In principle only water-soluble inks should be used in old fountain pens. Royal-blue Montblanc ink is unobjectionable for historical fountain pens with cork seals — reports of 'eaten-up' cork seals can mostly be traced to already pre-damaged material, not the ink preparation itself. Bordeaux-red Montblanc ink has, by contrast, in individual cases caused problems with old seals and is to be used with care. Pelikan royal blue (4001) counts as a tried and very compatible ink for vintage pens. Iron-gall inks and strongly colour-saturated specialty inks are to be avoided in old writing instruments.