Identifying and Sourcing Stone for Historic Building Repair

2006Identifying and Sourcing Stonefor Historic Building RepairAn approach to determining andobtaining compatible replacement stoneTechnical advice note

SummaryThis Technical Advice Note providesguidance for architects, surveyors,engineers, building managers, contractors,conservation officers and owners on thebest way of identifying and sourcing stonefor the repair of historic buildings andmonuments.Stone repair must be planned and carriedout with care and sensitivity, and requiresa sound knowledge of the characteristicsof the stone involved. The choice ofreplacement stone must be bothsympathetic and cost-effective. Whereverpossible compatible materials should beused – stone that closely replicates theoriginal in its appearance, chemical,physical and mineralogical properties,strength and durability.The aim should be to retain the maximumamount of original stone, wherever thisdoes not compromise the integrity of thebuilding. Maximum retention is often thepreferred option, but for inaccessiblestructures – such as spires, where thenecessary scaffolding is expensive – it maynot be the most cost-effective in the longterm.It is therefore acceptable to replacestone selectively to ensure structuralstability, or when it has decayed beyondrepair. Replacement may also beappropriate when the purity of thebuilding design is considered equallyvaluable, or more valuable, than theoriginal construction material – forexample, a 19th-century classical building.Successful repair also relies on a thoroughunderstanding of the historical context ofthe building and its surroundings.The selection process involves severalsteps:1 Establishing the historical and physicalsignificance of the building, and thelikely impact of any proposedintervention2 Assessing and understandingweathering and other decay processesaffecting the stone3 Undertaking an initial fabric/masonrysurvey to determine the need for stonereplacement4 Defining the types of stone used, byvisual examination in situ5 Answering any technical questions insteps 1–4 above, by using the mostappropriate analytical techniques onsamples taken from the site6 Obtaining samples of potentialreplacement stone, and testing thesewhere necessary7 Sourcing and securing replacementstone from existing quarries or (incertain cases) from quarries temporarilyre-opened for the purposeIn all but the smallest of projects, stoneselection for historic building repairdemands the combined skills andknowledge of a team of people experiencedin working with stone. Ideally theteam should be led by a stone consultant(usually a geologist or petrographer), whocan both identify the stone and find eitherthe most compatible petrographic matchfrom existing or new sources, or the mostclosely related alternatives. When stone isbeing sourced from a temporary quarry orfrom a new quarry, the stone consultantshould also be experienced in mineralplanning procedures.English Heritage supports the need forstrategic and sustainable sources of stonefor the conservation, repair, maintenanceand improvement (CRMI) of historicbuildings, and so is working with otherpartners to ensure that the environmentalimpact of quarrying is minimised.Addressing the wider issues arising fromsourcing and quarrying stone willcontribute as much to the long-termpreservation of our rich and diverseheritage as the building repair itself.Contents1 Introduction . . . . . . . . . . . . . . . . . 32 Evaluation and characterisationof the existing stonework . . . . . . 32.1 Visual and physical properties . . 32.2 Preliminary surveys . . . . . . . . . . . 43 Sampling and identification . . . . 43.1 Sampling . . . . . . . . . . . . . . . . . . . . 4Background informationrequired for sampling . . . . . . . . . . . 5Choosing sample locations . . . . . . . 5Preparing to take samples . . . . . . . 5Sampling fragile stone . . . . . . . . . . 6Planning stone sampling . . . . . . . . 6Photography . . . . . . . . . . . . . . . . . . 6Labelling samples . . . . . . . . . . . . . . 6Useful additional information . . . . 73.2 Examination and analysisof stone . . . . . . . . . . . . . . . . . . . . . 7Hand samples . . . . . . . . . . . . . . . . . 7Petrographic analysis . . . . . . . . . . . 7Physical parameters . . . . . . . . . . . . 8Other analytical techniques . . . . . . 84 Criteria for selectingreplacement stone . . . . . . . . . . . . 9Petrography . . . . . . . . . . . . . . . . . . 9Chemical characteristics . . . . . . . . . 9Appearance . . . . . . . . . . . . . . . . . . 9Geological age . . . . . . . . . . . . . . . . 9Porosity . . . . . . . . . . . . . . . . . . . . . . 9Compressive strength . . . . . . . . . . . 95 Sources of stone . . . . . . . . . . . . . . 95.1 Active quarries . . . . . . . . . . . . . . . 95.2 Disused quarries . . . . . . . . . . . . . 95.3 Recycled stone . . . . . . . . . . . . . . 106 Obtaining stone for historicbuilding repair . . . . . . . . . . . . . . 106.1 Specifying stone . . . . . . . . . . . . . 106.2 Samples of new stone . . . . . . . . 116.3 Testing replacement stone . . . . 116.4 Obtaining stone fromnew or temporary sources . . . . . 117 Sources of further informationand assistance . . . . . . . . . . . . . . . 117.1 Stone collections . . . . . . . . . . . . 117.2 Other sources of help . . . . . . . . 12Bibliography . . . . . . . . . . . . . . . . . . . . 12Useful contacts . . . . . . . . . . . . . . . . . 13Annex: A system for the fieldidentification of the morecommon building stonesused in England . . . . . . . . . . . . . 14Acknowledgements . . . . . . . . . . . . . . 152

