VOLUME 22

Lime harling Craig Frew

Craig Frew, MSc, IHBC, is a building conservation consultant, providing consultancy advice and training for the repair and conservation of traditional and historic buildings and structures through his company, Frew Conservation. He previously worked as technical consultant for a large stonemasonry contractor, as buildings consultant for a traditional building skills training centre, and as conservation officer for a local authority. Craig is a full member of the Institute of Historic Building Conservation and a committee member of the Building Limes Forum.

ime harling is a thrown, or cast-on, slurry of lime Perceptions and fashions change over time, and and coarse sand and aggregate mortar, used to Loudon’s appreciation of the external finish was not produce a rough-textured external wall finish. It shared by later Victorians. In the late 19th century, is the most common type of traditional surface external harling and render were commonly stripped finish found on traditional masonry buildings from the exteriors of older buildings in favour of Lin Scotland. In other parts of the UK and Ireland, where exposing the underlying stone or brick, and this it is often found in vernacular architecture, harling is practice is still all too common today. The vast more commonly known as ‘roughcast’ or ‘wet-dash’. majority of pre-Victorian-era buildings constructed Like other forms of lime render, harling provides a of rubble stone, certainly in Scotland, were generally weather-protective and decorative coating, commonly plastered, harled, and/or limewashed externally. There covering what is perceived to be less attractive and less are of course exceptions, for example the rear and weather-resistant rubble stone or brickwork. side elevations to many Georgian properties were This article attempts to provide an overview of lime often flush-pointed, with struck or lined-out joints to harling in both the historic and modern contexts. It formalise the appearance of the wall surface or, in some contains general information on materials and methods cases, to imitate ashlar stonework. for undertaking lime harling and associated works to The term ‘harling’ is generally understood to derive traditional masonry buildings. from the action of hurling wet mortar at a wall. In Scottish vernacular architecture, harling was almost The principal purpose for which any of these exclusively applied as a cast finish and not on to a processes [‘outside plastering’ including harling] is floated base coat, as is more common in England adopted on the outside wall of a cottage is, to keep (see ‘Recommended reading’, below) and on modern them dry; and a second purpose is, to render them cement roughcast work such as pebbledash. Cast-on ornamental, either by imitating stone, or producing textured finishes will generally give better adhesion a surface more curious or agreeable to the eye, than than trowel-applied coatings, particularly on rough or the rude materials concealed behind it. uneven surfaces and those with low suction properties. (J. C. Loudon, 1833 1) Cast-on coatings can also provide better weather resistance, as the mortar is generally better compacted

