When painting tefillin, are there any requirements regarding the ink (other than the need to be black)? Or is any ink okay to use?
Like in the comments to this answer, shoe polish is recommended. Is that indeed correct?
Keset HaSofer 21:14 rules that you can't put ground ivory into the tefillin paint as it is not from a kosher animal (an elephant). Accordingly, it would seem that one would need to make sure to use paint that has a certification ensuring that all ingredients are from kosher species.
See here "Halacha 4 How is ink prepared? One collects the vapor of oils, of tar, of wax, or the like, [causes it to condense,] and kneads it together with sap from a tree and a drop of honey. It is moistened extensively, crushed until it is formed into flat cakes, dried, and then stored.
When one desires to write with it, one soaks [the cakes of ink] in gallnut juice or the like and writes with it. Thus, if one attempts to rub it out, he would be able to.
This is the ink with which it is most preferable to write scrolls, tefillin, and mezuzot. If, however, one wrote any of the three with gallnut juice or vitriol, which remains without being rubbed out, it is acceptable."
I certainly wouldn't use shoe polish. How do you know what's in it? Kosher ink is not so expensive.