I far too frequently let ink dry inside one of my treasured pens. When simply flushing with hot water won’t clear them, I use this cleaning solution.
Simply draw some solution into the pen as you normally would ink. Shake it around and then let sit for 5-10 minutes. Flush the pen out several times with warm water, and you are back to an evenly flowing pen.
A good practice is to run warm water through the pen between fill ups. I often forget.
J. Herbin Fountain Pen Cleaning Solution 15Ml via Amazon