From http://en.wikipedia.org/wiki/Kilogram http://en.wikipedia.org/wiki/Kilogram : Since the IPK and its replicas are stored in air (albeit under two or more n

From http://en.wikipedia.org/wiki/Kilogram http://en.wikipedia.org/wiki/Kilogram : Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they gain mass through adsorption of atmospheric contamination onto their surfaces. Accordingly, they are cleaned in a process the BIPM developed between 1939 and 1946 known as “the BIPM cleaning method” that comprises lightly rubbing with a chamois soaked in equal parts ether and ethanol, steam cleaning with bi-distilled water, and allowing the prototypes to settle for 7–10 days before verification.