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The damage which can be caused to traditional solid-wall buildings by the use of inappropriate cement-based and other impermeable coatings is now well recognised. If correctly specified and used, lime harling can provide an attractive and durable weather- protective coating to most traditional buildings. It can also protect underlying masonry from decay and has the ability to improve the thermal insulation properties of the wall. Historic lime harling As with most traditional materials and techniques, knowledge and experience were passed down through generations and adapted to suit the local environment, materials, and labour availability. From surviving examples of traditional lime harling in Scotland up until the later 19th century, coatings would typically consist of a hand-cast, thin coating, usually no more than around 10 mm in overall thickness, but often less, applied in one or two coats. It is rare to find three or more coats of old lime harling, except in areas where overcoats were applied for the reinstatement of damaged or eroded harl, although such build-ups are often specified in modern-day lime harling work. The most commonly used traditional finish for harling was ‘as cast’, as this was the quickest and most e fficient option. There are, however, some historic examples where the cast finishes were gently pushed back with a float to remove the high points, or floated back more comprehensively to give a flatter finish. Flat finishes were often lined out to provide a more formal finish, the best examples imitating a high-quality ashlar stone facade. Such finishes became more common on town buildings and country houses throughout the 18th and 19th centuries, though over time, as these finishes were refined, they became part of the ‘plasterer’s’ skills rather than those of the mason/builder. Limewash finishes were typically, but not exclusively, applied over the Fig. 1 Craigievar and more uniform throughout its thickness than harling surface as a protective and decorative finish, Castle, Aberdeenshire trowelled-on coatings. incorporating natural earth pigments where colouring (built c.1626): It was the size and shape of the larger aggregate was desired. traditional hand-cast hot-lime harling with particles, allied to the degree of preparation and Analysis of historic and traditional lime harls has pigmented limewash, straightening carried out to the wall in advance of revealed that many of the mixes, as with the majority completed in 2010 application of the finish coat, which dictated the of building limes until the later 19th century, were hot- by Laing Traditional characteristics of the harling finish. lime mortars, made by slaking the fresh quicklime along Masonry Group Ltd Like many traditional building materials, lime with the coarse sand and water, and often used whilst (LTM Group). (Photo credit: LTM Group.) harling and other lime-based renders and coatings are still hot or warm. Commonly, the harling would be the vapour permeable, to various degrees, and can help same mix as that used for the masonry bedding mortar, to manage the e ffects of moisture on mass-masonry but wetter to permit casting on to the wall surface. walls. External lime coatings generally reduce the depth Historic harling and most other mortars in Scotland of rainwater penetration through the wall thickness, until the later 19th century were, with some exceptions, while the inherent permeability of the lime promotes non-hydraulic to feebly hydraulic and were typically evaporation from the underlying wall fabric and from used in relatively binder-rich mixes. Mixes of 1 part lime the harling itself under drying conditions. to 1 or 1.5 parts of sand were not uncommon.

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Surviving historic lime harling is becoming rare, It is entirely possible to match the texture and Fig. 2 (Above left) although remnants can often be found on buildings appearance of an existing or original lime harling. Lime harling which which were once harled but have lost the harling Analysis of harling samples can provide valuable is at least 150 years old on a church in through natural weathering and decay, or mechanical information on the type of lime and grading of the Orkney. removal. Where original harling does exist, e fforts aggregate, which can be matched with an available should be made to retain this and apply patch repairs source, bearing in mind that modern processed sands Fig. 3 (Above right) where necessary, rather than carry out full removal and may be quite di fferent from the original sands, which Recent lime harling, reinstatement. Grouting techniques can be used to were commonly used ‘as dug’ and rarely ‘processed’ hand-cast and lightly pressed back, with secure de-bonded harling, and patch repairs can be except, in some cases, by screening to remove pigmented limewash feathered in to the existing. oversized particles. finish, at the 16th- century Brodie Castle in Moray.

Fig. 4 Recent lime harling, hand-cast and lightly pressed back, with limewash finish, at St Peters, Sandwick, Orkney (built c.1836). The degree of ‘straightening’ of the wall allied to the degree of pressing back of the rough finish can result in a more formal coating; note the external corners are more defined.

