Reductite

Reductite is a vital tool used anywhere mages can be found, as it nearly completely blocks the link between a mage's mind and the world.

I don't like to use that shit. I'm a guard, not a torturer.

Making magic shackles

Reductite is made out of a special clay which can be found nearly everywhere in the world. It hardens easily with heat to become a sturdy, solid material.
What makes in the perfect material to put a magic-restraining spell on is its particular texture, weight, and smell. All of these properties make the clay really easy to picture in a mage's mind and thus makes the binding spells more powerful.
Aside from all the runes that could possibly hinder a mage's capacities, the reductite itself messes with the mind. Whenever a mage touches it, their brain either goes numb or "itches," rendering spell-casting nearly impossible.

I was kept prisoner with reductite for a few weeks. It's like your brain is continuously bombed by random thoughts and feelings. They took it off when I was too tired to think of anything else than sleeping and put it back as soon as I fell asleep. The nightmares kept waking me up.

Usages

Reductite is mainly used in jails for mages. The establishments keep a stock of clay and forge new handcuffs whenever a new prisoner comes in.
During wars, some regiments of archers are given reductite arrowheads and try to aim to mages. If the arrow manages to hit a mage during their incantation, there is a high chance that it will prevent them from casting any spell for a while.

Type

Ore/Mineral

Common State

Solid (clay)

The effects depend on the mage themselves. Some will only be bothered while others will live a nightmare. We don't know for sure how and why such variations between subjects occur.
A shared hypothesis among the main jails of the empire is that the domains in which the mage specialized are determinant in how they will be affected by reductite.