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Harling is a rough-cast wall finish consisting of lime and aggregate, known for its rough texture. [1] Many castles and other buildings in Scotland and Ulster have walls finished with harling. It is also used on contemporary buildings, where it protects against the wet Scottish and Ulster climates and eliminates the need for paint.
Harling as a process covers stonework using a plastering process involving a slurry of small pebbles or fine chips of stone. After a wall is complete and has been pointed and allowed to cure then a base of lime render is applied to the bare stone. While this render is still wet a specially shaped trowel is used to throw the pebbles onto the lime surface, which are then lightly pressed into it. Harl, being mostly lime render, cures chemically rather than simply drying. After this setting process, the harl is sometimes lime washed in a colour using traditional techniques. [2] [3]
It is not recommendable to replace more than around 20% of the lime content with cement. Cement-based render is very stiff and prone to crack or detach from the wall when subjected to stress induced by expansion due to solar radiation and moisture. It is also less permeable to moisture and water vapour. Water entering through fine cracks in the surface does not easily diffuse and can penetrate into the softer stone, thus causing the deterioration which harling aims to prevent. For similar reasons, modern barrier paints should not be used in place of traditional lime washes.
An example of the inadequate protection afforded by high cement harling is the Charles Rennie Mackintosh designed Hill House completed in 1904 using the newly available but poorly understood Portland cement harling. Over the years water had penetrated the harling and threatened the overall integrity of the building. The National Trust for Scotland, the building's owner, has (2019) enclosed Hill House in a transparent 'box' which will allow some rain to enter but which will eventually allow it to dry naturally. It is estimated that this will take seven to ten years, after which the box will be removed. [4]
The technique of harling features in a large number of famous Scottish buildings including:
Terrazzo is a composite material, poured in place or precast, which is used for floor and wall treatments. It consists of chips of marble, quartz, granite, glass, or other suitable material, poured with a cementitious binder, polymeric, or a combination of both. Metal strips often divide sections, or changes in color or material in a pattern. Additional chips may be sprinkled atop the mix before it sets. After it is cured it is ground and polished smooth or otherwise finished to produce a uniformly textured surface. "Terrazzo" is also often used to describe any pattern similar to the original terrazzo floors.
Mortar is a workable paste which hardens to bind building blocks such as stones, bricks, and concrete masonry units, to fill and seal the irregular gaps between them, spread the weight of them evenly, and sometimes to add decorative colors or patterns to masonry walls. In its broadest sense, mortar includes pitch, asphalt, and soft mud or clay, as those used between mud bricks, as well as cement mortar. The word "mortar" comes from Old French mortier, "builder's mortar, plaster; bowl for mixing." (13c.).
Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "render" commonly refers to external applications. The term stucco refers to plasterwork that is worked in some way to produce relief decoration, rather than flat surfaces.
Craigievar Castle is a pinkish harled castle or fortified country house 6 miles (9.7 km) south of Alford, Aberdeenshire, Scotland. It was the seat of Clan Sempill, and the Forbes family resided here for 350 years until 1963, when the property was given to the National Trust for Scotland by William Forbes-Sempill, 19th Lord Sempill.
Auchindoun Castle is a 15th-century L-Plan tower castle located in Auchindoun near Dufftown in Moray, Scotland.
Stucco or render is a construction material made of aggregates, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as a decorative coating for walls and ceilings, exterior walls, and as a sculptural and artistic material in architecture. Stucco can be applied on construction materials such as metal, expanded metal lath, concrete, cinder block, or clay brick and adobe for decorative and structural purposes.
Castles are buildings that combine fortifications and residence, and many were built within the borders of modern Scotland. They arrived in Scotland with the introduction of feudalism in the twelfth century. Initially these were wooden motte-and-bailey constructions, but many were replaced by stone castles with a high curtain wall. During the Wars of Independence, Robert the Bruce pursued a policy of castle slighting. In the Late Middle Ages, new castles were built, some on a grander scale as "livery and maintenance" castles that could support a large garrison. Gunpowder weaponry led to the use of gun ports, platforms to mount guns and walls adapted to resist bombardment.
Lime is an inorganic material composed primarily of calcium oxides and hydroxides, usually calcium oxide and/or calcium hydroxide. It is also the name for calcium oxide which occurs as a product of coal-seam fires and in altered limestone xenoliths in volcanic ejecta. The International Mineralogical Association recognizes lime as a mineral with the chemical formula of CaO. The word lime originates with its earliest use as building mortar and has the sense of sticking or adhering.
Lime plaster is a type of plaster composed of sand, water, and lime, usually non-hydraulic hydrated lime. Ancient lime plaster often contained horse hair for reinforcement and pozzolan additives to reduce the working time.
The Hill House in Helensburgh, Scotland, was created by architects and designers Charles and Margaret Macdonald Mackintosh. The house is an example of the Modern Style. It was designed and built for the publisher Walter Blackie in 1902–1904.
Roughcast or pebbledash is a coarse plaster surface used on outside walls that consists of lime and sometimes cement mixed with sand, small gravel and often pebbles or shells. The materials are mixed into a slurry and are then thrown at the working surface with a trowel or scoop. The idea is to maintain an even spread, free from lumps, ridges or runs and without missing any background. Roughcasting incorporates the stones in the mix, whereas pebbledashing adds them on top.
Plasterwork is construction or ornamentation done with plaster, such as a layer of plaster on an interior or exterior wall structure, or plaster decorative moldings on ceilings or walls. This is also sometimes called pargeting. The process of creating plasterwork, called plastering or rendering, has been used in building construction for centuries. For the art history of three-dimensional plaster, see stucco.
A plasterer is a tradesman or tradesperson who works with plaster, such as forming a layer of plaster on an interior wall or plaster decorative moldings on ceilings or walls. The process of creating plasterwork, called plastering, has been used in building construction for centuries. A plasterer is someone who does a full 4 or 2 years apprenticeship to be fully qualified.
Lime mortar or torching is a masonry mortar composed of lime and an aggregate such as sand, mixed with water. It is one of the oldest known types of mortar, used in ancient Rome and Greece, when it largely replaced the clay and gypsum mortars common to ancient Egyptian construction.
Lime render is the first coat of lime "plaster or the like" applied to the external surfaces of traditionally-built stone or brick buildings.
Damp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Dampness problems are among the most frequent problems encountered in residences.
Basement waterproofing involves techniques and materials used to prevent water from penetrating the basement of a house or a building. Waterproofing a basement that is below ground level can require the application of sealant materials, the installation of drains and sump pumps, and more.
Cement render or cement plaster is the application of a mortar mix of sand and cement, and water to brick, concrete, stone, or mud brick. It is often textured, colored, or painted after application. It is generally used on exterior walls but can be used to feature an interior wall. Depending on the 'look' required, rendering can be fine or coarse, textured or smooth, natural or colored, pigmented or painted.
An external render is, in its most basic form, a coating applied to the walls of a building, to provide a protective coating which would prevent rain penetration. It also acts as a decorative finish to enhance the appearance of a building.
Coxton Tower is a late sixteenth-century tower house in Moray, Scotland. Heavily fortified, it was built around 1590, with substantive repairs in 1635 and 1645, but its design is reminiscent of much older buildings. It has not been occupied since around 1867 except to house Canadian soldiers during the Second World War, but was renovated in 2001 to help protect the fabric of the structure, which is designated a Category A listed building.