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General principles for out the background suction across variable substrates. Traditional putty-based limes and hot-lime mixes are lime harling inherently more ‘sticky’ than most modern hydraulic limes and may not require this ‘pricking-up’ coat. As a basic principle, lime harling and other external Selecting the right mortar for the job is essential. coatings should generally be weaker than the A non-hydraulic (air) lime may work well when background material over which they are applied, and applied to a relatively dry wall, but could be at risk each successive coat should be weaker and thinner on a very damp wall as a saturated lime mortar will than the preceding one. The exception is in areas of not carbonate, and therefore not fully harden or severe exposure, where it may be advantageous to develop its full vapour-di ffusion characteristics. maintain consistency throughout the thickness. Rarely However, a hydraulic-lime mortar would also be at are stronger hydraulic-lime mixes appropriate for lime risk if maintained in a saturated state and subjected harling to traditional buildings – in most instances, to frosty conditions. Despite its initial hydraulic set, it weaker lime mixes are better suited, as they generally still relies on carbonation to achieve full strength, its provide greater flexibility and vapour permeability. required pore structure, and frost resistance. To avoid An appropriate method of work allied to a suitable failure, any damp, uncarbonated lime must therefore specification for materials and mix design is critical be protected from frosts through the winter. At the in achieving the best outcome for the building or risk of stating the obvious, frost requires moisture, so structure. a dry mortar in freezing conditions will not become When harling is applied to an existing masonry damaged by frost action. wall, several factors require careful consideration When contemplating winter working, bear in mind before a decision is taken on a suitable approach and that colder temperatures will slow down the curing specification. It is important to establish the function process (as with cement mortars), so greater care of the coating: is it to improve the appearance of a must be exercised in planning the works to ensure building, to protect vulnerable underlying masonry, adequate protection. Sca ffolding must be enclosed or to help dry out a damp building or wall? The and protected and background heating may be specification of both the materials and methodology required, hence the reason why, historically, lime work for undertaking the work can be designed to achieve was carried out within a ‘weather window’, i.e. April a particular outcome, and where appropriate address to September or March to October, depending on several, or all, of the foregoing. geographical location. The type and condition of the substrate will also Protection is essential at most times of the year – have a significant bearing on the specification. Most direct sunlight and drying winds can be as damaging importantly, any coating must achieve an adequate to new lime harling as rain and frost. The length of bond with its background. Where loose, friable time that the works should be covered and protected substrate materials exist, remedial repairs and/or will depend on the mixes used and the weather and consolidation of the substrate will be essential before temperature at the time of working. A close covering any lime coating can be applied. In some instances, a of damp hessian sheets and/or plastic sheeting can few applications of diluted limewash may be helpful on assist by protecting the work from external weather a weakened substrate. conditions and maintaining a humid environment Adequate bonding can be achieved through a within which the harling can cure more readily. well-controlled suction bond, a mechanical key, or a Di fferent limes have di fferent curing requirements combination of both. Inappropriate suction control – with hydraulic limes, the initial hydraulic set will is a significant cause of failure in mortar coatings typically take place over the first few days, so the – over-wetting as well as insufficient damping harling must be kept damp during this time. Thereafter, down will adversely affect the bond between the the carbonation set requires damping down and slow existing wall surface and the new coating. Where drying cycles over a longer period of time – fully dry the wall contains both dense impervious stone and or saturated limes will not carbonate. Air limes, of highly porous mortar, there is a need to thoroughly course, rely solely on this latter carbonation set for dampen the mortar joints, while ensuring that the their durability. stone surfaces are dry to the touch, prior to applying Where walls are found to be damp, as is often the any lime coating. case where a cement-based coating has just been A ‘pricking-up’, ‘bonding’, ‘tack’, or ‘scud’ coat may removed, specialist advice on materials selection be helpful on di fficult backgrounds – particularly should be sought. The time of working, length of those with low or variable suction. This is typically contract, and level of protection possible (for working a thin, binder-rich, gritty coating, which provides a and curing) are all important factors which must be good bond with the substrate and can help to even considered at specification stage.

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Application process There is no ‘standard’ methodology for harling an existing masonry wall, as there are so many variables. However, the works can generally be divided into the following applications, bearing in mind that all may not be required: 1 Substrate preparation 2 Dubbing (daubing/flushing) out 3 Pricking-up (tack/scud) coat 4 Straightening coat 5 Finish coat 6 Limewash (see ‘Maintenance of lime harling’, below)

