Yeah, that stuff is no bueno for fountain pens. It’s meant for dip pens.

So, the part of the problem with the W&N drawing inks is that all the retail art supply stores put them right in the same section as all of their fountain calligraphy pens, and there isn’t enough information available standing there in the aisle (or even written on the box) that these inks are not suitable for use in the fountain pens they’re inevitably sold right beside.

Fountain pen ink is formulated to be completely water soluble and is dye based and not pigment based, so they don’t permanently dry within the feed mechanism and can’t clog it with particulate matter. Dip pens have neither of these limitations, so dip pen ink can contain pigments (little particles of gumf floating in the ink as colorants) and can contain binders and hardening agents like gelatin and/or shellac. You don’t want this stuff in your fountain pen.

On the bright side, you can get a cheap dip pen set (the Speedball nib holders and their myriad of nibs are widely available at craft stores) to use up your Windsor & Newton ink. I have a row of the stuff sitting on my shelf as well, for use with my glass dip pen.