1 IntroductionStone – abundant, readily available,workable and strong – has historicallybeen the most widely used naturalmaterial for constructing buildings,monuments, sculpture and carveddecoration. Throughout the world,cultural stature is closely related to thenumber of large stone buildings andpublic monuments, and through themillennia the extraction, transportationand working of basic rock forms hasbeen refined, often with considerableingenuity. On a local scale, use oflocally available stone and local masonshas produced distinctive vernacularbuilding styles.Although stone is something of ametaphor for permanence and durability,this can prove far from the truth.Composition varies, even within a singlesource, and a building stone may proveunstable, brittle or soft. Many limestones,for example, are particularly prone todecay. To preserve an historic building, itis often necessary to replace some of itsstone and regeneration, repair andmaintenance all help the building’ssustainability. For intervention to besuccessful, however, it is imperative thatthe historic and physical significance ofthe building and its current condition bedetermined before the repair programmeis designed.Stone sourcing forms part of the designand specification stage of repair work,enabling the work programme to remainon course and within budget. Stonesourcing for historic building repair musttake the following factors into account:● the matching stone should be similarin colour, texture and physicalproperties to the original● any intervention must not harm theoriginal building fabric● the roles and responsibilities of thosewithin the design team must be wellunderstood● all work must comply with the ListedBuilding Consent framework andBritish and European StandardsMatching stone is often notstraightforward. Although it may bepossible to distinguish common stonetypes on the basis of colour and texture –sandstone from slate, Bath limestonefrom Ketton limestone – there will bemany subtle colour differences, texturalchanges and variations: striations andsedimentary structures, grain sizes,porosities and cementation, andmineralogical incompatibilities withingeneric stone types. It is exactly thesefine details that may be critical to asuccessful repair, but their recognitionrequires expertise and experience.Historic building architects, stone-tradeconsultants and masonry contractorswill often be able to undertake basicvisual identification and sourcing,especially with common stones suchas Portland limestone, or wherebackground information is available.However, specialist advice will beneeded to identify more complex stonetypes or to diagnose unusual problems.Stone consultants – geologists orpetrographers – can be useful guides forclient and contractor throughout theprocurement process.Stone sourcing should be part of thesub-contracted masonry package. If itis left out of the procurement process,there will be insufficient time for selectingand ordering, expediency will replaceplanning and unsuitable stones or stonesupplies may be selected. This couldhave a drastic visual and physicalimpact, and as the replacement stonecannot usually be remedied withoutgreat expense, it may remain as anuncomfortable example of bad design,or have a possible long-term disfiguringand damaging effect on the historicbuilding’s fabric.Expensive errors can be avoided byincluding a stone consultant in the basicproject team. Operating as a link betweenthe project architect or consultant,and the contractor and masonry subcontractors,the stone consultant canalso control quality when large amountsof stone are being supplied. On occasionsthey may work in conjunction with amasonry trade consultant.Common stone procurement routesOwner – Architect – (Planner) – Architect –Contractor – QuarryOwner – Architect – (Planner) – Architect –Contractor – Masonry Contractor – QuarryOwner – Architect – (Planner) – Architect –Project Management Company – MasonryPackage – Quarry2 Evaluation and characterisationof the existing stonework2.1 Visual and physical propertiesMost old buildings have a history ofalterations and additions. Often theoriginal fabric incorporated severaldifferent kinds of stone, and others mayhave been introduced during subsequentrepairs and restorations.In most cases the dominant stone is oflocal origin, but other stone types mayhave been needed for different elements ofthe construction – stone suitable forwalling may not be sufficiently durable forexposed areas, such as quoins or parapets.The flextural strength required for loadbearingelements such as lintels, and theneed for large block sizes for certaindesign requirements may also havedemanded different varieties. Somechoices may have been purely aesthetic.At times it may have proved necessary toimport suitable stone from outside thelocal area. The stone used at an earlierstage of building may no longer have beenavailable in subsequent phases: often theremaining stone in the original quarry wasnot suitable for construction purposes.It is also not unusual to find that stonehas been salvaged and re-used (Fig 1).Fig 1 Anadolu Kavagi, near Istanbul, Roman stone, restoredand reinforced by the Genoans c 1350. (David Heath)Every type of stone has unique properties,and a distinct manner of weathering anddecay. The rate and manner ofdeterioration is dependent upon notonly on the stone’s composition, but alsoon the manner in which it was quarriedand worked, its final use and theenvironment in which it is placed. Theremay be faults in the building design andconstruction that cause local decay.Even sound stone elements will be subjectto the various chemical processes andmechanical stresses that can, over time,lead to internal and superficialdeterioration, and eventually may causecomplete disintegration.3

The rate of decay will vary, especially forexposed stone, depending on theconditions around and within thebuilding: for example, wind, rain, thermalvariations, frost action, atmosphericpollution and biological activity. Theremay be a delicate balance between thestonework and the prevailing climate; ifthis is destabilised deterioration canaccelerate.Changes to the fabric, such as theinsertion of a different type of stone or theenclosure of an external wall (especially ina heated area), can have a harmful effecton the existing stonework. Thejuxtaposition of two incompatible stonescan lead to adverse chemical reactions: forexample, placing limestone above certainsandstones can lead to decay as alkalinemoisture leaching out of the limestonereacts with clay or reactive silica bindersin the sandstone.Natural decay processes will beaccelerated by poor design andconstruction, such as incorrect orientationof bedding planes (so that blocks areedge- or face-bedded), or permeable stonebeing used for rain-shedding features suchas copings and parapets. The resultingingress of moisture can lead todeterioration elsewhere in the building.Studying the construction of the originalbuilding and the current condition of thestonework and its surroundings istherefore essential to determine the needfor repair, and for choosing the bestmanner of replacing severely decayedelements.2.2 Preliminary surveysThe initial step is to survey the fabric ofthe building in detail, recording thelocation of the different types of stone,identifying and distinguishing the stonesused, and determining whether these areoriginal or later additions.Condition surveys by architects andsurveyors can help identify the factorsgoverning the choices of stone. If possiblearchival sources should be studied, tocollect as much information as possibleabout the building’s history, from originalconstruction to the present day. Suchresearch often provides useful insights intothe design, the sources of original andadded materials, the methods ofconstruction and the originalworkmanship and that of subsequentalterations.Fig 2 Shap granite showing the scattered large sedimentaryphenocrysts of feldspar (field of view 60mm wide).(David Jefferson)The fundamental fabric/masonry surveyand visual inspection must also determinethe need for, and extent of, any requiredrepair, identify the most suitable stonetypes for replacement and decide on whatskills and personnel the project will require.During the initial survey it is notnecessary to identify each stone type in astrict geological sense, although variationswithin a general lithological type – thatmay have resulted from similar stonesbeing obtained from different sources –should be recorded.Features to document include: banding,inclusions of different coloured materialwithin igneous or metamorphic rocksFig 3 Whitby Abbey, North Yorkshire: various large-scalefeatures, such as grain size variations and cross-bedding, on250mm high column drums. (David Jefferson)(Fig 2) and the orientation of bedding andchanges in grain size between layers insedimentary rocks (Fig 3).For sedimentary stones, the height ofthe masonry courses should be recorded.Unless the blocks have been used edgebedded,course height usually reflectsthe thickness of the beds in the quarry.The relative proportions of course heightscan be invaluable, helping a stoneconsultant or geologist to identify theoriginal source.Recordings should be drawn by hand onsite as accurate line drawings, for exampleproduced from a photogrammetric surveyof the building or documented on toenlarged digital photographs.3 Sampling and identificationReasons for professional analysis● to identify the type and source ofstone so as to obtain an acceptablereplacement● to understand the manner in which astone is weathering or decaying, toestablish measures to slow the rate ofdeterioration● to determine the nature and distributionof contaminants within the pores ofthe stone, and their possible effect onthe stone and on the surrounding fabricof the buildingInformation required by the analyst● the aim of the petrographic analysis,and how the information will be used● the history, age and condition of thebuilding● the nature and extent of visible decaymechanisms● the context and location of specificstones or areas of the building beingstudied3.1 SamplingSamples can be used to investigate severalaspects of the stone and its deterioration.The type of sample preparation, theanalysis undertaken and the resultingrecommendations will depend on theproblem being investigated. Sampling isinvasive, and should not be considered aroutine procedure.The following is a guide to the type ofinformation needed for differentinvestigations.4