1 Substrate preparation This will generally involve repairing any defects and filling any voids or cracks in the masonry. All masonry must be clean and free from dust or debris; soft, friable stone should be brushed back to a relatively sound surface. In some instances this may need to be consolidated with a diluted limewash application or the masonry repaired by some other method to ensure a satisfactory background. ‘key’ for the subsequent harling, hence the requirement Fig. 5 Hand-cast for this mix to contain a sharp, gritty sand. No attempt harling from a harling 2 Dubbing out is made to ‘straighten’ the wall faces with harling at (or scudding) trowel. A small scoop of This is the process of filling previously prepared open this stage (this should have been done as part of the harling mix, evenly joints and building out the mortar until flush with the dubbing-out process), as variable thicknesses can lead spread across the face of the masonry. As with most filling of rubble- to an inconsistent appearance in the finished work and trowel, is cast against stone masonry joints, pinning stones are incorporated can impact on its performance. the face of the wall where necessary to avoid the accumulation of with a flick of the wrist, spreading the large volumes of mortar. The mortar must be well 4 Straightening coat mortar thinly and compacted to fill any voids and can be finished fairly This may be required where the background masonry uniformly. (Photo rough ‘o ff the trowel’ to take up its initial sti ffening. Prior is particularly uneven and/or where a flatter finish is credit: LimeRich to hardening, the mortar is compressed back to ensure required. It is important to bear in mind that the final Stonemasonry & a tight bond with the adjacent stone. Suction control, finish coat will be a relatively thin coating of consistent Plastering.) as previously described, is critical in achieving a good thickness, so will not mask any unevenness in the bond between stone and mortar. Finally, the dubbing- background. Typically this coat is cast or sprayed on, out coat can be scraped back with the edge of a trowel for the best adhesion, the mortar being scraped from to open up the surface to aid the curing process, and the high points (a plasterer’s straight edge can be used) also to provide a good key for the pricking-up coat. The and the low points being filled as work progresses – it is dubbing out should bring the wall face to a suitable important not to overwork or ‘move’ the mortar around level for application of a thin harling coat. In certain too much on the wall as this can weaken its bond with circumstances, where the wall face is relatively sound the background and create denser patches, which can and flat, a single application of harling will be su fficient, reflect (‘grin’) through the finish coat. omitting the pricking-up and straightening coats. 5 Finish coat 3 Pricking-up coat Finally, the harl is applied to a consistent thickness This is applied in a similar manner to the harling, either and texture. Careful planning is required for large by the traditional method of hand-casting the mortar elevations without natural breaks, such as internal or using a specially shaped trowel, or with mechanical external returns or string courses. Visible day joints spray equipment (where appropriate). Pricking-up can be avoided by ‘fairing’ or ‘feathering out’ the coats generally comprise a thin, binder-rich, and gritty harling (gradually reducing its thickness) in a wave- coating which acts to even out the background like shape (not a straight line) and blending in the suction levels. This is particularly important where following day. It is also essential that the sca ffolding is high-capillarity lime mortars exist alongside impervious designed specifically for the purposes of harling, with stones, such as with granites and other impervious removable inner boards to avoid the risk of ‘lift lines’ igneous rock types. It also provides a good mechanical in the finish.

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Fig. 6 Mechanical The number of coats and relative thicknesses of ‘open’ edge. Harling is typically feathered out to almost spray harling harling will vary from building to building, depending nothing at the base of the wall to discourage moisture application. The on the type and condition of the substrate and the retention – another traditional detail which is often hand-held hopper part of the render gun type of finish and appearance desired. Similarly, the ignored in reinstatement work, in which, unsurprisingly, is used to scoop up timing between coats for curing and drying will vary this is a common failure point, particularly where wet the harling mix, which depending on the materials selection and the number ground conditions exist. is forced on to the and thickness of coats. Individual coats of harl should wall by compressed generally be no thicker than about 10 mm – often air, the hose being Maintenance of lime harling attached to an air traditional Scottish lime harling is less than this in overall compressor. (Photo thickness. Once the lime harling is complete, it should As with any external render coating, lime harling does credit: LimeRich be allowed additional drying time, after curing. Any require maintenance. The application of a limewash Stonemasonry & patchiness in the moisture levels across the harling may finish provides enhanced durability of the harling Plastering.) lead to an inconsistent appearance when limewashing. by soaking into its outer surface, filling some of the This will be more pronounced when using more open porosity and minor shrinkage cracks formed strongly pigmented limewash. during its curing and drying. Additionally, it provides Where stone dressings exist around window and an extra layer of protection to the harling by acting door openings and at quoin stones, these often have a sacrificially. Limewashes will weather more quickly in raised margin, up to which the harling is taken. Typically highly exposed environments, particularly with wind this is only a few millimetres in depth, which is suited to abrasion, so should be checked regularly. Small-scale a lime harling but not to cement-based coatings, which limewash repairs and wholesale re-limewashing are are generally much thicker. There are many examples generally cheap and simple, and will protect the harling of poorly detailed cement harling/roughcast on itself from damage or decay through exposure. traditional buildings, which stands proud of the stone Where access is difficult or would be expensive margins, detracting significantly from the appearance for painting and/or repainting, there may be of the building. Where no raised margins exist, lime instances where a potassium silicate (waterglass) harling can be feathered out to nothing, but more paint system would provide a suitable alternative typically would be carried round the corner to meet to limewashing. Such paints can be expensive the window or door, thereby avoiding a susceptible in comparison to a limewash, but when correctly

