Over the years I've had the odd bottle of ink go "bad." For me, this invariably manifests as a very strong ammonia smell from the ink. To date, this has happened to two bottles of Noodler's, an almost new bottle of Platinum Carbon Black (ouch!), and even some Carbon Black cartridges!
Most recently I got a bottle of Pilot Namiki Black from an Amazon seller that had this strong ammonia smell as soon as I opened it. In the past I'd just dump the ink down the drain, and maybe try for a refund if the return window was still open. But since reading FPN I've become familiar with the work of SamCapote and this famous thread:
So I decided to try and save it. This afternoon I rinsed out an old ink bottle and an Aeropress coffee filter with some denatured alcohol. I filtered the ink, and did indeed find a substantial layer of slimy film and sediment left behind. I added 6ml of 4% Phenol solution (from Natural Pigments as in the reference thread).
I will report back in 2 or 3 weeks with the results. I am not sure how well this will work - I have read that some Japanese inks have a PH of 8.0 or above. I don't know if Namiki Black is one of these inks, but effectiveness of Phenol is supposedly reduced by high alkalinity. So we'll see how it goes...