Background information requiredfor samplingThe nature of the problem being studiedmay make it necessary to sample in aspecific manner, so the analyst’s adviceshould always be sought before samplingbegins. In many cases – such as studiesof rates of weathering or deterioration,the effects of previous treatments or saltcontamination – it is imperative toexplain the precise nature of the problemwhen submitting the samples, as thismay influence the sample preparation.If previous treatments are being assessed,it is necessary to gather backgroundinformation, particularly about thematerials used. This should includechemical, physical and ageing properties,weathering and deteriorationcharacteristics of the treatments andpotentially harmful effects on both thetreated stone and on any surroundinguntreated stone.Choosing sample locationsIt is essential to obtain representativesamples of the stone to address therelevant problem. For instance, samplingweathered material can lead tomisinterpretations of the original natureof the stone. As an example, sandstonecemented with calcite will have completelydifferent properties from sandstonecemented with clay minerals. Whenweathering occurs, both will eventuallyconsist of sand grains separated by voidsfrom which the cement has been leachedand appear remarkably similar.Misinterpretation can lead to incorrectchoices of replacement stone, or toinappropriate conservation strategies forthe surviving stonework. Similarly,sampling only the non-decayed portionsof blocks is of little use if the problembeing investigated involves any interactionbetween the stone and the bedding orpointing mortar.The sample taken must be truly typical.If there are apparent variations in thestone – for example in colour, textureor type of weathering – wherever possiblesamples should be collected of eachvariation.Samples should always be taken directlyfrom the building and not by collectingfallen stone as its history and whether ornot it is representative will always be indoubt. If a piece of stone has detachednaturally from the fabric of the building,its weathering may well bemore advanced, and so thepetrographic characteristicsmay not be the same asthe stone still in situ.Preparing to take samplesSampling the stonework of animportant historic building orancient monument isdestructive, but the damageincurred by taking arepresentative sample must beassessed against the potentialdeterioration and loss ifunrepresentative samplingshould lead to inappropriatetreatment. Large samples canoften be sub-sampled for thelaboratory analysis and thenrepaired and placed backin situ during the repair work.Samples should be selected to have oneweathered face, with the remaindernon-weathered. The sample’s orientationmust be recorded (Fig 4): this caneither be indicated in the informationsupplied with the sample, or if thesample is sufficiently large, and where itwould not obscure any useful surfacedetail, marked directly on the outer faceof the stone.The optimum sample size will dependon the homogeneity and grain size of thestone. For uniform fine-grained stone,samples measuring 75mm 75mm 40mm are generally adequate, whereasa coarse-grained stone will requiresamples 150mm 150mm 80mm.Where a sample must be taken from thecorner of a block with two weatheredfaces, it must be large enough to includesome of the non-weathered inner core.If samples are required from carveddetail, a small-diameter core drill canbe used to collect material fromconcealed surfaces. The resulting holescan then be surface-filled by a specialistconservator.The site for a sample must be carefullychosen so as not to disfigure the building,although this must not result in thesample being unrepresentative.Permission to sample must be secured from therelevant authority: a faculty for Church ofEngland buildings, the equivalent forother religious authorities or ScheduledMonument Consent from the Departmentfor Culture, Media and Sports throughOrienting a sample.Having knocked the sample off the selected portion ofthe fabric, replace it in the block and mark the top andthe orientation of at least one face. If the top is not aflat surface, an upwards pointing arrow on one of theside faces can be used to indicate the top of the piece.Fig 4 Marking the orientation on a removed stone sample. (David Jefferson)English Heritage for ScheduledMonuments. For listed buildings, theLocal Authority Conservation Officermust be consulted. Ideally an officialobserver such as a Local AuthorityConservation Officer or an EnglishHeritage Historic Buildings Architector Inspector should be present duringsampling, to agree on sample locationsand sizes.It is always preferable that sampling becarried out by an experienced specialist.Before sampling, the area should bephotographed (see below). If large-scalecolour banding or grain size variationsexist, this should be captured withclose-up photographs, and separatesamples should be taken of each of thedifferent bands.European standards for samplingAlthough harmonised European Standards(BS ENs) exist to govern and provideuniformity and consistency in sampling,these are designed for quality and processcontrol.The number of samples requiredto achieve the confidence levels necessaryfor control purposes would not normallybe acceptable for understanding thestone from a historic building, for whichinvasive investigations must be minimisedas far as possible. Where a constructionmaterial must be identified or a weatheringproblem investigated, choosing the numberand location of the samples will demandexperienced professional judgement.5

Sampling fragile stoneIf the area to be sampled is deeplyfractured or has shear cracks, it may benecessary to apply a temporary facinglayer to it to prevent detachment, lossof loose surface material or furtherfracturing during sampling and removal.Acid-free tissue should be adhered to thesurface with a diluted solution of asuitable reversible consolidant (such aspolyvinyl alcohol, or an acrylic emulsion),applied by a qualified conservatorengaged by the client or stone specialist.If necessary, fragile samples can beprotected in polythene bubble wrap fortransportation; the protective layers andconsolidant should then be removed priorto the laboratory analysis.Planning stone samplingThe sampling strategy must be fullyplanned in advance. Ensure that allsampling locations have been identified,and that each location has been providedwith safe access. A range of tools andother equipment should be available,including:● small hammers● chisels● a range of sample bags● felt-tip permanent markers and pencils● tape measures● core-drilling equipment to obtain25–75mm solid diameter cores(Figs 5 and 6)Fig 5 Core-drilling rig with 75mm diameter core samplingmagnesian limestone, Howden Minster, East Yorkshire.Fig 6 Core drill (75mm diameter) and extracted core inprotective polyethylene tube for safe transportation.Checklist prior to samplingPhotography Have photographs been taken ofthe building or monument, andof all the sample areas?Sampling Is the sample truly representativeof the stone being studied? Is the sample being taken directlyfrom the standing structure? Does the sample include both aweathered and a non-weathered side? Is the sample approximately 75mm2 75mm 2 40mm? Has the orientation been markedon the sample?PhotographyAll locations must be photographedbefore sampling, preferably in colour.A general view of the building is alsorequired, also a context shot of thegeneral location of each sample, as wellas several close-up shots of the samplesite itself. A colour calibration chartand a scale should be included in theclose-up photographs.Direct lighting, from sunlight or flash, cansubdue the surface features, which mayassist in the interpretation of the sample.Therefore wherever possible, obliquelighting should be used, creating shadowsthat can emphasise subtle surface features.The surface of the sampling area maybe soiled, or covered with biologicalgrowth. If possible a small sample area ina concealed location should be lightlycleaned to reveal the true colour andtexture of the underlying stone. If not, thevisible condition of the stone should berecorded in addition to the photography.Labelling samplesEach sample must be given a uniqueidentifier, and all information regardingthe sample and its location must berecorded in a systematic manner. Toprevent error, samples should be labelledas soon as they are collected.Where only a few samples are taken,the identifier can be the name of thebuilding or site, and a sample number.However, where samples will be analysedby an organisation that deals withmaterial from many different sites, theidentifying system must be site-specific,and even organisation-specific. WhateverAfter samplingPhotography Have the sample locations beenphotographed after sampling? Has the sample been photographed?Labelling and packaging Is the sample clearly labelled with aunique identifier? Is the sample accompanied by astatement showing the building orsite name, street, town, county, thelocation on the building ormonument and the date of sampling? Is the sample securely wrapped andsecured in an appropriate bag, withadequate labelling both inside and out?system is used, the identifier should bemarked not only on the sample, butalso on any accompanying information,and on all photographs taken of thesample location. The date of samplingmust be included.Samples should be placed immediately ina sealable polythene sample bag, with theidentifier marked either directly on thesample, or where this is impossible on amanila tag placed inside the sample bag.If there is any concern over chemicalpollutants, a non-plastic container shouldbe used instead. When the identifier iswritten directly on the sample, take carenot to damage any features or any surfacecontamination that may be pertinent tothe investigation for which the sample wascollected. The outside of the sample bagor container should also be labelled, usinga permanent marker.As most stone is abrasive, if the sample issent for analysis by post or carrier, it mustbe packed in a manner that prevents itfrom moving around within the packaging– especially when more than one sample isincluded. The detachment of even a smallfragment during transit may result in theentire sample being damaged. This risk isparticularly high with samples ofweathered stone. In this case samplesshould be tightly bubble-wrapped, beforebeing packed into a padded postalenvelope, either with additional softpadding to stop the sample moving aroundwithin the envelope, or with the sampletaped securely to the inside of the bag.For samples too large to fit safely into apadded envelope, a card postal pack ora box may be required; any spare space6

