China [aka india] ink contains carbon and varnish [or perhaps shellac] to produce a distinctive long-lasting black coat which when dry is waterproof.
No amount of water will dissolve the hardened ink residue.
Shellac is easier to remove than varnish so test the pen first with denatured alcohol. If the ink liquefies, it contains shellac; if it gets soft but doesn't dissolve, it contains a mixture of shellac and lacquer. Test the pen again with lacquer thinner; if it liquefies, it's lacquer. Shellac can be removed with denatured alcohol, lacquer with lacquer thinner, and a shellac-lacquer combination with a 50-50 mixture of denatured alcohol and lacquer thinner.
If the ink contained varnish (which is most likely), you will need something to dissolve the varnish such as varnish remover. Let the parts soak in a solution in a glass jar. If you have access to an ultrasonic vibrator, put the whole jar into the water bath to increase varnish remover efficiency. Check it after several minutes to see how it's going. If the liquid in the jar starts to turn black, it's working.