Limewash is a traditional interior and exterior finish for many stone and brick buildings, applied either directly to the masonry surface or, more commonly, to a coating of lime plaster, render or harling. As a vapour permeable material, limewash is the most effective and appropriate finish available for traditional buildings: it is also the most beautiful.
As with other lime-based materials, limewash is a relatively environmentally friendly material that cannot burn or ignite and does not give off VOCs or other toxic gases. Limewash is UV-proof (ultra-violet light is the main agent of decay in modern exterior paint systems). There is no risk of adverse reactions between coats and, in fact, each addition binds and improves what is already there. Traditionally, limewash is tinted with cheap, local earth or mineral pigments, all of which are compatible with lime.
A key characteristic of any limewash is the size of the lime (calcium hydroxide) particles in suspension. All limewashes are suspensions of calcium hydroxide particles along with small amounts of calcium carbonate, silica particles, and other minerals. The typical size of particles in a limewash made from hydrated lime is likely to be around 200-300 microns or more (the actual particles of calcium hydroxide are smaller than this but they agglomerate when mixed with water and even intense mixing cannot fully break up the agglomerations).
A limewash made from slaking quicklime will have smaller particle size, perhaps 100-200 microns, which is why this material is considered better than hydrated lime. Smaller particle size gives a number of benefits:
- The suspension is much more stable and does not settle as
fast, making application and handling much easier and more
consistent. - The particles are able to penetrate into the substrate more easily
as they can exploit more (and smaller) pores and capillaries in
the surface, giving a more bonded and durable finish. - The greater surface area within the suspension allows the
limewash to hold more pigment enabling richer colours to be
achieved. - The greater surface area also means that the carbonation
reaction is rapid and consistent. - Finally, there is some evidence that vapour permeability of the
limewashed surface is increased with a decrease in the particle
size in the limewash.
In short, the smaller the particle size the better. One of the finest examples, White Peak limewash has an average particle size of just one micron. Made at Buxton Lime’s Tunstead Works in Derbyshire using techniques developed for industrial water treatments, its particles do not form agglomerations. A superfine suspension of calcium hydroxide such as this results in easy handling and application, excellent finish and durability, and the ability to hold rich colours and to handle a wider range of substrates than previously possible.
K Rend approved