should be filled with polystyrene chips,vermiculite or bubble-wrap.Useful additional informationFor cost-effective and efficient analysisit is best to provide as much informationas possible about the stone and thebuilding to the analyst along with thesample. Where something is known aboutthe original sources of stone fromgeographical regions, or better still thespecific building, this can be readilycompared with the results of the analysis.This information can help to identifypotential sources of replacement stoneas quickly as possible.In the study of deterioration mechanisms,it is important to record the preciselocations of samples, and the degree towhich the original stone was exposed.3.2 Examination and analysis of stoneHand samplesThe characteristics of many buildingstones can be readily identified in situ:this is especially true of the coarsergrainedvarieties such as granite, andmany of the sandstones and limestones.Preliminary determination of the generalstone type and mineralogy of a stonecan often be obtained in situ using ahand lens of about 8 magnification.However, the characterisation of certainother stones – for example basalts, slates,siltstones and fine-grained limestones –will require specialised laboratorytechniques. Detailed studies requirestereomicroscopy, using magnificationsof up to 40. This helps to interpret thestructure and composition of the stoneand any physical breakdown.Petrographic analysisPetrographic analysis precisely identifiesthe mineralogical composition of a stone,and the conditions under which it wasformed. This allows some assessment ofthe likely decay processes, especiallyas the manner in which stone weathersvaries according to its petrographiccharacteristics.A thin section of the stone is made bymounting a fragment of stone onto amicroscope slide and grinding it down toa thickness of 30 microns (1 micron =1 /1000 mm) and is viewed through apolarizing microscope. This enablescrystals and grains as small as one micronto be viewed in plane-polarized and crosspolarizedlight and so determine the typeof minerals present and any changes intheir composition (Fig 7).Fig 7 Left side: sample of Devonian sandstone from a medieval corbel, Kilpeck Church, Herefordshire, studied under ordinary. Right side: polarized light microscopy.,The width of each picture is0.72mm. (David Jefferson)The photomicrograph on the left was taken using ordinary light and shows that most of the mineral grains are transparent, although some can be speckled or slightly cloudy. In the same imageon the right viewed under polarized light, the nature of the sand grains is clearly visible.The quartz grains range from clear to black depending upon their crystal orientation relative to the angle ofthe polarized light. A grain of microcline feldspar, distinguished by its tartan twinning, is located on the bottom left and fragments of rock composed of a number of small grains of quartz arepresent in the centre, just above the central quartz grain, and in the bottom centre.The bright colours of the elongated grain in the centre top indicate it is muscovite or white mica.Fig 8 Left side: an oöidal limestone with a coarsely crystalline cement, the oöids have been partially dissolved away to leave a strong, sponge-like stone of high porosity, which is highlighted by theuse of blue-coloured resin within the sample. Right side: Ooidal Portland Whitbed under ordinary polarized light with, in this case, the intergranular pore distribution again shown by the bluecolouredresin in the sample. Width of both pictures is 1.4mm. (David Jefferson)7

This type of thin section analysis canalso provide an assessment of the porosityof a stone, which is not visible to thenaked eye (Fig 8). The identification ofporosity is important because the sizeand shape of the pores and whether theseare connected to form continuouschannels can markedly affect thedurability of the stone.The British Standard BS EN 12407:2000Natural stone test methods – Petrographicexamination is unnecessarily exhaustivefor characterising historic building repairstone, and its procedures are not suitablefor studies of weathering and soiling.Moreover, the thickness it recommendsfor limestone could be inappropriate forsome studies.Thin sections should thereforebe prepared in the context of the studybeing undertaken, and there should be norequirement that sections be preparedand analysed in the manner described inBS EN 12407:2000.Physical parametersAlthough petrographic study candetermine the composition and physicalproperties of stone, very subtle changes inmineralogy can considerably alter itsappearance. This is especially true ofcolour, which is often controlled by thequantity and variety of iron mineralswithin the rock. In sandstones, forexample, the grains are often coated with athin layer of an iron mineral. This coatingcan be less than 1 micron thick, and maygive the stone a slight pinkish tinge.Should the iron-rich coating be slightlythicker – even by only 1or 2 microns –the colour may be a deep reddish-brown.There are several common iron mineralsthat produce colour variations.The colour of the stone being studiedshould be recorded using the Munsell ®colour code (an internationally recognisedstandard system used for describing thecolour of all types of materials).Geologists and archaeologists use theMunsell system to describe rocks, soilsand ceramics, while architects, buildingsurveyors and general contractors will befamiliar with its use for classifying andspecifying paint colours. Colour isarranged into classes or categoriesaccording to three variables: hue, valueand chroma. It should be noted that moststones lighten or darken to some extentwhen wet. The colour of the stone shouldtherefore be recorded both wet and dry.Many stones that superficially look veryalike, such as sandstones, can be seenunder high magnification to besignificantly different. The microscopemay disclose that the natural cement inone sample is calcite, in another a claymineral, and in a third silica. Theweathering properties of all three stoneswould be very different, no matter howsimilar they appeared to the naked eye.Petrography can also identify mineralsknown to be unstable in the builtenvironment, or likely to react with othermaterials, such as mortars. For example,cryptocrystalline silica in a sandstone mayreact with highly alkaline lime mortar,resulting in the degradation of the stone.Such features are important not onlywhen determining the reasons for thebreakdown of a historic stone, but alsowhen selecting its replacement.The geology of the British Isles is wellcharacterised, so petrographic study canbe very useful for locating quarries andsources of compatible stone within theUnited Kingdom. Matching historic stoneimported from Europe and other parts ofthe world, however, demands both specificknowledge of the stone and carefulreference to standard samples.Petrographic analysis is essential whenusing commercially available stone, as thetrade names can be misleading. Forexample, Rosso Levanto Marble (Fig 9)is not a marble but a hard polishableserpentinised stone. Purbeck marble isjust one of the many so-called marblesavailable that are not in fact truemetamorphic marble, but are hardpolishable limestone. Similarly, someslates are actually highly compactedmudstones and siltstones that have notbeen subjected to the metamorphic forcesnecessary to produce true slate.Fig 9 Rosso Levanto Marble (field of view 95mm).(David Jefferson)Other analytical techniquesX-ray diffraction analysisThe diffraction of x-rays passing througha crystalline material produces patternscharacteristic of the crystal, and determinesthe mineralogy of a particular stone.Scanning electron microscopyMany of the reactions that damagebuilding stone occur at sub-microscopiclevel. A scanning electron microscopepermits magnifications well in excess of1000, and with the addition of an EDX(Energy-Dispersive X-ray Spectrometer)can determine aspects of chemicalcomposition (Fig 10).Fig 10 Environmental Scanning Electron Microscope (ESEM)images: top, pure gypsum on the surface of dolomitic sandstone(White Mansfield stone); bottom, individual oöids (roundedshapes) have developed a crust of gypsum within the surroundingpore spaces of Portland limestone. (David Jefferson)A simple chemical test for carbonatesLimestone and calcareous sandstone canbe distinguished from other forms ofsandstone and from other stone types byapplying a 5% solution of dilute hydrochloricacid to a small area of surface.Any calcium carbonate present withinthe stone will vigorously effervesce ascarbon dioxide is liberated by reactionwith the acid.Sandstones are commonly composedof quartz sand grains, but the cementbinding these together can vary incomposition. If effervescence is observedbetween the grains, the cement is likely tobe calcite. If there is no reaction, the binder8

