First, these caveats. [1] I am a bit reluctant to share, as I don't know if my method would meet the quality requirements of those on this forum who approach monochrome printing with scientific vigor, it probably wouldn't, though it could be a starting point for some. [2] I cannot speak to any long range effects on various printers, CIS systems, or refillable cartridges. I did my initial testing on a cheap, simple 4-ink Epson C88+, then when that worked I converted an old R2880 that had enough head clogging to be useless for good color. In other words, I've not used it yet on a valuable piece of equipment. [3] And, finally, although 50 years a professional photographer and illustrator, the look I now seek is more that of a high quality etching, lithograph or photogravure print, and not to emulate a silver darkroom print. My hack has given me exactly what I'm after.
rdeloe1 also replied to this thread with Paul's formula's using STS WJ1082, and I think that would be a better starting point for most people. I use many of the same ingredients; in general, distilled water, propylene glycol (instead of glycerin), a wetting agent (I'm not sure why two—PhotoFlo and Edwal LFN are used in that method), and, after trying a lot of different carbon sources, found a very rich and dense 100% carbon pigment india ink without varnishes added that had a good draw down and costs less than 10 bucks a quart.
I accept no responsibility for damage to cartridges or printers for anyone who would experiment using this information. I originally spoiled a few carts.
As for my exact carbon product and dilutions, I am creating a presentation on it for our Camera Guild and will be glad to share a link to it when that's done. The reason is, I've found people don't really understand when I just explain how I have achieved the grey mixtures using a greyscale file of a color Macbeth chart and then matching the equivalent grey tones to the various color ink cartridge position colors. But that won't be ready this week; I'm installing a new Canon ipf6400 in our Guild's network this week (we took advantage of their $300 rebate).