The perfect sphere that will be used to redefine the kilogram in terms of the Planck constant in 2018 has been shown to consist of the purest silicon ever made.
The kilogram is the only unit still based on a physical object. The ‘big k’ is a platinum–iridium cylinder stored in an underground vault in France. However, over time this artefact can gain or lose atoms, which is why the International Committee for Weights and Measures decided to redefine the unit using the Planck constant. This fundamental physical constant relates to mass through Einstein’s formula E = mc2