Gemara

(Rav Yehudah citing R. Meir): R. Akiva knew that I put vitriol in the ink. He never objected. I told R. Yishmael 'I am a scribe. I put vitriol in the ink, lest a fly (rest on and) remove the ink at the extension of a Dalet, making it a Reish!

R. Yishmael: You may not use vitriol. "He will erase" - the writing must be erasable!

Refutation (Beraisa - R. Meir): I used to put vitriol in the ink. R. Yishmael never objected. R. Akiva forbade this. (Rav Yehudah says that R. Akiva allows vitriol!)

(Beraisa #1 - R. Yehudah): Any ink may contain vitriol, except for the ink used to write Parshas Sotah;

R. Yakov says, any ink may contain vitriol, except for that used to write Megilas Sotah in the Mikdash.

Gitin 19a (Mishnah): We may write with anything, e.g. with ink, yellow dye, red lead, sap, vitriol or any pigment that lasts.

'Any pigment that lasts' comes to include the law of R. Chanina's Beraisa, that if one wrote a Get with Afatzim (gallnuts) or Mei Tarya (rainwater; alternatively, water in which Afatzim were soaked), it is valid.

Shabbos 23a (Rav Huna): Every Seraf (sap) is good for ink. The best is of balsam.

103b: If a Sefer Torah was not written in ink, it is Pasul.

Megilah 17a (Mishnah): One is not Yotzei Kri'as ha' Megilah if it was written with sap or vitriol. It must be written in ink.

Nidah 20a: R. Ami would break off pieces of ink to check blood.

Rishonim

Rashi (Shabbos 23a DH l'Gavel): To make ink, one smokes glass in steam of oil, which blackens the glass. He scrapes off the black and kneads it with a bit of oil. When it dries, it is smeared into the ink.

Rambam (Hilchos Tefilin 1:4): To make the ink, we gather the smoke of oils, tar, wax or similar things, and knead it with tree sap and a little honey. We soak it very much and pound it until it is like wafers. We dry it and store it. When it is time to write, we soak it in Mei Afatzim or similar things, so if it is erased, it will be erased. This is the best ink for Seforim, Tefilin and Mezuzos. If any of the three were written with Mei Afatzim and vitriol, which cannot be erased, it is Kosher.

Hagahos Maimoniyos (2): We concluded that any ink may contain vitriol, except for that used to write Megilas Sotah in the Mikdash. R. Tam says that even the opinion that forbids vitriol allows adrement. It is not vitriol (unlike Rashi in Gitin, who equates them). The Mishnah in Gitin allowed vitriol, and the Gemara added Shichor, i.e. adrement!

Hagahos Maimoniyos (3, citing Sefer ha'Terumos): R. Ami would break off pieces of ink to check blood. This applies only to hard inks, like ours. Ink of Afatzim is always wet. In Shabbos, we learned that all Seraf is good for ink. This applies to our inks, which we make by soaking bark in boiling water. The water absorbs the Seraf. It later congeals and dries and becomes ink. Seraf is not sap. It is moisture (of bark).

Hagahos Maimoniyos (3, citing Tosfos): R. Tam says that even French ink is not ink that Chachamim discussed. Even though it is dry, they do not put oil in it. They just use oil to smoke it. It seems to me that any ink is Kosher, as long as it lasts and is visible on the parchment.

Hagahos Maimoniyos (3, citing R. Simchah): Also ink of Afatzim is ink. Every black dye that lasts on the parchment is called ink, for we find no tradition from Sinai requiring congealed ink or of Afatzim. The Yerushalmi explicitly allows ink without Afatzim.

Perush ha'Mishnayos (Sotah 2:4): Megilas Sotah requires an ink that can be erased. Whenever we require ink, in a Sefer Torah and others, it must be erasable. I saw that in Eretz Yisrael they avoid writing a Sefer Torah with inks called Chiber. They use only real ink. Whoever gave this ruling holds (some texts - is correct. However, some say) that Seforim with inks that cannot be erased are Pasul. This is utterly wrong! The Gemara concluded that any ink may contain vitriol, except for that used to write Megilas Sotah in the Mikdash.

Hagahah: Chiber is ink of Afatzim. They put in it vitriol, and kind of earth that blackens and causes that it cannot be erased. R. Chananel, R. Tam and the Rambam in Hilchos Tefilin say that Stam ink does not have Afatzim or vitriol. It is made from coals of vines and smoke of oils. It can be easily erased.

Rosh (Gitin 2:10): (Mei Tarya and) Afatzim can be used, unless the parchment was treated with Afatzim, for then the ink will not be visible.

R. Tam (cited in Rosh Hilchos Sefer Torah Siman 6): Ink made with Afatzim is not called ink. A Mishnah (Gitin 19a) discusses ink and dyes Kosher for a Get. R. Chiya's (our text - R. Chanina's) Beraisa permits Mei Tarya and Afatzim. It adds to the Mishnah. This shows that Afatzim are not called ink!

Rebuttal (Rosh): Afatzim themselves are not called ink, until they are mixed with sap. Then, it can be used to write even on a parchment treated with Afatzim.

Hagahos Ashri (Gitin): The Riva says that the Beraisa permits Mei Tarya, i.e. Seraf, or Afatzim without Seraf. They are called ink only when together. The Or Zaru'a forbids vitriol, for it is (not ink. It is) dye.

Poskim

Shulchan Aruch (YD 271:6): A Sefer Torah must be written with ink made from smoke of oils soaked in Mei Afatzim.

Rema: L'Chatchilah, it is good to make the ink only from tree products.

Darchei Moshe (5): The best ink is of smoke, like the Rambam says. Maharil (119) says that a Kosher ink should be used. The Zohar connotes that only tree products should be used.

Shulchan Aruch (ibid.): If it was written with Mei Afatzim and vitriol, it is Kosher.

Beis Yosef (DH u'Mah she'Chosav b'Shem): Sefer ha'Terumos and the SMaG agree with R. Tam. People conduct like the Rambam and Rosh, who permit. It seems that Tosfos and R. Simchah agree.

Shach (11): The ink must be black.

Gra (OC 691:2): The Shulchan Aruch also allows a Megilah written with Mei Afatzim and vitriol. We must say that this refers to both together, for Megilah 17a disqualifies vitriol by itself. Also, Gitin 19a connotes that Mei Afatzim is not ink, for the Mishnah allows ink, and a Beraisa allows also Aftza.