What is Fountain Pen Friendly Ink?
This question was posed by someone new to fountain pens, and it's a really good one. It sounds silly at first: if the ink is made for fountain pens, shouldn't one assume that the ink is fountain pen friendly? The answer is a resounding NO!
First, you must realize that fountain pens come in modern, vintage, and antique varieties:
Modern pens are generally made from various synthetic resins (plastics), and most of them can handle a wide range of modern inks, in terms of both pH and innovative special-purpose formulations. The ink reservoir in most modern pens is a cartridge or converter that is disposable and easily replaced should the pen fail to write because of clogging or drying out. Most of these pens use a plastic feed that can take a bit of abuse (but some are subject to destruction by certain inks).
Vintage and antique pens were made of materials that might not be able to withstand exposure to the innovative formulations of some modern inks. There is also the age of the material to consider: over time some vintage and antique materials have become more brittle than when they were produced. We have mentioned pH, and although it is not the only factor in how friendly a given ink might be to pens, it is definitely one of the factors. You can delve deeper into pH by reading: Inks: Inks: Report on the pH of More than 60 Inks
Because of the influx of "boutique" inks over the last several years, you need to understand dye saturation. Fountain pen ink is a solution, and the more heavily saturated the solution is, the more intense its color will be. The problem with very heavy dye saturation is that it tends to clog a pen over a relatively short period of time. Surfactants are used to help overcome this but can only do so much. (Too much surfactant causes the ink to bleed and feather on the paper). We have become a disposable society, and fountain pens are no exception to the phenomenon. There are literally dozens of fountain pens that can be purchased for under $20. Use one for a few months, and when it gets clogged to the point that it no longer writes reliably, just toss it out and ink up a new one. This leaves our vintage and antique pens in a precarious situation when we use inks of this type. Let's face it, no vintage pen, no matter its value, is disposable. There are some people who use these types of inks with no problem because they maintain meticulous pen hygiene, almost to the point of being OCD. They use a pen, and as soon as they are done they empty it and flush it clean. More in-depth information on these and other inks can be found in the article: Inks: Inks: The good, the Bad, and the Ugly
We never really answered the question, did we? Fountain pen friendly ink can be defined as ink that works well in modern, vintage, and antique pens without causing damage to them. As a general rule, inks sold by companies that make pens tend to be safer and more friendly than inks made by companies that offer only ink. Three reliable go-to brands are Waterman, Sheaffer Skrip, and Parker Quink. Most of the Diamine inks are safe, but there are a few colors that are better avoided. Understand that no matter what advice we give and what we say can happen with any given ink, you might experience different results. That doesn't mean that the ink is safe and friendly for every user, it just means that it's working for you. Use caution when trying new inks; it's a good idea to try a new ink in an inexpensive pen, one that you consider disposable if things to south. Mixing inks can also present problems, and our suggestion to try new inks in cheap pens holds double there. No matter what inks you use, treat your pens well and practice good pen hygiene.