After joining the metal and wood that make up the mbira, one more step must be taken before the mbira is complete. Oil from the hands must be rubbed on the wood until it becomes smooth and shiny. And the metal keys must be repeatedly struck and scratched with the nails of the hand before they too become smooth and shiny, softening the mbira's firm tone. The sound then takes on a certain sweetness. This process can be compared to the ripening of a fruit, as a fruit cannot be eaten until it fully matures and changes its colors.
By the way, the mbira is made of wood from the "mukawa" tree, which only grows in Zimbabwe. The quality and age of the tree have a big influence on the sound and tone of the mbira. In the past 500 years, many mbira makers have gone through trial and error, experimenting with the wood from other trees, but none have been proven to be as suitable for mbira as the mukawa.