Wash the rice and black gram and soak them separately for 4-5 hours.
Drain the water and grind the rice into a coarse paste with little water.
Grind the soaked lentils into fine paste by adding water little by little.
Mix together the rice and lentil paste. The batter should be thick dropping consistency.
Keep the batter in a warm place for 10 hours. After fermentation the batter will rise and become sour.
Add salt before making the idlis. Mix the batter gently.
Coat the surface of idli moulds with little and drop spoonful of batter in each mould.
Add about two cups of water in the idli steamer and place the idli stand in the steamer and close. Steam on high for 5 minutes.
Reduce heat and steam on low for 10 minutes. Remove the steamer from heat and let it stand for 10 minutes. Open and remove the idlis from the mould with a knife.
Serve warm with hot sambar or chutney.