Collectors report that over long idle periods, ink residues accumulate in the cap and dry into a concentrate, since ink consists predominantly of water and the remaining pigment sinks to the bottom when cleaning. When cleaning, one should be careful: with some old celluloid models, black ink in particular attacks the material from inside, so that the cap can even break in a water bath. Careful rinsing under running water, followed by a water bath, is usually sufficient to clean the cap without damaging the pen.