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Adding grout, a fine textured version of cement mortar, will fill in the spaces between your mosaic pieces. It also unifies and enhances the design. Grout comes in many colors and you can even color the grout yourself to get the perfect match for your mosaic. It looks best if you pick a contrasting color so your design will stand out. If you used very textured or porous pieces, like shells or very uneven stones, you don't need to grout your mosaic. If you are in doubt about the grout color, black is usually the best choice, and pure white often drowns out the mosaic design. If you want a lighter color, try a cream-colored grout. Check the label on your grout to see how long it will need to set. It should be set in about 20 minutes. Then, wipe the mosaic with a clean sponge and warm water. Rinse out the sponge regularly as you wipe away grout from the surface of the tiles. If there is any stubborn grout residue left on the mosaic, or if you wait too long after it sets to wipe it off, clean the excess grout off with a clean lint free cloth or a piece of crumpled newspaper. Remove big pieces of grout stuck to the mosaic pieces with a non-scratch nylon scouring pad or a small wooden stick.
Choose your grout. Let the grout set and wipe away the excess.