This 2012 presentation by the Bureau International des Poids et Mesures
describes the process by which the prototypes are manufactured and
how they are washed and cleaned and then calibrated.
The platinum-iridium mass standards are cleaned with chamois leather
soaked for 48 hours in a mixture of equal parts ethanol and ether after
which the absorbed solvent is wrung out of the leather.
They are then steam cleaned with water.
Cleaning the prototypes can remove between 5ug and 60ug of "contamination"
depending upon when they were last cleaned.
http://www.bipm.org/ws/CCM/MeP_2012/...1nov_final.pdf