Soak urad dal overnight in water and grind to a fine paste. It should have the consistency of cake batter. Put it n a vessel and whisk it very well so that the mixture is fluffy. Heat oil well in a deep frying pan. Test if the oil is ready by dropping a little of the mixture into the pan if it fluffs up and floats to the surface the oil is ready. Take a piece of wet cloth on your left palm. This allows the vadas to slide off into the pan without difficulty. Put some of the mixture onto the cloth and flatten it, shaping it into round one-inch vadas. Slide these off one by one into the oil, and deep fry. They should be fried through and through, and have a golden brown colour. Take care not to make the vadas too thick.Keep a tawa on the fire and put the crystals of hing on it. Take a matka or an earthenware pot, and as soon as the hing emits an aroma, turn the matka upside down on the tawa to soak up the smell. Take it off the fire and fill it with warm water. Mix the rai, salt, red chilli powder and whole red chillies into the water. The fried vadas are now put into the matka. The top of the matka is covered with a clean muslin cloth and securely tied. The matka is left out in the sun during the day for eight days. It is then ready to be served.