Back in 2009, as Morning Edition reported, some scientists were worried that the small metal cylinder used as the world's standard for what a kilogram should be might be losing weight.

But there were also those who thought the problem might have nothing to do with that carefully sealed away cylinder in Paris — that the real problem might be that the 40 replicas sent around the world are gaining weight.

The theory is that surface contamination is building up on the replicas, adding in some cases 50 or so micrograms to their weights compared to the original. They're kept in controlled conditions, of course, but get taken out more often than the original in Paris.

But you can't just clean them any old way. If a kilogram is going to be the same everywhere — an important thing for many scientific endeavors — the cleaning procedures have to be the same everywhere. And they have to do the job without scraping away any important material.

Well, as Wired reports, "scientists at Newcastle University have developed a high-tech way to clean the standards. ... It's based not on manually scrubbing the metal chunks but on exposing them to ultraviolet light and ozone about once per decade. Recently, [Newcastle's Peter] Cumpson fine-tuned the procedure and added another step: a pure water rinse to remove dust particles. The final recipe is now available online in the journal Metrologia."

Worrying about the replicas may some day seem quaint. As Wired has previously reported, scientists are also working on a way to craft a perfect kilogram out of silicon. To do it, though, they had to count the number of atoms in a kilogram of silicon — and then try to reproduce it. But even after two years of grinding and polishing, they still only reached "near perfection."

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