I’ve always wanted to directly address this question because it’s one I get a lot (including multiple times today in-shop). Many pen companies - especially luxury purveyors such as Montblanc - make a point of emphasizing in their warranty materials that you should only use their in-house brands of ink. For example, only use Montblanc ink in a Montblanc pen, Pelikan ink in a Pelikan pen, etc. The reason for these disclaimers are straightforward: If you damage your pen by using another brand’s ink, they shouldn’t be held responsible for you using a product over which they have no control. But how likely is it that you will actually harm your pen by mixing pens and inks of different brands, or from using any given ink in general? In my experience, not very likely, and I can count on one hand the number of times I’ve seen someone do truly irreparable harm to a fountain pen by using the “wrong” ink.
I have no issue using boutique inks (like the Wet Pen inks shown here) in any of my pens. With any fountain pen ink, typically the worst thing that can happen is that the pen clogs, which a flush with water will fix. If you have any doubts, test with a glass pen or steel dip pen first.
Most Standard Fountain Pen Inks Are Perfectly Safe to Use in Any Pen
Fountain pen inks are generally water-based, so unless you are using calligraphy ink or ink with some sort of special chemical property (discussed further below), you are unlikely to cause permanent harm to your pen. Note that when I say that an ink is “safe,” I mean that it will not destroy the nib, feed, or filling system to the point where you will need to send it in for any sort of warranty repair. The pen will continue to write. HOWEVER, if you are using any color other than black or standard blue - and especially brighter colors like red or purple - you do run the risk of staining clear acrylics or lighter-color materials. My personal recommendation is that if you have a pen you are extremely concerned about staining, use only standard blacks and blues made by companies like Pelikan, Waterman, Herbin, etc. These inks tend to be on the less saturated side, and I have found them less likely to stain.
Exercise Caution When Using Inks with Special Properties
By “Special Properties,” I mean things like permanence, heavy shimmer, no-freeze, super-sheen, and other attributes that you increasingly see advertised in today’s market. While most of these inks will not cause issues in modern fountain pens, if an ink clogs or otherwise causes an issue with your very expensive piston filler, the original manufacturer may decline to cover the pen under the warranty if they didn’t make the ink themselves. Again, while issues with these inks aren’t common, and the worst that usually happens is a clog that can be flushed out with water at home, you do proceed at your own somewhat low, but not zero, risk.
Don’t get me wrong, I also have a selection of calligraphy inks and other non-fountain pen safe supplies. I just keep a selection of dip pens close at hand.
The MOST IMPORTANT RULE IS TO Only Use Fountain Pen Inks in Fountain Pens
By far, most of the stories I hear about inks damaging pens involve people attempting to use calligraphy and India ink in a fountain pen, which is a big no-no. Not only do these inks stain, but some contain glue-like substances such as shellac. For obvious reasons, introducing these inks into a fountain pen is a bad idea because you will literally never get them out, and any integrated filling system will likely be ruined. (It’s basically the equivalent of using paint.)
So, the short-form, TLDR version of this answer is “Don’t worry too much and enjoy your pens, as long as you are using inks by a reputable company advertised for use in fountain pens.” Clean your pens semi-regularly and don’t allow the ink to evaporate. If you have a very expensive or sentimental pen that you are concerned about damaging (especially staining), consider only using standard blues or blacks, and never, ever, use India ink in any fountain pen!
I periodically take a day to answer reader questions, some of which you can find archived in the FAQ section of the site. If you’re in the Nashville area, we’re always happy to answer your questions in person at our Nashville shop.