If you are installing porous tiles, such as marble, granite or slate, always seal them prior to grouting them. If the tiles are not sealed, the grout can seep out the many tiny holes and crevices in the tile and fill them. Once the grout begins to set, it can be difficult to get the grout out of these crevices again. You can remove dried grout from tiles within 2 to 3 weeks of the installation. After this time, the grout will cure. Depending on the type of tile, type of grout and how long it has been on the tile, you may need to take multiple steps to get the grout up.
Pour some alkaline cleanser directly onto the tiles and work the area with a green scouring pad, moving the pad in a circular motion over the grout. Rinse with water by pouring fresh water over the surface repeatedly as the grout begins to come up. Wipe up excess water and grout with a grout sponge.
Scrape the grout from the tiles with a long-handled 4-inch razor blade. Push the blade across the tiles toward the grout joints, taking care to stop just at the joint to avoid removing the grout from the joint.
Wash the tiles again with an alkaline cleanser and scouring pad. Rinse well with fresh water, and clean up the area with a grout sponge.
Dissolve one cup of sugar into a gallon of warm water. Soak several paper towels in the sugar water, and lay them over the grout-covered tiles. Leave the paper towels on the tiles to soak into the pores for two hours.
Rinse the tiles with fresh water to remove the sugar, and wipe the tiles clean with a grout sponge.
Apply an acid grout stripper to the tiles to remove any stubborn grout. Test the stripper in an inconspicuous area first to make sure it will not remove the finish from the tiles as well. Use the stripper with a green scouring pad to pull the grout from the tiles, wearing gloves as you work with the acid.
Rinse the tiles with warm water, and allow them to air dry.
Sprinkle a layer of baking soda over the tiles to neutralise any acids remaining on the tiles before you allow bare skin to come into contact with the tiles.
Rinse the tiles with fresh water and allow to air dry before you allow contact with the tiles.