This is a sketch done by Leonardo da Vinci to record how Verrocchio went about creating the bronze ball. Mirrors were used to reflect solar rays on the pieces of metal to fuse them together. The metal was also hammered so mercury guilding could be used. This method was adopted to prevent the issues the first artist had when creating his palla. It is not confirmed, but widely thought that the issue must have been with the guilding of the palla and not the hammering of the metal itself. Unfortunately, the original palla was damaged when it fell and no longer rests atop the lantern.
Works Cited:
Neilson, Christina. Practice and Theory in the Italian Renaissance Workshop: Verrocchio and the Epistemology of Making Art. Cambridge University Press, 2019.