What is the black spot on the mridangam?

The black spot on a mridangam is a permanent load on the right, treble head, called soru, saadham or karanai in Tamil usage. Makers bind an iron-bearing mineral powder with cooked rice or rice flour and build it up in many thin layers; Wesleyan records up to 50. The load gives the head a definite pitch and a metallic timbre.