessary(Brereton, 1995, p. 7) and, as little as possible (Feilden, 2003, p. 235).

The decision to replace deteriorated or defective masonry must be based upon need.A badly eroded facade may not necessarily require intervention. Conversely, if themasonry has eroded to such a degree that the masonry is loosening and beginning tocollapse or structural integrity is in question then intervention will be required. Thisdecision should be undertaken with the input from a suitably qualified conservationaccredited structural engineer. See Plate 1.

An extremely good example of over zealous masonry repair can be seen at theKeep at Newcastle-upon-Tyne (Plate 2). The decision to reface large sections of thetower, were probably driven by cost considerations, but the intervention has detractedfrom the integrity and authenticity of the building. The unnecessary works have led tomore historic fabric being removed than was necessary.

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Clearly all masonry components will eventually require intervention. It is the authorsview that prioritisation for selective replacement of badly deteriorated cornices, stringcourses and hood moulds would be easy to substantiate as they are weathering details,offering protection to the underlying substrate. That being said, these componentscould be repaired using less intrusive methods that would enable the retention of agreater 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 tradeoff situation is present, namely, retention of fabric against longevity of repair.

The repair options available to those attempting to repair historic masonry, rangefrom overzealous (leading to potential cost savings, but, indefensible conservation) tothe puritan (potentially philosophically good conservation, but costly). An example ofthis situation could be the use of pinning and dowelling techniques that would enablethe retention of a higher degree of historic fabric, rather than replacing masonry. Itmust however, be emphasised that good conservation need not be expensive and a

Plate 1.Eroded masonry facade at

Doune Castle


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puritan philosophical approach can in many cases be the least costly option. It is alsowell recognised that regular maintenance can be the most effective method to reducedecay, and as William Morris, stated stave off decay by daily care (SPAB, 2008).

When confronted with decaying masonry substrates, the following options may beconsidered:

. 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 and flaunching.. Consolidation and open wall head treatment.. Rebuilding.

Minimal (least) intervention: no intervention and descalingIn situations in which the masonry appears aesthetically poor but does not requireintervention doing nothing can be a defensible position. If concerns are however raisedabout falling masonry, de-scaling could be an appropriate action. Either of theseapproaches may create an unhappy aesthetic outcome.

Descaling is the process of removing loose, delaminating and friable sections ofstone from the building. However, the amount of stone that could be removed withoutreplacement could eventually lead to structural concerns. In most cases, descalingshould be the superficial removal of surface masonry only.

That being said, the cultural significance that is placed upon a building wouldgreatly influence the decision to remove delaminating stone or to consolidate (seePlate 3). It is clear that cost would greatly increase if pinning, grouting and flaunching

Plate 2.Newcastle-upon-Tyne,

Keep

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techniques were to be employed. This issue goes to the heart of philosophical driveninterventions as one would be unlikely to use such crude conservation techniques suchas descaling on monuments of international importance.

Minimal (least) intervention: doweling and flaunching of natural stoneRetention 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 ofsmall 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 arethen capped with a pigmented lime mortar to disguise the repair.

The lime based grout used as a consolidant can be modified with the addition ofcasein to increase its workability properties. The lime grouts set to form a calciumcaseinate 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 andcan be considered as being a least intervention approach when compared withdescaling or replacement stone.

Minimal (least) intervention: replacement stoneReplacement 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

Plate 3.Delaminating argillaceous

(clay rich) sandstone,Doune Castle


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longer life expectancy than alternative techniques. However, this may only be achievedby cutting back sound masonry to a point at which sufficient bearing can beachieved 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 interms of retaining the highest degree of original masonry, (as you are not cutting backmasonry to the same degree) but could be criticised for introducing a non traditionalrepair process (i.e. cramping). Additionally, the longevity of the repair will be shorterthan replacing thicker sections of stone. Building in a stone without cramps requires aminimum of 100 mm of stone to be cut back from the surface, to enable safe bearing ofthe stone on the lower masonry units. It can be seen that a trade off situation mayarise where cost out-balances loss of historic fabric. An alternative approach is toutilise lime based plastic repair techniques.

