QUARRYING provided blocks for building, but also a material to help fix them in place. Lime was a key component of many traditional mortars, renders and washes used externally and internally on buildings. It was generally derived from limestone or chalk, though sea shells were sometimes used in coastal areas. These materials, which have a high calcium carbonate content, were burnt in a lime-kiln, releasing carbon dioxide (CO²). Lime burning was often small scale and localised; it could also be a farming sideline since lime was useful for agriculture as well as building. Larger, more industrialised kilns, offering economies of scale, gradually came to dominate. The mid-nineteenth-century Annery lime kiln at Weare Gifford in north Devon combined three kilns in one structure. It was located next to a river for ease of transportation and an adjacent row of cottages provided accommodation for workers. Primitive kilns might merely be a chamber dug in the side of a quarry and usually worked on the principle of an ‘intermittent’ burn resulting from layered calcium carbonate and fuel. The lime had to be removed at intervals from the kiln base as the burn proceeded. More sophisticated kilns kept fuel and calcium carbonate separate, allowing a continuous burn. The lime burner looked for a cherry red flame indicating temperatures around 1,000 degrees centigrade at which point carbon dioxide would be released from the calcium carbonate.
The product of lime burning was calcium oxide, otherwise known as ‘quick’ or ‘lump’ lime. Quicklime is notorious for its caustic effects and was used to accelerate the decomposition of buried bodies. For building, the quicklime needed to be put through one further process – slaking – which was usually carried out in a pit. Violent bubbling occurred when water was added to fresh quicklime. Sometimes, for uses where consistent quality was less important, the ‘hot lime’ method would be employed where quicklime was mixed dry with aggregate and water was then added. The slaking process, however, yielded calcium hydroxide or ‘milk of lime’, which settled to form lime water and, most importantly for building, lime putty. Lime putty is white with the consistency of soft cheese and is known as ‘non-hydraulic’ because it does not set underwater. Rather, when left in water to mature it ‘fattens’, a process requiring at least a month but ideally more. When removed from water for use, the lime putty gradually carbonates in the air, returning to its original chemical state of calcium carbonate. This is how lime mortars and renders harden.
The now-ruinous Annery lime kiln at Weare Gifford, north Devon. The kiln was at the centre of a complex which included workers’ cottages and had easy access to the river Torridge. When built in the nineteenth century the kiln had a decorative crenelated parapet.
Quicklime is slaked in water to produce lime putty for use in mortars, renders and washes. The reaction is violent, generating considerable heat.
Building lime can exist in two other forms. Powdered hydrated lime is found in all modern builders’ merchants and is used principally to plasticise cement mortars. Just enough water is added to quicklime during manufacture to allow slaking and then all moisture is driven off by steam. A further powdered form is hydraulic lime – not to be confused with its hydrated cousin. Hydraulic lime can be produced from calcium carbonate sources such as Blue Lias limestone. This liassic limestone includes clay impurities that have the effect of increasing the setting properties of the lime. Hydraulic limes will set under water like modern Portland cement. Lime putty has a much slower speed of hardening but this rate can be increased by the addition of materials such as low-fired brick dust (known as a ‘pozzolanic’ additive). The Romans used a volcanic ash called Pozzolana for this purpose.
The lime mortar pointing of this random rubble walling in south Somerset is traditionally finished flush with the face of the stone.
A mechanical roller pan mill used to mix lime-based materials for use in modern building work.
Lime mortars, plasters and renders are similar in composition, involving a mix of lime – in one of its forms – with aggregate. The ratio is often quoted as one part lime to three parts aggregate (mentioned in writings as ancient as those of the Roman, Vitruvius), though this will vary depending on the type of aggregate and its intended use. Water is not normally added where lime putty is used. Lime mortar, if not mixed by hand, requires a mortar mill or pan mixer rather than a drum mixer of the kind used for modern cement-based materials. A traditional lime mortar, when used for rubble or brick walling, includes well-graded sand particles – that is sand in a range of sizes. The sand has to be ‘sharp’ (of angular form) so that the particles lock together to form a matrix which helps reduce shrinkage as the mortar sets. The aggregate, which bulks-out the more costly lime, was usually obtained from rivers or pits. Beach sand was sometimes used at the coast, but the high salt content could be problematic. The colour of the aggregate influenced the colour of the mortar. Where joints between blocks were slender, as with gauged brickwork or precisely cut ashlar blocks, a finer aggregate was used, such as silver sand, or stone or brick dust.
Mortar joints separate stones or bricks and allow even courses to be laid where the masonry blocks are of irregular size. Lime-based materials have advantages for traditional construction over harder cement-based products in being relatively ‘breathable’. Their softness in comparison with the masonry they surround allows evaporation of moisture to be concentrated in the joint. This makes the mortar ‘sacrificial’ but it is more easily replaced, through periodic repointing, than the stone or brick. Lime-based mortars can also accommodate some structural movement in masonry, which was particularly necessary as foundations of traditional buildings were often shallow.
Render being lined out in imitation of ashlar stonework. To the left the exposed base coat can be seen with its surface scratched to make a key for the finishing coat.
