How do you define one kilogram? Easy: it's the exact mass of a metal cylinder called the International Prototype Kilogram, IPK for short, kept in controlled conditions in France. But the standard kilogram has gained weight since its creation in 1875. To trim it and its many replicas down to size, we need to clean them. The report is in the journal Metrologia. [Peter Cumpson and Naoko Sano, Stability of reference masses V: UV/ozone treatment of gold and platinum surfaces]

When scientists analyzed the surface of an IPK replica, they found it carried tens of additional micrograms. These changes are due to contaminants building up on the kilograms, despite their careful storage under glass. The additional mass is equal to just a few grains of sand. But the kilogram is such an essential unit that tiny changes to different replicas are important.

To remove the contaminants, we need a standard and reproducible cleaning method. Researchers exposed one kilogram measure to ozone and ultraviolet light, which removed the carbon-based contamination without damaging the metal. But to permanently standardize the kilogram, we'll need to define it in terms of fundamental constants, not a lump of metal in a jar.

—Sophie Bushwick

[The above text is a transcript of this podcast.]