The depth to which a stone needs to be built back into the substrate is of keyimportance. If a thin piece of stone is built into a wall without fixing restraint (say40 mm) it is possible that upon erosion of the mortar joint, rotation leading to falling

masonry could occur, with potentially fatal consequences. Figure 2 diagrammaticallyrepresents this hypothetical situation.

Plate 4 illustrates the use of replacement natural stone built in a minimum of100 mm and substrate preparation for lime based plastic repairs.

Minimal (least) intervention: indenting natural stoneIndenting 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

Figure 1.Dowelling techniques fordelaminating stone

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stone, replacing it with newly dressed and/or carved sections. Plate 5 illustrates a goodexample of a natural sandstone indent. These repairs are clearly least intervention innature and enable the continuity of the aesthetics and integrity to be achieved. Theyare also initially honest as the intervention can be clearly read. However, overtime theblurring of the old and new fabric may occur as patination develops.

Figure 2.Insufficiently thicker

replacement stone, leadingto potential rotation

Plate 4.Built in replacement

natural stone withoutcramps


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When approaching the recarving of a sculptural carved enrichment, good reproductioncan 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 orbonding using epoxy resins being commonly adopted.

The indent shown in Plate 6 highlights that even the smallest sections of stone canbe reattached and carved in situ. In this case the fingers of a statue in Dresden, havebeen reattached.

A well executed indent should be almost seamless with the adjacent stone. Thepoorly executed indent shown in Plate 7 is clearly of a different geological compositionto 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 markscan be seen in the right hand lower corner.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 probablygreater than the use of resin, however, a case for resin bonded indents could be made ifit is sparingly used (BS7913, 1998).

Over-hanging indents for repairs to cornices and string courses must be welldesigned, executed and utilise dowelling techniques. If this is not achieved thelikelihood of falling masonry is increased. Alarmingly, 1,275 incidents of fallingmaterials and debris from buildings were reported by 25 local authorities, over atwo-year period, and of these 80 per cent involved buildings greater than 100 years old(Soane, 2008). Masonry accounted for 40 per cent of the reports, with the greatest

number of issues relating to external walls.

Minimal (least) intervention: plastic repairThe use of plastic repairs may have advantages in terms of retaining the highestquantity of original fabric. Ashurst and Ashurst (1988, p. 36) establish criteria to aidthe decision making process for the utilisation of plastic repairs, amongst which thefollowing questions are to be posed will the use of mortar enable more originalmaterial to be retained than if stone was used? And will the use of mortar avoid

Plate 5.Indent to sandstonecarved enrichment(Frauenchirk, Dresden)

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disturbing critically fragile areas? Disadvantages of these forms of repair include thatthey will not be as durable, or as aesthetically acceptable as replacing natural stone.

The use of Ordinary Portland Cement based plastic repairs, are clearly indefensibleas they are technically incompatible with the masonry substrate due to their tendencyto entrap moisture and being relatively inflexible. These factors can effect the bondbetween the plastic repair and the masonry substrate (Forster, 2002) increasing theprobability of laminar debonding (sheet failure) (Meek, 1996). Exacerbating thissituation, a quantity of the masonry substrate is often still bound to the rear of theplastic repair, increasing the loss of historic masonry fabric.

When undertaking plastic repair (see Plate 8) various techniques can be utilised toensure bond to the substrate. Suction bonds are the generally accepted method forrepair for non-overhanging interventions. These require the drawing of fine particlesfrom the fresh mortar into the pore structure of the substrate, that upon setting andhardening 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. Thestrength of these forces are related to the permeability, the pore size and pore sizedistribution of the host masonry.

It must be emphasised that suction bonds are difficult to achieve in relativelyimpermeable masonry substrate types due to the inability of the binder to be drawninto the pores.

