The right answer depends mainly on the piston material. Pens with plastic pistons, typically models from the 1960s onwards, can be stored empty without problems after a thorough cleaning. With cork pistons one must distinguish whether the cork has been freshly renewed; a freshly fitted cork soaked in wax stays tight for years even without being filled with water. Older, already used cork should likewise be stored dry, since even distilled water can promote bacteria over time, which in extreme cases leads to mould on cork or celluloid. If the pen is to be used again later, it is enough to fill it with water a few days before use so the cork can swell again. Ink is regarded as the most harmful thing that can be put in a fountain pen; therefore it is advisable to fully disassemble pens before storage, fit a new cork seal or a new ink sac, soak the parts for at least a week in water, and use only distilled water for filling. To test nib operation it is enough to dip the nib briefly in ink; the feed will draw enough for a writing sample. Pens with polyethylene seals, used by Montblanc from the model 252 onward, i.e. from the 1960s, do not need any moisture for storage.