Older celluloid fountain pens should be filled with water roughly every three months during longer storage; some of the water visibly disappears, partly given off through condensation and partly absorbed by the celluloid, which prolongs the service life. Writing instruments made of celluloid and hard rubber must in general be protected from light, especially sunlight, since the material otherwise ages; celluloid was the first industrial plastic and tends to self-decompose, as is also known from old film reels. Hard rubber was historically only limitedly dyeable, which is why almost all pre-celluloid pens are black, while celluloid made strong, colourful tones possible. Pens of newer construction, particularly from around 1960 onward, can be stored empty but should first be flushed with water so that dried ink does not damage the piston seal lip in later use. Celluloid and hard-rubber pens must on no account be placed in an ultrasonic bath, as the colour can dissolve; polishes are also to be avoided because many products soften the surface. Hard rubber can be cared for by rubbing it with olive oil or gun oil such as Ballistol, which restores a darker surface. For material identification, celluloid is mostly very colourful and brightly dyed, while hard rubber shows muted tones such as black, dark red or rarely orange and can discolour to grey-brown or fawn under light influence; safe identification is only possible via a burn test, which, however, is not an option on a whole pen, since celluloid is extremely flammable.