In situations where the efficacy of suction bonds are potentially limited, armaturescan be included. Traditional armature methods include:

Plate 6.Indent to hand of

sculpture (Dresden) priorto being carved in-situ


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. 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.

Plate 8.Execution of plastic repairto ashlar facade,

Edinburgh

Plate 7.Poorly executed indent toashlar sandstone

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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 (ifapplicable), 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 andmoisture movement. This is opposed to stainless steel that will have a greaterco-efficient of linear expansion than the substrate.

The use of lime based plastic repairs are philosophically and technically moredefensible than those previously mentioned, as lime was the traditional binder forinitial construction. Although all plastic repairs are limited in design life whencompared with replacement stone they are more often than not, cheaper to execute. Themajor advantages of utilising a lime mortar for plastic repair is that they have a higherpermeability than OPC and are therefore more breatheable(Forster, 2002; Hughes,1986) and they have better flexural response (Allen et al., 2003). They are thereforemore compatible with the lime and masonry built substrate.

Colour matching of plastic repairs to the substrate can be achieved by variousmechanisms. Traditionally, the colour of the lime mortar was derived from theaggregate and more specifically, the fines (Gibbons, 2003). It is generally difficultto ascertain a true colour match with the surrounding stone, and hues will varysubtly. This could be argued to be an honest approach due to the differentiation ofthe old and new. The counter argument could be made that this debases theintegrity of the building as a whole and a better match would go some way torectify this situation.

Minimal (least) intervention: profiled plastic repairProfiled repairs can be a suitable type of intervention that can lead to the conservationand retention of carved enrichments and straight through work, such as stringcourses and cornices. This type of repair is executed by building up thin layers of limemortar around an armature system, with the final profile being established with theuse of a horse.

It is the authors view that plastic repairs are in most cases less suitable thanreplacing natural stone for weathering details. This is due to the exposure levels andincreased incidence of surface water run off that they will encounter. Premature failurecould be correlated with corresponding likelihood of falling masonry. In situationswhere the profiled repair overhangs, (for example, cornices) the likelihood of fallingmasonry may be significantly increased due to the dead load of the material. It is clearthat even if a case was made to undertake such repairs on philosophical grounds, theincreased risk of falling masonry, and the implications for health and safety legislation

must be paramount in the designers mind.

Minimal (least) intervention: sculptural stone replacementWhen replicating sculpted and carved stone it is clear that a great deal of evidence isrequired 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 havebeen externally located are eroded to greater or lesser degrees. The replication of these


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objects are clearly prone to be a conjectural interpretation, with the newly carved piecebeing potentially misguiding and dishonest.

A decision to remove a deteriorating externally located sculpture must not be takenlightly. 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 thecommissioning of a contemporary piece could be considered as opposed toreproduction.

Hill (1995) believes that the philosophy of repair for carved and sculpted piecesraises complex issues that do not require consideration for dimensional stone. Thatbeing said, the author believes that philosophically the same problems occur in bothforms of repair as there is potential for conjecture in each. It is the authors view that ifinsufficient documentary evidence exists to guide the replacement of a sculpture then acontemporary piece should be commissioned, that does not detract or compete with thebuilding. This approach would be honest, support masonry craft skill, and if wellexecuted, form an integral part of the buildings history. This type of modernintervention would undoubtedly be the product of talented and creative minds andpotentially result in a product that may add to the listing description of a building(Maguire, 1997).

Minimal (least) intervention: reuse of existing pinningsWhen undertaking repointing to rubble masonry it is important, wherever possible toretain the original pinnings stones (also known as Galleting) for later reuse. Thelocation of the pinning stones should be noted prior to raking out in an attempt toensure the correct sequence for reinstating (Gibbons, 2003). This approach conforms tothe principle of least intervention and will ensure that the masonry construction style isnot compromised. Additionally, it increases the speed of the repair as the contractordoes not need to source, sort or cut new pinning stones.

