THE PRINCIPAL APPLICATIONS FOR LIME MORTARS ARE: Repointing If original lime mortar has decayed or been replaced by unsuitable materials good quality repointing in traditional lime mortar will enhance both the appearance and the weathering performance of your building. Unfortunately, repointing, more than any other maintenance operation is frequently poorly executed using inappropriate materials. The performance of traditional masonry buildings depends on the ability of the building fabric to transmit moisture vapour. Both stone and mortar naturally absorb a certain amount of water and, importantly, allow it to dry out again. If this process is inhibited or thrown out of balance by the use of impermeable mortars a number of problems may be encountered including accelerated stone decay and water penetration. In addition, modern cement mortars appear harsh and unsympathetic in conjunction with traditional stonework.
External lime coatings In Scotland, lime harling (rough cast) and to a lesser extent, flat renders, were widely used as external protective and decorative coatings. Harling was frequently applied to rubble masonry, first
brought out to a level surface with pinning stones (smaller stones, usually offcuts from the main masonry units, placed between the masonry units to level up courses of masonry and reduce the overall volume of mortar in any one place). The technique of casting material onto a wall surface in the form of harling was widely used and some harling was finished by pressing back to achieve a flatter surface. Where a more formal finish was required, often in urban areas, a trowel applied lime render, or stucco, might be used, lined out to imitate ashlar stonework.
Internal lime plaster In traditional wall construction, lime plaster, either on wood lath or directly onto the masonry, provides a healthy breathable finish. For the repair of lime plasterwork, including decorative work, the use of similar materials is essential. Most early materials and techniques can be matched for repair and conservation work. Later decorative work often incorporates a gauging of gypsum.
Lime plaster also possesses good thermal insulation properties.
Limewash Limewash is a traditional external and internal finish for many stone and brick buildings, applied either directly onto the masonry surface or, more commonly, to a coating of lime harling, render or plaster. A good quality limewash, well applied and cured, will last for many years. Properly applied, limewash will penetrate the surface of a lime render or harling and form a bond with this backing. Limewash will successfully fill small cracks in renders and harling, and subsequent small fissures developing over time will be sealed by the redeposition of lime. As a vapour permeable material, limewash is the most effective and appropriate finish available for traditional buildings. Limewash is also the most beautiful finish and appears to ‘glow’ at sunrise and sunset.
For further information contact: Scottish Lime Centre Trust Tel 01383 872722
admin@scotlime.org www.scotlime.org
Fax: 01383 872744
Listed Heritage Magazine January/February 2020 105
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