Celluloid pens should, during long storage, be filled with water about every three months; some of it is absorbed by the material and prolongs its life; the rest evaporates via cap-residue condensation. Hard rubber and celluloid must be stored light-protected, since especially sunlight leads to discolouration (hard rubber turns grey to fawn through sulphur oxidation) and material decomposition. Celluloid is typically recognised by powerful, colourful colours; hard rubber by dark, muted tones like black, dark red or muted orange; reliable distinction of black pieces is visually hardly possible. Functioning collector pieces should not be stored with ink but with water, since ink on drying damages mechanism and seals; before storage, thorough flushing is needed. Writing instruments after 1960 can be stored without water filling but should be flushed beforehand.