The omission of pinning stones in lime pointing works will lead to failure of themortar due to excessive shrinkage (Gibbons, 2003). From a philosophical perspectivethe use of pinnings are essential to enable the integrity of the fabric not to becompromised. See Plate 9.

Plate 9.Rubble masonry with highquality pinning work

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Legibility (honesty and distinguishability) and honest repairArticle 12 of the Venice Charter (1964) states that Replacements of missing parts (offabric) must integrate harmoniously with the whole, but at the same time must bedistinguishable 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 constructiontechniques to highlight new repairs to the host masonry. This approach can obviouslycreate tension as it may visually detract from the integrity of the structure andintroduce different performance characteristics into the building. Earl (2006, p. 108)points out that Aggresively visible repairs can distract attention from the veryqualites that mark out a building for preservation. A little discretion may be no badthing. The degree to which any repair is clearly distinguishable from original work, isoften subjective. For example, when assessing the replacement of well matched naturalstone, it is evident that the identification of new work from existing fabric may bedifficult to distinguish especially post weathering. A geologist, may upon close

examination of the masonry be capable of determining new from old, but to the layperson the fabric may appear seamless. This poses the question, is thedistinguishability of repairs meant only for expert interpretation, or is it meant to beread by the lay person? The use of a stone type that is clearly different from the hostwould be readable, but would create technical performance deficiencies. Theseproblems are generally associated with incompatible physical characteristics of theselected stone, leading to accelerated decay of the original fabric (Wilson, 2005).

A philosophically, defensible intervention could be to utilise an alternative masonrytype that is clearly distinguishable from the historic fabric. One such example of thiscan be seen in Plate 10 in which the decayed stone has been replaced with red brick.Hill (1995, p. 16) is critical of this approach believing that making repairs in bricktoday for philosophical reasons serves simply to advertise that philosophy.

Obviously, brick, tile and stone slip repairs are very honest, but are criticised for beingaggressively 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

Plate 10.Honest repair to Roman

walls in Barcelona


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repairs in so much as they have the potential to alter the performance of the wall, andmore specifically, the moisture handling and load bearing characteristics. In theory if thehistoric fabric is composed of masonry that has a relatively high permeability functionthen the newly inserted impermeable brick will prohibit moisture transfer, leading to the

alteration of the breatheability characteristics of the wall as a whole.That being said, alternative specification with high permeability, low strength

bricks could act in a sacrificial manner and therefore aid the ultimate performance andsurvival of the historic masonry, whilst fulfilling the principle of honesty anddistinguishability.

Honest repair: rebuilding masonryReconstruction and rebuilding of masonry must be based upon solid evidence. Ifsignificant sections of masonry have deteriorated to such a degree that rebuilding isnecessary 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 reconstructionusing alternative material and/or masonry styles, thereby, enabling honestinterpretation of different construction phases.

The approach taken clearly has ramifications for the integrity and aestheticoutcome of the building. The author has seen several examples of masonrystabilisation and reconstruction undertaken in obtrusive red brick inserted into thestone substrate. This approach has clearly been driven by the concept of honesty anddistinguishability. However, an alternative approach could have been to utilise naturalstone slips or an alternative masonry bonding style with less detriment to the integrityof the structure.

Honest repair: profiled masonry

The replication and reconstruction of tracery and other moulded masonry must be basedon evidence (i.e. sufficient existing profile to enable templating). If the deterioration hasoccurred to such a degree that this evidence is tenuous then it may be necessary to adoptan honest repair approach creating a new moulding detail or a contemporary design. Inmost situations moulding detail may be sufficiently evident to enable templates to beproduced without conjecture. Clearly, templates would be taken from the mostrepresentative surviving sections of masonry, reducing the degree of conjecture.

