State of the Avogadro 28Si spheres

The initial determination of Avogadro´s constant based on measurements of ²⁸Si material ended up with an overall uncertainty of 3×10^-8. The results were clouded by an unexpected effect: the surface of both silicon spheres were contaminated by small amounts of copper (Cu) und nickel (Ni). Although the contaminations were quantitatively measured and their impact into the different quantities was taken into account, it left an uncertainty component which could not be neglected. A special chemo-mechanical technique was applied with which the metallic contaminations could be removed. The decontamination, the subsequent measurements and their ramifications on the overall uncertainty of Avogadro´s constant are described.