Just found this ...
Mordent is one of the few words doing the rounds here that is completely new to me.
Mordant, on the other hand, I encountered when using a microscope at the age of 11. When using stains to enhance tissue contrast under the microscope, a mordant is a substance used to act as a sort of "glue" between the tissue and the stain. As an example from that past foray into microscopy, a stain that's used for plant tissues is iron haematoxylin, but you need to soak the tissue to be stained in an alum solution for the stain to bind. Otherwise, the stain washes right out when you counterstain with another stain, such as carbol-fuchsin.