Selective replacement voussoirs have been undertaken at St Magnus Cathedral,Orkney. It has been assumed that sufficient detail existed to enable restoration to beachieved with a minimum of conjecture. That being said, if the degree of erosion in thevoussoirs has resulted in no meaningful evidence of carved enrichment being left thenthe decision to recarve and replace may lead to philosophical problems. On one level itcould be assumed that as this is a Norman Cathedral, following an architecturaltradition, with common decorative forms, then it could be extrapolated that dog toothenrichment would have been most likely adopted. However, if in reality the masonsdeviated from the norm and created a transitional or freeform detail, would it still bethe correct approach, or would this be conjectural and dishonest? (Plate 11)

As mentioned in Paper 1, this problem confronted those entrusted with the repair ofYork Minsters West door. The decision to utilise a contemporary design, rather thanattempting to recreate the existing, avoided this situation.

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Honest repair: tile repairs

SPAB were pioneers in the development and adoption of honest repair techniqueswith tile repairs for masonry substrates being one solution. These repairs utilisetiles or natural stone slips that are built in a coursed manner. They are clearlydistinguishable to the lay person and could therefore be considered philosophicallydefensible (Plate 12).

Plate 11.Recarving of voussoirs to

door arch, St MagnusCathdral, Orkney

Plate 12.Honest repair in the form

of plain tiles


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As previously mentioned criticism could, however, be made that they detract from theintegrity and beauty of the structure. It is the authors view that in certain masonrytypes these repairs can work aesthetically well, and it must be emphasised that a widerange of options are also available when specifying honest repair methods. For

example, the use of stone slips in natural stone substrates are clearly honest, whilstbeing less obtrusive than the clay plain tile option. These repairs do not detract fromthe 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 labourintensive, and therefore cost ineffective especially when compared to replacing naturalstone. There may be a certain amount of validity in this view, however, it must beemphasised that if cost were the basis for our repair strategies then many indefensibleinterventions would be undertaken.

Honest repair: replacement stoneThe utilisation of tile repairs are clearly a response to concerns of deviation from theprinciple of honesty and distinguishability. This raises the question, is all replacementstone ultimately dishonest? What motivates some practitioners to select obtrusiverepair over less distinguishable types of materials? These philosophically driveninterventions are clearly in the fore of the minds of the professionals who choose toutilise tile repairs, in favour of replacement stone.

Hill (1995, p. 16) discusses replacement stone, believing with some it has become anarticle of faith that no stone should ever be replaced on the grounds that the originalfabric is sacrosanct. Also, that to put in new stone is an act of deception on the public, ifnot the present, then of the future. Hill (1995, p. 16) continues if it is essential for anypart of a building to be replaced, then the replacement should be in a foreign materialsuch 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 isbeing achieved by the insertion of tile, brick, or other foreign materials into an ancientbuilding. Structural stability, yes, but, at the expense of the visual satisfaction that thepublic is entitled to expect. To carry this view to its logical conclusion, the collapse of atracery window calls for the insertion of a single sheet of glass in a timber frame. It isimportant 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, withoutcomplete compromise of the buildings integrity.

It is clear that the main argument for the replacement of natural stone is that itensures the integrity of the building and this appears to be especially important inliving buildings such as neo-classical ashlar built structures, whilst the use of honestrepairs upon dead buildings appears acceptable to Hill (1995).

Hill (1995, p. 17) takes a negative stance towards honest repair to masonry,highlighting that English Heritage, the guiding guardian of historic sites andbuildings, indulges in the academically suspect practice of improving the appearanceof 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 materialin order to preserve the original design? The overriding consideration must surely befor the integrity of the original design. It seems extraordinarily arrogant to say thatours is the last generation that will, instead, see only weathered and barely

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recognisable features in place of the crisp outlines that were intended, set in a facadepatched with brick and tile.

Hill (1995, p. 18) clearly favours replacing fabric in natural stone highlighting thatthere 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 substantiateunnecessary replacement of fabric, on conjectural grounds.

