As infrequently as possible.

Weights should routinely be dusted, before use, using a clean, soft-haired brush but they should not be more extensively cleaned unless it is absolutely necessary. This might be, for example, if they are clearly contaminated with oil.

All weights have some contamination on their surfaces, albeit usually a small amount, and to a degree the amount of contamination remains constant, or in equilibrium, in given surroundings. Extensive cleaning, such as solvent cleaning, will change the surface chemistry and disturb this equilibrium. Initially it is likely that a cleaned weight will have a mass lower than before but it will probably increase as 'normal' air-borne contamination inevitably re-deposits on it. Either way, cleaning usually makes the mass of a weight less stable - at least for a while.

Before cleaning a weight

Be as certain as possible that cleaning, beyond routine dusting, really is necessary. Cleaning will not only invalidate any current certificate of calibration but will destroy any continuity of calibration history that may exist and this may have a significant knock-on effect in a measurement uncertainty budget. If the weight belongs to someone else, get their agreement that the weight should be cleaned and remind them of the potential consequences; it might also be appropriate to suggest to them how the storage and handling of their weights could be improved.

Weights should be calibrated before cleaning if it is important to measure the change in mass value caused by the cleaning process.

Cleaning procedure priorities

1. Lightly dust the weight with a clean, soft-haired brush (this should be undertaken routinely).
2. If Step (1) fails, lightly rub the weight with a soft clean cloth (a washed chamois leather is ideal).
3. If Step (2) fails, or is likely to further contaminate the weight (for example if there is an oil patch on the weight that would be spread around by rubbing), then solvent cleaning should be used as a last resort. In general this should take the form of wiping the weight with a clean cloth that has been soaked in solvent.
4. Other more elaborate forms of cleaning include the use of ultrasonic baths and soxhlet apparatus.

After cleaning

After cleaning a weight (excluding category 1 cleaning above) it should be allowed to stabilise before being re-calibrated. The stabilisation time will vary according to the class of weight and the extent of cleaning that has been undertaken, but it can range from a few hours to several days. Such weights should not be used until they have been re-calibrated because, as mentioned above, the previous calibration certificate will have been invalidated by the cleaning process.