The surfaces of a disc forming a filler element of a fountain pen (see Group XV) may be "Lucite" (methyl methacrylate resin), "Pyralin" (R.T.M.) (cellulose nitrate plastic) nylon or polystyrene and may be treated to improve their wettability by ensuring that all the surfaces are free of foreign matter such as oils, grease and wax. Where the material is "Lucite" the discs are immersed in 20 per cent fuming sulphuric acid at room temperature for approximately 10 minutes and then removed and washed in distilled water. Alternatively, the discs may be immersed in concentrated sulphuric acid for 15 minutes and then washed with methanol, or the discs may be immersed in a saturated solution of chromic acid at room temperature and then washed in distilled water followed by methanol. In certain cases the wettability can be increased by including in the material a suitable wetting agent. In the formation of Lucite from 2 1/2 -10 per cent "Aerosol" O.T. (di-octyl sodium sulfosuccinate) is mixed with the methyl methacrylate while the latter is in monomer form and the latter is then polymerized. In place of "Aerosol" other wetting agents such as 7 1/2 -12 per cent benzoic acid, 5 per cent glycerine, 7 per cent methacrylic acid or 2 1/2 -10 per cent di-ethylene glycol may be used.