It is clear that this is a contentious issue, and the author would like to emphasisethat both methods of repair are suitable in the correct situation. It would be unhelpfulto restrict the range of repair options available on the basis of dogma.

Honest repair: surface finishes (tooling) to natural stoneThe principle of honesty of repair also poses questions about finish techniques tonatural stone. The majority of natural stone would have been finished with toolingeffects derived from the masons chisels and force with which they are used.

Chisels claws and punches all create different finishes and give valuableinformation relating to the construction and reduction techniques employed by themasons.

In a situation, where natural stone has been used as the repair material then thesurface finish to be applied should take guidance from the surrounding existingmasonry. It is evident that very similar finish techniques applied to geologicallysimilar replacement stone, could lead to indistinguisability. The use of variation intooling finishes could be applied to differentiate the new from the old. However, theclear ability to distinguish the original from the repair may be diluted as the finishesweather back. The decision to identify new works through finishes may only thereforebe satisfactory within shorter time frames.

Plate 13 illustrates very poor quality surface finish to natural stone. The finish hasbeen created with the use of an angle grinder and bears little relation to the tooledfinish associated with the original masonry. It could however, be argued that it ishonest and clearly distinguishable. This view does not however draw upon the need tosupport traditional skills.

There is no excuse for poor quality or ill-conceived interventions that could beargued to be honest. This could be used to substantiate poor workmanship in almostany application. Good, well-executed honest repairs require high standards ofworkmanship and design creativity.

Honest repair: refacing traditional mass masonryThe complete replacement of a dressed masonry facade could be required if the

structural integrity is compromised, however, this situation is not common. Thedecision to replace the complete facade is generally aesthetically driven. This approachis philosophically difficult to defend as it contravenes the principle of leastintervention, reversibility, respect for historic patina and eventually honestly anddistinguishability.

That being said, if one were to assess this approach applying Hills (1995) concept ofthis being a living building then the ethical concept of integrity of structure wouldpotentially override the aforementioned principles.


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Honest repair direct datingA common method of denoting repairs is to directly date the work. This is achieved bycarving the date of construction onto certain elements of the structure. This approachis 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 notindicate 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 conflictbetween the principles of honest repair, and like for like materials replacement. Aspreviously discussed the selection of natural stone for the repair of a building is thecorrect 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 apuritan approach may be to deviate from this stance and utilise clearly different repairmaterials and techniques.

Clearly, repair options and finish effects to natural stone could go some way toavoid the philosophical problems previously mentioned, however, when we attempt toapply philosophical parameters to lime mortars it becomes somewhat difficult.

The replication of historic mortars on one level is very easy to achieve as all mortarsare composed of a binder, an aggregate and water (if hydraulic lime is used as opposedto non hydraulic putty limes). This oversimplification of the materials is however,unhelpful. That being said, mortar specification requires a great deal of considerationand should always be directed by analysis that is ideally multi phase in nature. Thesample of existing mortar should determine the physical characteristics such as, the

Plate 13.

Poor quality appliedfinishes to natural stone,bearing no relation to theoriginal finish (ortechniques used to applythem)

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nature of the lime (whether, hydraulic or non hydraulic), the grading and type of theaggregate 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 andrecrystalisation of the material altering the amorphous and crystalline nature of thebinder, and pore structure (Forster, 2007). In addition, the complexity of historic mortarsis considerably greater than their modern counterparts, with variability in the firing ofthe material leading to alteration in reactivity of the binder, yielding different hydratedforms of Calcium Silicate Hydrates (C-S-H), calcite and portlandite (Forster, 2004b). Theincorporation of fuel waste, such as ash is also common, further complicating thesituation. The aggregates within these historic mortars can also be more varied thanthose aggregates available for modern conservation works (Hughes et al., 2005).