is probably silica, iron oxide or a claymineral. Granite and most other igneousrocks will not effervesce in the presenceof the dilute acid, as they do not containcarbonates.The majority of metamorphicslate is siliceous, but certain slates (forexample some from the Lake District) docontain some calcium carbonate, and willtherefore effervesce slightly.Testing stone with hydrochloric acidThe acid should be diluted to an approximately5% solution by volume (1 partconcentrated hydrochloric acid to 19 partswater).To avoid the acid spitting violently,the acid must always be added to thewater rather than the water to the acid.To conform with the 1988 COSHHRegulations (Care of Substances Harmfulto Health), a proper risk assessment mustbe made and there should be adequatesafety measures and sufficient personalprotection available. Safety spectacles anddisposable nitrile gloves should be usedwhen decanting and diluting the acid.To test, the stone is viewed through abinocular microscope as a small amount ofthe dilute solution is dropped on thesurface with a pipette.Effervescence indicates that calciumcarbonate is present in the stone.4 Criteria for selectingreplacement stoneThe following criteria – in descendingorder of importance – should be used forfinding replacement stone:PetrographyThe constituent grains should be of thesame type, size, angularity andproportions as in the original stone.The binding material must also be similar.The ratio of binding material toconstituent fragments or mineral grainsand the porosity must also be alike.If the mineralogy of the replacement stoneis similar to the original, then the chemistryof the stone will effectively be the same.Chemical characteristicsThe chemistry of the replacement stonemust be similar to that of the original,particularly the concentrations of silica,magnesium and iron. The iron contentand the form in which iron occurs areparticularly important, as this willlargely determine the final colour ofthe exterior surface, especially afterweathering.The composition of the fluids travellingthrough the building fabric will beconditioned by the chemistry of thestone through which they are passing.Different stones will contain pore fluidsof different composition and whenpassing into a dissimilar type of stonethese can generate adverse reactions.AppearanceThe appearance of the stone viewed inblock form must be close to the original.If the petrography and chemistry aresimilar, then the small-scale appearanceshould also be the same, but large-scalefeatures such as bedding and veiningmust also be considered. The bed-depthwithin the quarry must permit theextraction of suitably sized blocks for theintended purpose.Geological ageIn the case of sedimentary rocks, thereplacement and the original stoneshould be of a similar age, and from asimilar sedimentary environment.Metamorphic building stones shouldbe of a grade corresponding to theoriginal, and igneous rocks should befrom the same family.PorosityBoth the overall value of the porosity andthe pore-size distribution should be asclose as possible to those of the originalstone. If a stone with the same porosityand permeability cannot be found, useone with a higher rather than lowerporosity. Any subsequent degradation isthen more likely to occur in the new stonerather than in the original fabric.Compressive strengthWhere it is not possible to match thecompressive strength of the replacementwith the original stone, the replacementstone should be weaker rather thanstronger, so that it is the more likelyto fail.Satisfying all these criteria would ideallyrequire the replacement stone to befrom the original quarry, or at least asource in very close proximity to theoriginal quarry. Failing this, the new stoneshould meet as many of the above criteriaas possible, with the first three being themost critical.5 Sources of stone5.1 Active quarriesThere are approximately 220 quarriesproducing building stone in Englandlisted in a biennial publication TheNatural Stone Directory. Other quarriessupplying building stone can be found inthe Directory of Quarries & QuarryEquipment and in the Directory of Minesand Quarries. The British GeologicalSurvey and freelance stone consultantscan also help locate appropriate currentsources of stone.Some quarries have been working thesame strata in the same location forcenturies, for example, the Portlandquarries have been producing relativelyconsistent limestone since the 18thcentury. Some smaller quarries are alsoworking long-established building-stonebeds, but for others the output haschanged over the years. The modernstone available from a particular quarrymay retain the identifiable visual featuresof the original, but be petrographicallydifferent from the stone extracted in thepast. Such differences could provesignificant when a stone is inserted inthe existing fabric of an historic building(Fig 11).5.2 Disused quarriesIf the source of stone has been tracedto a particular quarry but the quarryis now closed, it may be possible toobtain the stone by reopening theworkings for a short time. Land andmineral-rights owners are frequentlyamenable to extracting stone from anold quarry on their land, especially ifthe building or monument beingconserved is a local feature. MineralPlanning Authorities are also normallysympathetic to proposals to obtainsupplies of the correct stone fromoriginal sources, especially for therepair of historic buildings.To determine whether the stone from aclosed quarry can be economicallyextracted, it must be thoroughly sampledin situ. If any lithological variation issuspected, representative samples mustbe obtained from all the beds of rock,and from different parts of every bed.The thickness of the different beds,any lateral variation in thickness, andthe distribution and angle of jointpatterns within the rock must berecorded, as these features determinethe sizes of the blocks that can be9

