This is very true, the pen, ink, and paper work together to form a system. They all have some effect on all of the variables such as flow, ease of movement, drying time, etc. For the most part modern ink are better in every aspect compared to classic inks.

I'm not sure I understand the difference between modern and classic inks. Is there a definition somewhere?

(I have 20-25 ink bottles of ink and by trial and error I've roughly figured out what ink works in what pen. I probably should know more about inks given that I write a lot, but I haven't made any systematic approach to figuring this out :-))

> I'm not sure I understand the difference between modern and classic inks. Is there a definition somewhere?

I'm not sure if there is an official definition. But what I mean by this, is that classic inks (like iron gall) were usually a combination of pigment and an acid as a carrier. Modern inks are usually alcohol or water carriers with nano-pigments or dyes, combined with surfactants which helps ensure the ink flows well without any issues across varied paper types.

Ink has moved forward rather significantly in modern times, but I am not a chemist so I can't really get into details. I've used (out of some desire for the archaic) some correct reproductions of classic inks like iron gall or other pigment inks and it is kind of nasty and clogs the nib of the pen, but leaves a very permanent mark on the paper that has a somewhat unique appearance. I know some people prefer something anachronistic, but I am a fan of modern ink for sure.

One data point i can contribute is calligraphic inks. Using normal fountain pen ink with a square nib dip pen will produce writing that will spread even when dry. The binding agent in the ink is designed for much smaller strokes.

Calligraphic ink (i use Windsor Newton) is quite different and forms something like a scale on the paper when dry holding everything in place. You can feel the texture of the writing when it's drym