During this plot, an Inmac purple pen (in the Pen 5 slot) pretty much ran out of ink:
It printed the legend perfectly and started the trace solidly enough, proceeding upward from the far right, but after ten circuits around the center it returned dragging a very faint line behind it.
Ten drops = 0.05 ml of more-or-less reddish purple blended inkjet ink restored its good humor:
If you get ’em before they fossilize, they’re refillable!
The Smell of Molten Projects in the Morning 
Have you tried more aggressive solvents like MEK on the crusted felt?
Not yet, but another ceramic pen just ran dry and won’t restart: I have a victim on tap.
I should test the solvents on a dried ink spot to see if they move the pigment / dye / whatever. If the spot doesn’t change, then there’s no point in soaking the nib.
So many projects, so little time… [grin]
I just got a pen holder for a Cameo Silhouette “craft cutter”. You need a razor blade holder for the 7475A. Then you can use the craft cutter as a 3D printer. Just stack up the laminations and you’re golden.
It’s about as tedious as it sounds. :-)
I’m wondering if I’m sick enough to use the pen holder to hold a dremel flex shaft… probably a really bad idea. Probably.
The Squidwrench Silhouette cutter reminded me of that trusty old HP 7475A… and now here we are.
Turns out 19 grams of downforce ain’t enough to push the blade through anything, but I sure tried hard to make it so! [sigh]