Ordinal's Cabinet of Ephemera

It is halakha from Moses at Sinai that one writes sifrei Torah, tefillin and mezuzot with ink alone. Ideally it is made thus: one combines ash from fat, pitch, wax and suchlike (that is, פלאמרוס or קיהנרוס) with tree resin (gum) and a little honey, and crushes it well until it cakes, whereupon he dries it. Before writing, he dissolves it in gallnut-juice or similar, and writes with it, so that if he comes to blot it off, it will go away. This is the ideal ink with which to write sifrei Torah, tefillin and mezuzot. If one wrote any of these three with gallnut-juice and kankantom (copperwasser) [ferrous sulphate solution], which doesn’t blot off, it is valid, and this is how ink is generally made these days - gallnut-juice, gum [arabic], and kankantom - but one must take care that it is properly black from the start [gallotannic inks darken on exposure to air]. The Mosaic halakha specified “ink” to preclude use of other colours - red, green and so on; if one wrote even one letter in another colour, or in gold, it would be invalid. So too one may not write Tanakh in anything other than ink; some say that the requirement of ink is limited to sifrei Torah.

Recipe for Kosher ink - Keset ha-Sofer. Iron gall ink lasts for extremely long periods of time; many cultures use it for permanent archival.