Fig 11 Grinshill Quarry, Shropshire: left side, old disused quarry; right side, modern working quarry. (Don Cameron, British Geological Survey. © NERC. All rights reserved IPR/74-54C)extracted. Possible block sizes must becarefully compared to the masonrysizes required for the repair.Most building stone tends to berectangular, but if the joints in the naturalblock of stone are not at right anglesthere will be a high level of wastage whenthe stone is cut, and the resulting blockswill be considerably smaller blocksthan those extracted from the quarry(Figs 12 and 13).5.3 Recycled stoneWhere no suitable replacement stone iscommercially available, but only smallquantities of material are required, forexample, minor repairs to window mullions,it may be acceptable to recycle stone.The provenance of stone being re-usedmust be established and must have beenobtained from a lawful dismantling of abuilding or other structure to preventunlisted vernacular buildings being robbedfor their stone.Great care must be taken when selectingstone for re-use. Avoid weathered ordamaged material, and ensure that theblocks are large enough to accommodatethe dressing-back necessary to remove theoriginal exposed surfaces.Fig 13 Grinshill, Shropshire, waste stone after cutting, andthe range of colours within one quarry source.(Don Cameron, British Geological Survey. © NERC.All rights reserved IPR/74-54C)6 Obtaining stone for historicbuilding repair6.1 Specifying stoneReplacement stone should be specified tomeet the criteria defined in Section 4,with samples of potential replacementscarefully matching the original material.Fig 12 Clipsham, Lincolnshire, irregular-shaped stone prior to being sawn into rectangular blocks. (David Jefferson)All stone varies to some extent, but thevariation in the replacement stone shouldnot be greater than the variation in theoriginal fabric of the building, to avoiddifferences in petrography, porosity andcolour of the repair stone. For example,limestone taken from a single quarry canrange from very fine-grained to coarse andshelly, depending on which beds are beingworked. If some of the stone produced bythe quarry is not suitable, the exact stone10

equired must be clearly specified. Thegrain size of sandstones can also varywidely, so the acceptable range should bespecified, along with the acceptableporosity. This is especially important if thestone is to be inserted into the existingfabric of a building. Experienced quarrymanagers and their staff can assist thespecifier with these aspects.Finding blocks of the correct size for aparticular purpose may require contractorsto pre-order and pay a deposit, as it maytake the quarry some time to locate andearmark the appropriate blocks. Thelead-in time the quarry requires to sourceand supply blocks must be taken intoaccount when designing the repairprogramme. The logistics should beplanned well in advance of when the stonewill be needed on site.BS EN 1469:2004 (E), Section 4.2Requirements of natural stone for cladding(pp 10–11) suggests that the minimum sizeof reference samples should be at least100mm 2 – 250mm 2 in face area, and shouldindicate typical appearance.The StoneFederation Great Britain can supply theirinformation sheet (entitled Stone Samples),which provides useful guidance on samplesizes, finishes and labelling. Labelling is inaccordance with BS EN 12440:2001 Naturalmaterial – denomination criteria: thetraditional name, followed by thepetrographical family, the typical colour, andthe place of origin (BS EN 12440Introduction clauses 3.1, 3.2, 3.3 and 3.4).6.2 Samples of new stoneAll quarries producing building stone willsupply samples on request. These typicallymeasure 100mm by 100mm, and areabout 10mm thick.As stone is a natural and variable material,a single sample cannot completely representthe variations that probably occur withinthe stone. This is often indicated in thesupplier’s literature, but it should beremembered that stone naturally changescolour as it weathers. New or replacementstone should therefore never be selected onthe basis of supplier’s samples alone, andthese samples should not be sent foranalytical comparison with samples ofexisting stonework.If a supplier’s sample appears suitable fora particular project, the quarry itselfshould be visited. There the likelyvariation in the stone can be determinedby examining the quarry faces, and blocksextracted and stored at the quarry.Samples from varying blocks can becollected in the same manner as for in situsampling (Section 3.1), and these shouldbe analysed to determine the suitability ofthe stone for its intended purpose.6.3 Testing replacement stoneMany of the established tests for buildingstone are designed to investigate theirperformance in the context of modernbuilding practices, for example, the breakingload of a fixing used in stone cladding.Other tests – salt crystallisation, saturationcoefficient, porosity, and freeze/thawcycles – were originally designed to givean indication of durability. As these teststend to be carried out on small cubes ofstone within a laboratory, they take intoaccount neither the environment withinand around the building, nor the effectof the mortar between the blocks. Thusit can be difficult to relate the test resultsto the behaviour of stone in situ. Suchtests may, however, be useful forcomparing the behaviour of the potentialreplacement with that of the originalstone, or with that of a stone whoseproperties in a similar environment arewell understood.Fig14 Stone testing chamber, Sheffield Hallam University:top, stone wall being built; bottom, testing in progress.(Chris Wood)Large-scale environmental testingWhere major repairs are required, andthe replacement stone chosen is notpetrographically identical to the original,large-scale environmental testing can beuseful, if costs permit.Walls constructed in a test chamber can beexposed to simulated weathering conditions,based upon real meteoro-logical data,including wetting, drying, and driving rain.Large chambers can accommodate mortarbondedmasonry units up to 2m or 3mlong and 2m high.The smaller freeze-thawchambers used for testing bricks can also beuseful for testing stone. Environmental testingis relatively expensive, but for major repairor replacement projects it can prove costeffective(Fig 14).6.4 Obtaining stone from new ortemporary sourcesIn recent years a number of small, disusedquarries have been temporarily re-openedto supply walling stone, stone roofingslates and even stone for crushing (to helpreplicate medieval mortar mixes). Openingup quarries requires professionalassistance from a stone consultant or anexisting quarrying company.The first step is to identify the landownerand, if they are not the same, the mineralrightsowner. The Mineral PlanningAuthority (MPA) should be approached todetermine whether planning consent isrequired. This may be dependent upon thescale of the proposed operation; if thequantity of stone required is relatively small,the MPA will frequently waive planningconsent and permit temporary works in amanner agreed in detail with them.Once the necessary consent has beenobtained, the overburden and topsoil iscarefully removed and stockpiled. Afterstone extraction is completed, theexcavation is backfilled with any wastestone, and the stored overburden andtopsoil is used to restore the site. Thevegetation usually regenerates rapidlyfrom the natural seed bank in the soil.7 Sources of further informationand assistance7.1 Stone collectionsThere are a number of sample collectionsof historical and contemporary buildingstone, which are useful for understanding11

