Lime plaster is type of plaster composed of hydrated lime (slaked lime), sand and water. Lime plaster is similar to Lime mortar, the main difference is based on use rather than composition. Traditional lime plaster also contains horse hair for reinforcement.
Improvement in the slow curing time of lime plaster, especially in cold and/or wet conditions was done in Roman times by adding volcanic ash (activated aluminium silicates) to produce hydraulic lime and hydraulic cement. Today, cheaper power station fly ash or other waste products are used to make hydraulic lime plaster or cement instead of volcanic ash.
Lime plaster is sold as 'bagged' powder or hydrated lime; or is available as lime putty. Lime putty is generally considered to be more suitable for pure lime application. Lime renders are mixed and composed of a sand / lime mix on site, however pre mixed bagged Lime Renders are available but one should remember it was nearly always mixed on site.
Lime renders were traditionally applied to give protection to walls built of poor quality rubble stone or porous brick or to walls in exposed locations facing driving winds. They help by acting like a sponge, absorbing rainfall then allowing it to evaporate rather than soak into the wall. Most cottages and houses built of rubble stone would have been rendered originally and they tend to suffer from penetrating damp if the lime render is removed or replaced with a cement rich render.
There is a very wide range of types of lime rendering. Rubble walls of many vernacular buildings were often treated with just a single coat of render, amounting to not much more than a very full, flush pointing. Such a render is thicker in the hollows and very thin over the stone faces. There was no attempt to create a flat surface so the undulations of the wall and even some of the stones themselves were not concealed. For a smarter finish or on more prestigious buildings the aim would be for a more uniform render achieved by applying a scratch coat to fill the hollows and take up some of the unevenness followed by one or two more coats which were worked to a flatter surface. Sometimes joint lines were ruled into the damp top coat to create the illusion of ashlar stone, but a common finish for many houses and cottages was a rough-cast where the final coat consisted of a mortar slurry containing coarse grit applied by throwing from a special trowel. For interiors a fairly smooth surface could be obtained using a coarse render mix, but for top quality internal plastering the final coat would be richer in lime and polished up to a smooth, close finish.
Lime Renders and lime plasters can be applied to a variety of backgrounds including earth (which should nearly always be rendered), stone and brick. Lime plaster is also applied to wooden laths for ceilings and internal partitions.
By carefully selecting appropriate aggregates it is possible to match existing renders and successfully repair failed patches without the need for complete re-rendering. Hollow or detached plaster can sometimes be consolidated and saved and further advice should be sought before replacing it, especially if it is very old. .