The 6th century doctor Aetius provides us with a rare look into how the ancient Romans applied tattoos. His meticulously written record gives many interesting details of the ink ingredients, procedure, and more! The Roman word for "tattoo" was "stigma" as Aetius expressed it, a word that still applies in modern English.
As Aetius writes in Medicae artis principes:
Stigmates are the marks that are made on the face and other parts of the body. We see such marks on the hands of soldiers. To perform the operation they use ink made according to this formula:
Egyptian pine wood (acacia) and especially the bark, one pound; corroded bronze, two ounces; gall, two ounces; vitriol, one ounce. Mix well and sift.
Grind the corroded bronze with vinegar and mix it with the other ingredients to make a powder. Soak the powder in two parts of water and one part of leek juice and mix thoroughly.
First wash the place to be tattooed with leek juice and then prick in the design with pointed needles until blood is drawn. Then rub in the ink.