As more fountain pen enthusiasts venture into the world of vintage pens, the question often arises -- what inks are safe? We've had a discussion of this posted for some time in our website FAQ, and I'd encourage readers to click through to read it. What I'd like to emphasize here is that vintage pens are not all alike.
Older pens can differ radically in both materials and construction. Hard rubber pens that are filled manually with a dropper -- mostly dating to the 1920s and earlier -- can handle any ink around. Hard rubber is extremely resistant to chemicals of all sorts, more so in fact that any modern pen plastic. Hard rubber retracting-nib safety pens can even be used with India inks.
Pens with a built-in filling mechanism which have hard rubber sections and feeds and which hold their ink in a rubber ink sac fall into a different category. No harm is going to be done to the hard rubber section assembly, and since the ink isn't in contact with other parts such as the cap or the barrel, they don't have to be worried about. The concern with this class of pens is pretty much exclusively the ink sac -- a relatively cheap and usually easily-replaced part. The standard material for sacs is latex rubber, and some inks are known to play badly with latex. In some cases this can be gotten around by using a sac made from a more resistant material such as silicone or PVC. For more information on this, see our FAQ discussion linked above and our Pen Sacs Primer.
Those vintage pens for which ink selection is most important are the ones which hold their ink directly in contact with celluloid or cellulose acetate parts. These materials are more permeable than hard plastics such as acrylics, and are thus vulnerable to staining. Examples of such pens are Parker Vacumatics, Sheaffer plunger-fillers, and older German piston-fillers, as well as a multitude of pens that use ink sacs but which have celluloid sections with transparent ink windows. In such pens you will especially want to avoid alkaline inks, which can attack both celluloid and latex rubber sacs. Many popular Japanese inks are strongly alkaline.
Finally, it should be noted that it can be a real pain to change inks with certain older pens. Perhaps the most extreme example would be the capillary-filling Parker 61, but earlier pump-filling Parker 51s and to a lesser extent Vacumatics also take a lot of work to empty and rinse. These pens were not designed for regular ink-switching, so best to select another model if you plan to change inks frequently -- or just buy more of them!
