To prevent the building from direct action of rain and moisture, lime plaster is used for exterior plastering. For this plastering, lime has to be prepared in a certain specific way.
Economically sustainable and it provides water resistance and breathability.
‘Kankar lime’is bought from the market. It is having 25-30% lime and rest of it is impurities. These impurities act as aggregates in the mix.
9 baand of lime and 15 baands of quarry dust (m sand ) is mixed well, to make one batch in the lime chakki.
WHY QUARRY DUST?
We want to avoid usage of river sand as it is not ecological. And quarry-sand has sharp edges which make for a compact arrangement.
9kg kadukkai (powdered or crushed) and 10kg jaggery to 300 to 400 ltr of water and allowing it to ferment for a minimum of 2 days.
This mix is then put into Lime-Chakki and Kadukkai-Jaggery water is poured over it. The entire thing is made into a paste by running the chakki with the help of bulls. It is estimated that after about 150 rounds of the chakki, the mix will attain the required consistency (from traditional knowledge).Kadukkai and jaggery mix gels and binds lime and quarry sand very well.
TEST FOR CONSISTENCY:
Take the mix in hand, leave little gap between the fingers and allow lime to fill in those gaps. Turn the palm upside down. If the mix won’t fall down if the mix is good.
The prepared mix is then stored for future use. Kankar lime as mentioned earlier is hydraulic lime and it will set very soon if exposed to air. Thus lime plaster is covered with kaudukkai- jaggery water for two weeks (Lime plaster also sets under kadukkai- jaggery water but in a very slow rate. Thus it should be ideally used up after 2 weeks of storage).
The stored lime is taken out in required quantities and is mixed with kadukkai-jaggery water, to make a more liquefied paste and it is applied over the walls as the first coat (scratch coat).
The wall surface must be made moist before applying the scratch coat ideally by spraying water on the surface or by splashing water with a mug.
The mix should be thrown onto the walls with good force, using a trowel in a swing so that it will grip the wall. The technique takes quite some time to be mastered.
First coat fills in all the gaps and undulations on the wall.
When the first coat is dried, 1.5″ thick second coat (levelling coat) is applied using the same mix, but having more amount of kadukkai-jaggery water, and the walls are smoothened.
A final coat (finish coat) is applied to cover all the minute cracks and to give a smooth surface finish.
At the places like edge of walls that are pointing the lime plaster should be rounded. If it is finished with pointed edge plaster will crack after undergoing expansion due to carbonation (setting).
Required colors can be achieved by mixing lime with various natural pigments and colored sand, and applying as a final lime wash.
Note: If hair line cracks appear then they should be gently hammered with a wooden hammer with rounded edge. This will seal the gaps and we can achieve a crack free surface.