Rather than attempting to replicate these materials it may be best to adopt aperformance-based specification approach. These types of specification utilise theperformance characteristics of the existing mortar and host masonry to guide therepair material. These characteristics include, permeability, pore size and pore sizedistribution, compressive strength, flexural response, capillarity, and resistance tofrost and salts. However, this does not help with the key issue of distinguishability andthe question could be asked is there sufficient ability to identify existing lime mortarfrom 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 potentialanswer could be to encourage different surface finish effects, but this still does notreconcile the problem of honesty once the finish has deteriorated.

ReversibilityThe principle of reversibility is easy to achieve with certain intervention types, such as

freestanding secondary structural frames for alleviating loading etc. However, whenwe attempt to evaluate the reversibility of masonry repairs the situation becomes morecomplicated. BS 7913 (1998, p. 3) defines reversibility as the Concept of work to abuilding, part of a building or artefact being carried out in such a way that it can bereversed 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 bycutting out the relatively soft lime mortar, and the replacement of natural stone thatcould also be cut out. One of the main issues of irreversibility in this area is directedat the use of overly hard cement based mortars on weak backgrounds. This situationhas 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 twentieth century have seen the

widespread use of epoxy resins. These repairs are irreversible and if used zealouslytheir relative impermeability can cause moisture entrapment. The limited use of epoxyresins can have benefits for retaining nylon and stainless steel dowels. However,alternative approaches can generally be utilised with similar results. Well specified andexecuted consolidation techniques using lime grouts are considered philosophicallymore defensible than epoxy resins as they utilise traditional materials that are out ofplace in historic structures. That being said, BS 7913 (1998) indicates that in certainsituations, the isolated use of epoxy resins can be useful.


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Earl (2006, p. 172) cites the Burra Charter (1999, article 15) indicating that Nonreversible change should only be used as a last resort and should not prevent futureconservation action. An example of an intervention of this nature is that of remedialworks to mass masonry wall core voiding and subsequent grouting techniques.

The treatment of wall core voiding is both technically and philosophically difficultto remedy. If the wall core is to be grouted, the hydraulicity of the grouting materialwill need to be relatively high. Therefore the hydraulic set will be required to be theprimary set mechanism, as opposed to carbonation as a side reaction (Hughes andSwann, 1998) to ensure that the material sets sufficiently and creates the structuralcapability that is required from the intervention. This set will derive little from thecarbonation set process as it would be difficult, if not impossible to achieve within thedepth of the wall core. Once the grouting process has been undertaken it would bevirtually impossible to reverse without the removal of large sections of externalmasonry. In addition, the performance of the hardened grout will tend to besignificantly different to that of the original wall core material (Beckman, 1995), and itis therefore not too unrealistic to assume that it would exhibit alteration ofpermeability, flexural response and compressive strength characteristics. Wall coremortars are generally, not dissimilar to bedding and construction mortars, in that theytend to be lime rich (Forster, 2004a) and may have been manufactured using hot limetechniques. The authenticity of grouting materials are clearly questionable, however,these remedial works do pose technical and philosophical quandaries.

Reversibility: structural repair and stabilisationDeflecting, bowing and leaning masonry walls may require remedial restraint to ensuresurvival. These interventions, should not be assessed in isolation and require evaluationof additional factors that have caused the structural defect (for example, failure offoundations 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 atFyvie Castle. The use of visually imposing straps is clearly honest, technically suitableand reversible, however, claims could be made that the intervention detracts from theintegrity of the building and could be utilised to substantiate an alternative solution.See Plate 14.

The use of tie rods and externally located pattress plates are traditional methods ofrestraining lateral forces imposed upon the structure (Beckman, 1995). Theseinterventions are reversible and are clearly honest as they visually express the repair.Alternative approaches, such as the construction of secondary buttressing to theexternal face of the masonry may also be reversible, but may be considered as beingoverly visually intrusive, detracting from the integrity of the building.