different stone types or for comparingstones. Some collections are open to thepublic, although admission may be charged.The National Collection of Buildingand Decorative Stones, Natural HistoryMuseum, London: reference collectionsmay be inspected by appointment.The John Watson Building StoneCollection, Sedgwick Museum Building,University of Cambridge: assembled atthe beginning of the 20th century, thiscollection includes building stone, stoneroofing slates, flagstones and road stone.The British Geological Survey (BGS),Keyworth, Nottinghamshire: thiscollection of more than 50,000 stones isavailable for public consultation, anddetails of the collection are accessiblethrough a computer database.Contact details can be found below underUseful contacts.7.2 Other sources of helpLocal Planning Authorities and theirConservation Officers can often helpwith technical advice on local naturalbuilding materials.The BGS can also provide informationon the geology of building stones,including the location of both active anddisused quarries.Independent consultants who specialisein the identification, selection andassessment of stone for the repair ofhistoric buildings can be located throughthe Natural Stone Directory, theGeological Society or the Institute ofQuarrying (see Useful contacts).BibliographyAshurst, J and Dimes, F J 1977 Stone inBuilding: Its Use and Potential Today.London: Architectural Press— 1990 Conservation of Building andDecorative Stones. London: ButterworthHeinemannBrereton, C 1995 The Repair of HistoricBuildings: Advice on Principles and Methods,2 edn. London: English HeritageBritish Geological Survey and NaturalEnvironment Research Council (NERC)2001 Building Stone Resources Map of Britain.Nottingham: British Geological Survey— 2002 Directory of Mines and Quarries.Kingsley Dunham Centre, Keyworth,Nottingham: BGSClifton Taylor, A 1972 The Pattern ofEnglish Building, 3 edn. London: Faberand FaberGeological Society of America 1991Rock Color Chart. Colorado: BoulderHart, D 1988 The Building MagnesiumLimestones of the British Isles. BuildingResearch Establishment Report.Watford: BRE— 1991 The Building Slates of the BritishIsles. Building Research EstablishmentReport. Watford: BREHolmes, A (P M D Duff, ed) 1993Principles of Physical Geology, 4 edn.London: Chapman and HallHowe, J A 1910 The Geology of BuildingStones. London: Edward Arnold; Shaftsbury:Donhead, reprinted in facsimile 2001Honeyborne, D B 1982 The BuildingLimestones of France. Building ResearchEstablishment Report. Watford: BRELeary, E 1983 The Building Limestonesof the British Isles. Building ResearchEstablishment Report. Watford: BRE— 1986 The Building Sandstones ofthe British Isles. Building ResearchEstablishment Report. Watford: BREQMJ Publishing 2005/6 Directory ofQuarries & Quarry Equipment.Nottingham: QMJ Publishing Ltd— 2006 Natural Stone Directory.Nottingham: QMJ Publishing LtdSchaffer, R J 1932 The Weathering of NaturalBuilding Stones. Department of Scientific andIndustrial Research, Building ResearchSpecial Report 18. London: HMSO;Shaftsbury: Donhead, reprinted 2004Shore, B C G 1957 Stones of Britain.London: Leonard Hill (Books) LimitedSmith, M R (ed) 1999 Stone: BuildingStone, Rock Fill and Armourstone inConstruction. Geological SocietyEngineering Geology Special Publication16. London: Geological SocietyWarnes, A R 1926 Building Stones:Their Properties, Decay, and Preservation.London: Ernest Benn LimitedWatson, J 1911 British and Foreign BuildingStones. Cambridge: University PressWhitten, D G A, and Brooks, J R V 1972 ThePenguin Dictionary of Geology. London: PenguinWinkler, E M 1977 Stone in Architecture,Properties, Durability, 3 edn. Hamburg:Springer-VerlagWood, C (ed) 2003 ‘Stone roofing: conservingthe materials and practice of traditional stoneslate roofing in England’. English HeritageResearch Transactions 9. London: James andJamesWoolf, D O, 1960 The Identification ofRock Types, rev edn. Washington, DC:United States Government Printing Office12

Useful contactsBritish Geological Survey (BGS)Kingsley Dunham CentreKeyworthNottingham NG12 5GGtel: 0115 936 3100fax: 0115 936 3200e-mail: enquiries@bgs.ac.ukwebsite: www.bgs.ac.ukBritish Standards Institution (BSI)389 Chiswick High RoadLondon W4 4ALtel: 020 8996 9001fax: 020 8996 7001e-mail: orders@bsi-global.comwebsite: www.bsi-global.comBuilding Research Establishment(BRE)Bucknalls LaneGarstonWatford WD25 9XXtel: 01923 664000e-mail: enquiries@bre.co.ukwebsite: www.bre.co.ukDirectory of Quarries and QuarryEquipmentQMJ Publishing Limited7 Regent StreetNottingham NG1 5BStel: 0115 941 1315fax: 0115 948 4035e-mail: mail@qmj.co.ukwebsite: www.qmj.co.ukEnglish HeritageCustomer ServicesPO Box 569Swindon SN2 2YPtel: 0870 333 1181fax: 01793 414926e-mail: customers@english-heritage.org.ukwebsite: www.english-heritage.org.ukGeologist’s Directoryavailable from The Geological Society,address belowGeological Society of America RockColour ChartGeo Supplies, Limited49 Station RoadChapeltownSheffield S35 2XEtel: 0114 245 5746fax: 0114 240 3405e-mail: sales@geosupplies.co.ukwebsite: www.geosupplies.co.ukInstitute of Quarrying7 Regent StreetNottingham NG1 5BStel: 0115 945 3880fax: 0115 948 4035e-mail: mail@quarrying.orgwebsite: www.quarrying.orgJohn Watson Building StoneCollectionSedgwick Museum of Earth SciencesUniversity of CambridgeDowning StreetCambridge CB2 3EQtel: 01223 333456fax: 01223 333450e-mail: sedgwickmuseum@esc.cam.ac.ukwebsite: www.sedgwickmuseum.orgNational Stone CentrePorter LaneMiddelton by WirksworthDerbyshire DE4 4LStel/fax: 01629 824833e-mail: nsc@nationalstonecentre.org.ukwebsite: www.nationalstonecentre.org.ukNatural History MuseumCromwell RoadLondon SW7 5BDtel: 020 7942 5839 (Customer Services)website: www.nhm.ac.ukNatural Stone DirectoryQMJ Publishing Limited7 Regent StreetNottingham NG1 5BStel: 0115 941 1315fax: 0115 948 4035e-mail: mail@qmj.co.ukwebsite: www.qmj.co.ukThe Geological SocietyBurlington HousePiccadillyLondon W1J 0BGtel: 020 7434 9944fax: 020 7439 8975e-mail: enquiries@geolsoc.org.ukwebsite: www.geolsoc.org.uk13

