Tincture is form of potion.
Physik- In Physik, Marcellus Pye tries to create a tincture, that would give eternal to him. Queen Etheldredda however, also knew about this, making her think of herself as eternal queen of Castle and therefore causing murder of her daughters. Marcellus Pye needed an seventh son of a seventh son to produce it and that's why he lured Septimus to the past. However the Tincture was never completed, because Etheldredda drank the incomplete version of it and Septimus, Jenna and Ullr returned to their own time. Septimus then produced a a new tincture for Marcellus and took it to him and Marcellus then turned to 30 years old likewise.