Back when I got my Jinhao 250 set (four pens, all quite nice, varying in nib size from fine through medium fine to true medium), I was very much a fountain pen newby. I made the newby mistake of thinking that bottled ink was bottled ink, and nearly ruined three pens using India ink, which contains shellac.
One didn't get as much abuse as the others, and Odysseus has been using it (the medium fine nib in the silver pen body). One was worse than the others, and had major issues with drying out and skipping...and I will admit I lost my patience with it a bit, and when the cap cracked on one of my favorite pen barrels, I switched the bad nib into that pen body and said to hell with it. I'd just about decided to leave it un-inked, once I got the ink used up out of the converter.
Yesterday, I decided to use the ink up. I found it a lot worse than I remembered, and utterly lost what patience I had with it. I pulled the nib and feed...and found chunks of the India ink clogging bits of the feed almost totally. It was bad enough that it took a toothbrush and some elbow grease to get the chunks cleaned up. And time. It took enough effort that I was about ready to set it down and go get my gun cleaning picks.
I also taped the cracked cap on the inside of the cap.
The pen writes beautifully, now. My only complaint is that it really is a true medium on the nib, and very generous with laying down the ink--traits I don't particularly care for.
1 hour ago