Printer ink is useful because each of the C, M, and Y inks is a perfect filter of exactly one colour. C filters red, M filters green, and Y filters blue. Commercial fountain pen inks almost never have this kind of absorptive specificity. They’re usually a mix of two or more dyes—a mix that you don’t control. Once a dye is in the mix, you can’t just take it out. The best you could do is dim all of the other colours, but then you lose saturation.
Here’s a specific example. Suppose you have a commercial ink of 5C:1M and you want pure C. You’re stuck with that 1M. The best you can do is add 1Y to make 5C:1M:1Y = 4C:1K. You’ve got a balanced C, but that extra 1K is going to make everything look a little grey. Ew. And that is assuming you can even get pure Y in the first place. No ink manufacturer in their right mind would try to sell a pure Y on purpose. It is very difficult to read. (Except under a pure blue light. It’s super awesome actually. This has been an underhanded privacy-invading tactic of the government for some time now. Yellow microdots are printed on all commercial inkjet printouts.)
These inks have also been designed to be mutually equally absorptive of their respective light wavelengths, so an equal ratio of 1C:1Y makes a perfectly balanced green. These inks has also been designed to stay in solution even when mixed. There are no chemical reactions that could cause precipitate to form, thus totally fucking the pen. Achieving this with commercial fountain pen inks would be difficult, and potentially dangerous.
However.
That’s actually not the reason why I started using printer ink. I was in Oulu, I had just run out of fountain pen ink, and all I could find was a print shop. Here is the whole story of my Oulu trip.I did a little research online before actually doing it. Other people have done it before. You just have to make sure to use dye-based ink and not pigment-based ink. I was able to confirm from Timi that it was dye-based. And prepare for the possibility of having accidentally turned your pen into an ink firehose because printer ink bleeds like three motherfuckers. It needs at least three parts water to calm it down.