So fountain pen ink, which has to be thin so as not to clog the workings of the pen, is formulated differently from India ink (like what you use for calligraphy with a dip pen and nib), which has binders that make it a tiny bit thicker and, when dry, more permanent. In my experience, at least, fountain pen ink doesn't work as well for dip pens and in some cases doesn't work at all, as it flows out too much and bleeds and feathers on paper that would otherwise be suitable.
This made me very sad, as there is a far wider range of fountain pen inks. So I got a bottle of gum arabic, a traditional paint binder made from tree sap, and adding just a drop to a little bit of fountain pen ink makes it much more tractable!
A nice side effect is that it makes the sheen in sheening ink really pop.
