Four methods for cleaning stainless-steel 1 kg weights were investigated as part of research aimed at improving the long-term stability of such mass artefacts by means of cleaning. Weights were repeatedly cleaned at intervals of 3 to 4 days and changes in mass measured by comparison with other standard weights. The most promising method is cleaning in a Soxhlet apparatus using ethanol, with a measured standard deviation of 2.5 mg for the change in mass due to cleaning and no resolvable net change in mass after consecutive cleanings. Similar repeatability was measured for cleaning in an ultrasonic bath containing ethanol, and for boiling in water, but average net losses in mass per cleaning of (1.6 ± 0.5) mg (1 ) and (1.1 ± 0.6) mg (1 ), respectively, were observed for these methods. Cleaning in a Soxhlet apparatus using water caused mass variations of up to ±40 mg. Repeated cleaning produces a recognizable pattern of mass changes that can be used to confirm successful cleaning of a weight after only two cleanings. Results on the increase in mass following cleaning are also presented.

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