Glazed ceramic tile has a glassy coating baked onto it, providing moisture protection to walls and floors where it's applied. If you're thinking about a project that involves removing that glaze, the first thing you should do is think twice. It's very difficult to get the glaze off without damaging the tiles themselves, and in most cases you're better off just replacing the tiles. However, if you decide you want to paint the tiles or undertake another project that requires de-glazing, and you're not afraid of a little work, it is possible to dull that shine.
Put on your dust mask. Set up the belt palm sander with 100-grit silicon carbide sandpaper (this is a very hard variation of sandpaper). Begin sanding the surface of the tiles from one corner of the tiled area and working your way out across the room. Move the sander very slowly across the tiles, forward and back in the same direction.
Clean up the dust from the surface, using a sponge and warm water.
Put on your rubber gloves and goggles. Mix a solution of one gallon of warm water to one cup of TSP powder in a bucket. (If it's a small area of tile, cut the volume in half, so it's 1/2 gallon of warm water and 1/2 cup of TSP.)
Scrub the tiles thoroughly with a stiff brush, using the TSP solution. Start in the same corner as you did before. Work every tile individually and scrub vigorously.
Allow the TSP solution to sit on the tiles for about 30 minutes, then rinse thoroughly with a damp sponge.