Lime or Whitewash is one of the oldest and cheapest methods of coating exterior and interior surfaces. It is created from Slacked Lime. Slack Lime has good opacity and so covering property.
Lime stones (CaCO3), are heated to temperature (above 825 °C) to form Quick Lime (CaO) which is then quenched by adding water to form Calcium Hydroxide (slaked lime-CaOH2). Lime bearing materials are lime-stones, marbles, chalk, sea shells.
A lime-wash of slaked lime or CaOH2, on evaporation of water has very little binding property. A lime wash for first fortnight has almost no strength, however, over few days of atmospheric exposure it begins to absorb CO2 from air and converts itself to CaCO3 – a reaction called carbonation.
A white wash retains 85 % of its original reflectivity for consecutive 120 to 180 non rainfall days. However, the operative life may stretch more than one monsoon season on exteriors and possibly two or more monsoon seasons in the interiors.
A hydrated Lime often contains impurities like magnesium carbonate, calcium sulphate, chalk. The quenching or mixing water may also contain soluble salts and sediments. Both the types of impurities increase the chances of flaking (poor surface adhesion). To improve the adhesion materials like Casein (a milk protein), gums, glues, cooked starches, molasses, sugar, alum, common salt, oils, tallow, fats have been used. In modern age polymers (emulsions) are added. All these old and new additives supposedly improve the adhesion in the shorter run. These substances invariably hamper re-coating after ageing.
Additives to Lime Wash may provide little more water repellence but they always reduce the process of carbonation of the material, resulting in chalking on the surface. For extra brightness ultramarine blue colour is added, and for other colour alkali resistant pigments like iron oxides are used. If the pigment additive is not alkali resistant, it will get faded out.
A Lime wash is considered air purifying treatment. It removes stench and rancidity of the enclosed spaces. American barns were regularly white washed with lime for the antimicrobial properties that provide hygienic and sanitary atmosphere. Limes washes, were used over tree trunks for their anti bacterial properties.
To day Lime-wash is considered ideal restoration-conservation coating as it does not harm the masonry surfaces. It is also a sustainable product.