Reversibility: surface stone consolidationProblems associated with irreversible interventions are highlighted when assessingthe use of brethane which is a alkoxy silane consolidant (Ashurst and Ashurst, 1988,p. 96). These interventions prevent the masonry from breathing and also lead to themigration and formation of damaging salts that crystallise in the masonry.

Paraloid B has been used as a stone consolidant for many years and is supposedlyreversible with the use of a solvent to the treated area. However, when paraloid enters

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the pore structure of the porous material it will diffuse into the body of the material. Itis 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 masonryand the process is repeated several times. The calcium hydroxide, is converted tocalcite via carbonation with a corresponding binding effect between the sand or limegrains. The degree to which this process is reversible is questionable, however, incalcareous based sandstone and limestones the principle binding matrix is composedof calcite. These types of sandstone and limestones should in theory benefit fromlimewater consolidation techniques. That being said, this technique does createproblems as it introduces a great deal of water into the host material and can causeadditional problems (Quayle, 1996). Sandstones that are argillaceous, or silaceous innature are bound in different ways and the introduction of calcite into these materialsmay not be suitable.

Recording and documentationRecording is defined in the Stirling Charter (Historic Scotland, 2000, p. 7) as thedescription, 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 conditionor use

BS 7913 (1998, p. 10) discusses recording and documentation stating Records ofwork done, and of the fabric before, during and after the work should be maintained,

Plate 14.Honest repair in the form

of stainless steel tyingstraps at Fyvie Castle


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and properly deposited and stored. One of the most common uses of recording anddocumentation for masonry is that of templating undertaken prior to the workscommencing. This is the process of tracing the outline of the masonry onto sheets ofdimensionally stable tracing paper in conjunction with rigorous labelling/numbering

system and photographic records. This can enable reasonable replication of the fabricwhen rebuilding is required.

An alternative approach for masonry dis-assemblage (taking down) is to use arudimentary numbering system with the numbers being painted (with non permanentpaint or chalk) directly on to the individual stones. The numbering should beundertaken in a logical manner to aid reconstruction. Once the numbering process hasbeen completed a framed grid is then placed over masonry and photographs are taken.This combination of numbers and grid, enables relatively accurate reconstruction tooccur.

When dealing with fragments of masonry, a system of labelling can be used todirect rebuilding.

SustainabilitySustainability has two meanings in the context of building conservation philosophy,namely, a green agenda and also the perpetuation of a buildings utility. The abilityof a building to be in continuous use is essential for its survival, however, change mustbe 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 ofthem for some socially useful purpose.

Clearly alterations and additions to the masonry fabric, enabling this sensitivechange to occur may be necessary. If these interventions are well designed, they shouldbe readable, reversible and not detract from the integrity of the building.

SummaryThis work does not profess to have definitive answers for masonry repair and shouldbe seen as a mechanism to stimulate discussion between all parties involved in theseprocesses. It is the authors view that a great deal can be gained from bettercommunication between craft and professional alike as both have valid contributionsto make.

Various techniques exist for the repair of historic buildings, some of which may bemore defensible than others. That being said, they all have their place in theappropriate situation. The attitude and education of those specifying and undertakingthe works may also vary considerably with consequent ramifications for the natureand sensitivity of the repairs selected. The selected repairs will not only depend on thetechnical issues but also the philosophical views held by the practitioner or overseeingstatutory bodies.

It is evident that the ethical concept of integrity (ostensibly discussed in Paper 1) ofa structure is paramount in most practitioners minds. This potentially takesprecedence over the principle of legibility or honest repair for those buildings that arestill inhabited, or also known as living buildings (Hill, 1995). For those buildings thatare uninhabited, or in a ruinous state (also known as dead buildings (Hill, 1995) theuse of honest repair may achieve a higher degree of acceptance and be more widelyadopted.

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The author is confident that if all parties involved in the repair of historic masonrystructures undertake an evaluation of their repair strategies, set against the philosophicalprinciples, 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

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Corresponding authorAlan M. Forster can be contacted at: [email protected]

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