Lime-based plasters and renders typically had a ratio of lime to aggregate very similar to mortar. They provided a protective, unifying finish for walling of all types, including earth, masonry and timber. The plaster or render was normally applied in two or three coats, with the top coats thinner and containing finer aggregate. On masonry, the base coat would serve to form a good bond with the underlying wall, and would level some of the more pronounced irregularities of the surface. The float coat above would provide a level surface, and would be scratched to improve adhesion of the final finish coat. Like the base coat it would include hair from an animal, generally goat, cow or horse, chopped into short lengths and spread evenly throughout the mix to provide extra cohesion. The finish or setting coat would contain fine aggregate and in a smooth plaster or render would be neatly trowelled. A contrasting finish could be obtained from ‘roughcast’ render, or ‘harling’ as it is known in Scotland. This ‘wet dash’ employed coats containing coarse aggregate that were thrown forcibly by hand against the wall, with a clutch of flexible twigs or in Scotland with a special ‘harling trowel’, producing a textured surface.
A façade with rough and smooth textures; the wall is finished with a lime-based roughcast render while the shell-hooded porch and hood-moulded window surrounds are finely detailed in carved stone.
In lime roughcast, known as harling in Scotland, the top coats contain a coarse aggregate and are applied by being thrown forcibly at the wall.
Lime plaster or render could be applied directly to the daub panels in a traditional timber frame, but if applied across the whole surface of the building, timber laths would be fixed first. These thin strips of timber were usually of riven (split) oak, but sawn softwood later became the norm. They were nailed to the primary structure, positioned so that a gap of an index finger’s width remained between them. This gap allowed a ‘key’ to be formed between the laths and the base coat of the lime plaster. The plaster pushed through, creating a ‘nib’, and anchored behind the lath as it set. Much lime plaster on timber frames had a finish smoothed with a wooden float but in East Anglia, in particular, a tradition developed of decorative pargetting. The technique’s origins seem to lie in the sixteenth century and may draw on Italian stucco traditions, reinvented for their British context. Pargetting could involve simple, decorative surface patterns. Fan shapes, basket weave, chevrons or small indentations, sometimes within outlined panels, were common. Until about 1700 figurative or architectural forms were sometimes created in high relief. Projecting armatures and pre-moulded shapes might be required to produce these, and backing coats sometimes incorporated straw or wood shavings as well as aggregates and animal hair.
Decorative dated plasterwork in the gable end of a house at Reigate in Surrey.
A member of expert pargetter Bill Sargent’s team finishes a panel of figurative, lime-based pargetting. The pargetting tradition has been revived in East Anglia, but not always with the use of lime-based materials.
A common final finish for plaster and render, but also for masonry, brick and timber framing, was limewash. This was made from lime putty, sieved and diluted with water to the point where it had the consistency of thin cream. Natural earth pigments such as ochre, or even bull’s blood, could be added to produce subtle colours. Limewash provided a unifying and protective finish. In west Wales it was even applied to thatch. In comparison with modern masonry paints, it was labour intensive, since, as Smeaton remarked in 1837, ‘it is better to put two thin coats on a wall than one thick one.’ Limewash also needed to be renewed regularly, yet in comparison with impermeable modern paints limewash is ‘breathable’ allowing the moisture movement essential to traditional walling. It also offers attractive soft textures and subtle variations in colour across a wall surface.
Despite their compatibility with traditional building methods and materials, by the early twentieth century lime-based mortars, plasters and renders were being steadily superseded. Their disadvantages were relatively slow setting times and vulnerability to frost damage. Lime work had to be protected against the effects of frost, or over-rapid drying in hotter periods and could quickly fail if used inappropriately. Replacements for traditional lime-based materials, based on natural cements, had been in production from the end of the eighteenth century. Parker’s Roman Cement of 1796 included materials with high clay content, yielding an accelerated rate of set. Loudon wrote in 1834 that ‘The plasterer ... is now a very important artisan, having the sole employment of that material which produces such magical effects on exteriors, Roman cement. In consequence of the discovery of cements of this kind, we are now enabled to erect buildings of brick, coated over with this material, which are as handsome as those of stone, and much stronger and more durable’. The last point was debatable, but Roman and other natural cements were widely employed in the early to mid-nineteenth century for smart stucco-fronted houses. John Nash, one of the pre-eminent architects of this period, commented on how these stuccos might be ‘coloured and jointed in imitation of Bath stone.’ An even more significant development was the invention of Portland cement, patented in 1824 by Leeds bricklayer, Joseph Aspdin. It was named ‘Portland’ for its supposed likeness to Portland stone.
Limewash is a ‘breathable’ protective finish that also offers a soft texture and beautiful variations in tone.
External walls of many rubble-stone buildings were finished with a lime-based mortar, flush with the face of the stone, before being coated with layers of limewash to create a uniform surface. This example comes from the Cotswolds.
A weaver’s cottage at Dyserth in Denbighshire, North Wales. Limewash was exploited throughout Britain as an easy way to protect external walls while simultaneously smartening their appearance.
A manufactured material that could be used for mortars and renders alike, Portland cement had a rapid set and was more forgiving of hostile weather and unskilled workmanship, but its relative hardness has caused considerable damage to traditionally constructed walls. In the twentieth century, Portland cement became the preferred constituent of mortars and renders with lime, if used at all, relegated to a secondary, plasticising component. In the twenty-first century, however, lime has enjoyed a revival, boasting better ‘green’ credentials than cement, as well as great advantages when used with other traditional materials.
Stucco with vermiculated (worm-like) decoration to the facade of a mid-nineteenth century seafront house in Brighton.
An early Ransome cement mixer. Portland cement, which was patented in the 1820s, gradually superseded earth and lime-based materials. It offered a quick set but was much less compatible with traditional constructional materials and methods.
Mathematical tiles on the façade of Brighton’s fashionable Royal Crescent, built in 1798. Mathematical tiles provided a simple, smart finish to a lightweight timber frame and were well-suited to the local marine environment.
