This is the metal cap of a Sheaffer Imperial. I think it can handle something stronger than the water/ammonia mix.

What can I do? Soaking and flushing has not been able to cure this. It seems that A LOT of ink is dried up in there. Like, a whole converter dumped in there and then dried for ten years.

Even though it's metal, any stronger acids or alkalis might stain or corrode the metal. I sometimes have good results with Windex, but I think that it is also a mild ammonia solution with a bit of soap.

It there is a solid lump of dried ink in there, you may have to see if someone with an ultrasonic bath can help. Try your local jewelery shops.

See how you go,
David

I recommend Koh-I-Noor pen cleaner. Squirt it in the cap, scrub with a test tube brush. The stuff is amazing.

Shine a light in there and make sure there is not a piece of cotton or fabric saturated with ink.

Can you believe this? It's like some sort of prank. I do not think that a month of soaking would have caught this. I stuck my dental pick in there and scooped out all the saturated material and viola, cap is clean, clean, clean.

Thanks for the advice!