Ink was a material used for drawing or writing.
Anak had what appeared to be a bottle of ink. However, the ink was actually the microbes of the survivors of two warring planets. (PROSE: The Lampblack Wars)
The Hobothy were going to introduce special ink into the first British stamp, the Penny Black. They planned to influence people through the ink to kill. They were stopped by the Tenth Doctor and Rose. (PROSE: Stamp of Approval)
Professor Pierre Aronnax took barrels of ink collected by Captain Nemo from the Nautilus. (AUDIO: The Wreck of the Titan)