As a potter I have made terra cotta tiles. To get a glaze on them you need to cover them with the glaze and re-fire them in a kiln. The tiles would have to come off your wall in order to do this. It would probably be easier to take them out and install new tiles. ...One thought, terra cotta is porous so I would think they would have had some glaze or protective covering on them if they were used in a kitchen. If so you might just be able to paint them white.