This Japan ink is really fine.
I did a little comparison between the Japan ink and my go to ink--McCaffery's Penman's Ink (Black).
Both the inks are new (despite the aged appearance of the McCaffery's bottle, I refilled it from the master bottle with fresh ink), used the same nib, paper, and, of course, the penman, if I may use the term, were the same.
The Japan ink is actually blacker, which can be seen on the finest hairlines--see the first of the slanted straight lines.
The Japan ink is also absolutely flat, whereas the McCaffery's has a very slight shine to it. This is just noticeable on the swells. I would imagine the difference would be more noticeable with larger letters and with broad brush /point strokes. The Japan ink looks, to my eye, almost like stick ink used in Japanese calligraphy.
They seem to be equally water resistant. I smeared the c in these lines with water--it had no apparent effect, so good for envelopes.
Definitely recommend this ink. Perhaps you could make another batch?