379. The Ink
Tefillin, Mezuzah v’Sefer Torah 1:4
Ink is made by gathering ingredients like oils, tar, wax, etc., and kneading them with tree sap and a drop of honey. It is moistened, crushed into flat cakes, dried, then stored. When one wants to write with it, he soaks one of the cakes in gallnut juice or something similar, after which he would be able to write with it. If one tries to erase it, it would erase. This is the most desirable ink to use when writing sefarim, tefillin and mezuzos, but if one use gallnut juice or vitriol, which cannot be erased, it is valid.
Tefillin, Mezuzah v’Sefer Torah 1:5
Since one need not use the most-optimum ink, what was excluded by the halacha l’Moshe MiSinai requiring ink? This law excludes other colors, like red and green. If even one letter of a sefer Torah, tefillin, or mezuzah is written in another color or in gold, it is invalid.