They called the sphere smooth at the beginning of the article and flat at the end of it. A centrifuge was used to separate isotopes, so the sphere would be exceptionally pure.
http://www.nist.gov/pml/si-redef/kg_new_silicon.cfm
Well, flat would be a strange word for the surface of a sphere. Maybe something was lost in translation.
The watt balance is mentioned. It would need to be used in a vacuum to get the precision needed (i.e. to remove the factor of the ball’s slight buoyancy in air).
You see? Iran is merely helping out the Weights and Measures guys who are working on this kilogram problem.
-- John Kerry