Annex: A system for the field identification of the more common building stones used in England (based on a scheme developedby the Division of Physical Research, Bureau of Public Roads, U.S. Department of Commerce by D O Woolf)Preliminary classification groups1 Glassy (wholly or partially) 2 Not glassy, dull, homogeneous, so fine-grained that the grains cannot be recognised3 Distinctly granular 4 Distinct foliation, no effervescence with dilute acid 5 Clearly fragmental in composition, rounded or angularpieces or grains cemented togetherGroup 1 ● glassy lustre, hard, conchoidal fracture, colourless to white or smoky quartz 1Glassygrey, generally brittlerocks ● cellular or frothy glass pumiceGroup 2 Sub-group 2A ● particles almost imperceptible, dull lustre, homogeneous, clay odour, little shaleVery fine- not scratched by a if any effervescence with dilute acid, laminated structure, breaks into flakesgrained fingernail, but readily ● brisk effervescence with dilute acid, little if any clay odour although a limestonerocks scratched with a knife bituminous odour may be discharged if the stone is struck; may containfossil fragments● brisk effervescence with dilute acid, no clay odour, soft, normally white in chalkcolour, can leave white powder on the hands● little if any clay odour, brisk effervescence with dilute acid only when the magnesianrock is powdered or the acid is hotlimestoneSub-group 2B ● very hard, pale colours to black, no clay odour, conchoidal fracture, waxy chert 2not scratched by a knife, or horn-like appearanceor scratched only with ● heavy, dark colour, may be finely crystalline when viewed with a hand basaltdifficulty, no efferv- lens, may contain small cavities which can be open or filled with crystallineescence with dilute acid mineralGroup 3 Sub-group 3A ● brisk effervescence with dilute acid; granular material may be broken fossil limestoneGranular easily scratched with fragments, angular material or small spherical grains, the granular materialrocks a knife may be mixed with fine-grained material● brisk effervescence with dilute acid; grains may be large and interlocking, marblecompact, colour may be white, cloudy grey or coloured, often banded orvariable● grains are deep red or brown, embedded in a similarly coloured material; ironstone 3may be slight effervescence with dilute acid; hard bands or patches of veryfine-grained red or brown mineral may occur;. may stain hands redSub-group 3B ● mainly quartz and feldspar in relatively large crystals, although mica granitehard, not scratched commonly present, usually grey, pink or redwith a knife or scratched ● mainly quartz in distinct grains often clearly set in a matrix, can fracture sandstonewith difficulty, grains round the grains; no distinct red colouration, can be buff, grey or greenish greynormal of approximately ● mainly quartz in distinct grains often clearly set in a reddish coloured ferruginousequal size matrix, can fracture round the grains; may stain hands red sandstone● mainly red-stained quartz, embedded in a relatively large quantity of red or ironstone 3brown coloured material; hard bands or patches of very fine-grained red orbrown mineral may occur; may stain hands red● mainly quartz in distinct grains often clearly set in a pale-coloured matrix calcareouswhich effervesces with dilute acid, can fracture round the grainssandstone● mainly quartz in distinct grains in a hard siliceous matrix, stone fractures quartzitethrough an appreciable quantity of the grainsGroup 4 ● medium to coarse grain; roughly foliated gneissFoliated ● very fine grain, splits easily into thin slabs, usually dark grey, green or black slaterocksin colourGroup 5 ● rounded pebbles embedded in a cementing medium which can be reddish conglomerateFragmentalin colour● angular fragments of rock embedded in a cementing medium which can be brecciareddish in colour● fragments of volcanic (fine-grained or glassy) rocks embedded in compacted volcanic tuffvolcanic ashor breccia● quartz grains, possibly together with fragments of rock and mica, rounded sandstoneor angular, less than about 1mm in diameter, cemented together● quartz grains, possibly together with fragments of rock and mica, rounded or gritstoneangular, between about 1mm and 4mm in diameter, cemented together14

Notes for Annex opposite1 Although quartz is a mineral rather than arock, it is included as a building stone becausequartz cobbles from rivers and beaches havebeen used in rubble stone buildings.2 Chert includes flint, which is the traditionalname given to the chert bands and nodulesfound in some Cretaceous chalk.3 Ironstone is a term traditionally used forboth calcareous stones rich in iron minerals,which effervesce with dilute acid, andextremely iron-rich sandstones that do notreact with acid.AcknowledgementsThis Technical Advice Note was written byDr David Jefferson (Jefferson ConsultingLimited), and Seamus Hanna and BillMartin (Building Conservation andResearch Team, English Heritage). We areindebted to Sasha Chapman’s previouswork on this publication. The authorswould like to express their thanks toEnglish Heritage colleagues John Fidler(Conservation Director), Robert Gowing,Robyn Pender and Chris Wood (SeniorArchitectural Conservators, BuildingConservation and Research Team), GilesProcter (Historic Buildings Architect,Yorkshire Region) and Dr David M Jonesfor editing the text for press, as well as toJohn Bysouth (Independent StoneConsultant), Peter Harrison (HarrisonGoldman), Graham Lott (BritishGeological Survey), and Nigel Sunter(Purcell, Miller, Triton) for their helpfulcomments on the manuscript.15

English Heritage is the Government’sstatutory adviser on the historicenvironment. English Heritage providesexpert advice to the Government aboutall matters relating to the historicenvironment and its conservation.For further information (and copies of thisleaflet, quoting the product code), pleasecontact:English HeritageCustomer Services DepartmentPO Box 569SwindonSN2 2YPtelephone: 0870 333 1181fax: 01793 414926e-mail: customers@english-heritage.org.ukLONDON23 Savile RowLondon W1S 2ETtel: 020 7973 3000EAST OF ENGLANDBrooklands House24 Brooklands AvenueCambridge CB1 1DJtel: 01223 582700EAST MIDLANDS44 DerngateNorthampton NN1 1UHtel: 01604 735400NORTH EASTBessie Surtees House41–44 SandhillNewcastle upon Tyne NE1 3JFtel: 0191 269 1200NORTH WESTSuits 3.3 & 3.4 Canada House3 Chepstow StreetManchester M1 5FWtel: 0161 242 1400SOUTH EASTEastgate Court195–205 High StreetGuildford GU1 3EHtel: 01483 252000WEST MIDLANDS112 Colmore RowBirmingham B3 3AGtel: 0121 625 6820YORKSHIRE37 Tanner RowYork YO1 6WPtel: 01904 601901Cover figure:(top left) Grinshill, Shropshire: disused quarry. (Don Cameron, British Geological Survey. © NERC. All rights reserved IPR/74-54C)(top centre) Howden Minster, East Yorkshire: new magnesian limestone replacement on the south doorway arch. (Seamus Hanna)(top right) Clipsham, Lincolshire: modern working quarry. (David Jefferson)(bottom left) Thin section of an oöidal limestone with blue coloured resin highlighting the porosity. (David Jefferson)(bottom centre) Truro Cathedral, Cornwall: replacement stone in a decayed frieze. (Seamus Hanna)(bottom right) Rosso Levanto marble: a hard polishable serpentinised stone. (David Jefferson)Published March 2006Copyright © English Heritage 2006Edited and brought to press by David M Jones,English Heritage PublishingDesigned by Mark SimonsProduced by English Heritage PublicationsPrinted by: The ColourhouseProduct code 50912