Ink, as you mentioned, is not something that invalidates your wudū' (ablution). The following are nullifiers of wudū' :
"...1 – Any discharge from the front or back passage (urine, stools, wind, etc.), except for wind emitted from a woman’s front passage – that does not break wudoo’. 2 – Emission of urine or stools from anywhere other than the urethra or anus.
3 – Losing one’s mind, which may mean losing it altogether by losing one’s mental faculties, which is insanity, or by losing it temporarily for a certain length of time for some reason such as sleep, unconsciousness, intoxication, etc.
4 – Touching one’s penis, because of the hadeeth of Basrah bint Safwaan, who heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “Whoever touches his penis, let him do wudoo’.” (narrated by Abu Dawood, al-Tahaarah, 154. al-Albaani said in Saheeh Sunan Abi Dawood, no. 166, it is saheeh).
5 – Eating camel meat, because of the hadeeth of Jaabir ibn Samurah, who said that a man asked the Prophet (peace and blessings of Allaah be upon him), “Should we do wudoo’ after eating camel meat?” He said, “Yes.” (Narrated by Muslim, al-Hayd, 539)..."