CW wrote:

India ink fades. Tried it.

India ink doesn't fade (it's lamp black!) - however much of it these
days isn't real India ink and they'd obviously used a fugitive modern
dye instead of the genuine pigment.

India ink also contains shellac, which tends to limit penetration. You
can get a much more hard wearing surface if you use something that
soaks in further.

PS - iron and vinegar does outgass. It's not toxic, but it will burst a
glass jar. Knock a few nail holes in the lid first.