30 VOLUME 22 specified and applied can provide a highly durable paint finish to lime harling. They do however require a dry substrate, so freshly harled buildings and/or those in wet and exposed environments may prove challenging in terms of meeting such requirements. The correct specification and application of such paint systems is essential, as many ‘mineral’ or ‘silicate’ paints contain polymer binders, which can reduce their vapour permeability. Where the harling itself becomes damaged or decayed, patch repairs can, and should, be carried out swiftly. Lime harling which has lost its outer surface will be more prone to accelerated decay. The repair methodology will depend on the nature of the damage or loss, but repairs are typically feathered into the existing, sound harling. The state of the art Reinstatement of previously stripped harling finishes to traditional and historic buildings is now becoming more prevalent. One of the challenges over the last two decades has been relearning the almost-lost traditions in terms of both materials and methods. Much of the early lime-revival harling work carried out in the early 1990s was founded on the use of non-hydraulic, putty- based mortars, often utilising practices employed in the application of cement harling/pebbledash or plasterwork, where relatively thick, float-applied, Many of the materials and methods used for Fig. 7 Wardlaw base and straightening coats were common. This lime harling over the past two decades bear little Mausoleum, Kirkhill, unfortunately led to a number of failures in lime harls resemblance to traditional lime-harl coatings. We still highlighting typical surface weathering which adversely impacted on their reintroduction. have much to relearn from traditional practice. Harling of a ten-year-old lime Today, much of the harling work carried out utilises which has survived for over a century on many of our harling and limewash excessively strong modern hydraulic limes, possibly traditional and historic buildings can tell us much about on a moderately to overcompensate for some of these earlier failures. what we should be doing today, particularly in the way exposed site. Whilst the properties and characteristics of many that hot-lime mortars, renders, and harls had been used modern hydraulic limes are known and tested, it is successfully. often argued that the properties and characteristics of the more traditional lime mixes, including air limes and This article was first published in The Building hot limes, are more variable and less reliable. History Conservation Directory 2013 and has been updated and and tradition, however, tell us otherwise. adapted for inclusion within the BLF Journal. Mechanical spray application of lime harling is also becoming more commonplace. There are some advantages of spray application in that a lower water content is required for application, minimising the risk Reference of drying shrinkage and creating a more compacted 1 J. C. Loudon, Encyclopedia of cottage, farm and mortar finish. The action of spraying under pressure, villa architecture , 1833. when it is well applied, also helps to ensure a good contact between the substrate and the lime coating. Recommended reading However, if reinstating a harl finish to a traditional or Technical advice note 15: External lime coatings on historic property, it can be di fficult, if not impossible, traditional buildings , Historic Scotland. for a sprayed-on coating to match the texture and appearance of a traditional hand-cast finish. In such A. Henry and J. Stewart, Practical building circumstances, a hand-cast finish should be specified conservation: Mortars, renders and plasters , Ashgate for the finish coat, even if the underlying coats are to be Publishing for Historic England, 2012. mechanically sprayed.

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