ILMI

Milled ilmenite is an iron-titanium mineral, ground fine. It brings iron and titanium into a glaze together, which is why it darkens a surface at the same time as it breaks it up.

Milled ilmenite colours and variegates a glaze, and in much smaller amounts starts crystals growing in one. Much of what is mined has weathered in the ground, which strips iron out and leaves the grain richer in titanium, while material worked fresh from hard rock runs leaner, so the iron and titanium are not in fixed proportion. It does not melt at the temperatures a kiln reaches; in an oxidising firing it oxidises from the outside of each grain inwards and dissolves into the melt from there. The titanium comes out of solution as the glaze cools and gives crystals something to grow on, which is much of where the mottled and streaked surfaces come from; the iron darkens them.