Rutile and ilmenite are the materials behind most of the broken, mottled and variegated glaze surfaces potters buy commercial reactive glazes for. Both carry titanium, both encourage crystals to form as a glaze cools, and both do more to a glaze's texture than to its colour.
What rutile does
Rutile is mostly titanium dioxide with some iron. In a glaze it promotes crystal growth during cooling, which breaks a flat surface into streaks, mottles and the pale runs that reactive glazes are sold for. It also warms colour towards tan and honey. Because the effect depends on cooling, the same glaze can vary noticeably by kiln position.
What ilmenite does
Ilmenite is an iron titanate and carries much more iron than rutile, so it gives darker speckle rather than pale mottling. Coarse ilmenite is used deliberately to produce dark specks scattered through a glaze or a body, which is one of the standard ways to make a plain glaze look less industrial.
Using them well
Both are usually added at a few percent to an existing base rather than used as primary colourants. Rutile added to a stable glaze is the classic route to a reactive one, and it is worth doing on test tiles first, because a glaze that becomes more fluid as well as more interesting will run onto the shelf. Cooling rate matters: a slower cool gives larger crystals and more pronounced effects.
Granular and powdered grades
Suppliers sell both fine and granular forms. Powdered rutile disperses and gives an even mottle; granular rutile leaves visible specks that bloom in the firing. The same distinction applies to ilmenite. Recipes usually specify which, and substituting one for the other changes the surface more than any percentage change would.
Questions people ask about rutile
What does rutile do in a glaze?
It promotes crystal growth as the glaze cools, breaking the surface into streaks and mottles, and warms the colour towards tan.
What is the difference between rutile and ilmenite?
Ilmenite carries far more iron, so it gives dark speckle where rutile gives pale mottling and variegation.
Will adding rutile make my glaze run?
It can, because it changes the melt as well as the surface. Test on a tile with a vertical face before using it on work.