Grout haze is the leftover particles of cement that have settled onto the surface of the tile and dried there. Grout haze can occur on any tile, even those that have been rinsed with water after grouting. Haze that appears on slate tiles can have additional concerns, since slate has a textured surface that can trap excess grout. Since slate is a natural stone and subject to etching, most chemical haze removers or vinegar treatments will harm the stone without eliminating haze. Therefore, the most effective way to remove haze from slate is with scouring pads.
Crumple a paper towel into a loose ball. Use the ball to gently brush or dust the slate tiles. If the slate has been well sealed, this may be enough to remove a light film of grout haze from the slate by brushing the dried particles off of the sealed slate.
Pour a small amount of water onto slate tiles that have more significant grout haze and wet a nylon scouring pad. Work the pad in circles over the slate tiles. Pay attention to the clefts, fissures and pits that occur in the stone as these are all areas where grout may have settled.
Rinse the scouring pad frequently in fresh water. Pour additional fresh water over the slate tiles that have been scoured to rinse away excess grout.
Wipe dry one or two tiles that have been cleaned with an absorbent cloth and check these tiles against dry, loose, ungrouted tiles to check for hidden haze. If the tiles are clean, continue to scour the surrounding tiles in the same way. If the tiles are still hazy, pour more water on the tiles and continue to work the tile with a fresh, clean scouring pad until no haze can be detected.