Grout is the coarse substance that is placed between tiles when they are laid on a surface. Grout serves a dual purpose: it fills in the spaces between the tiles to help hold them together and it prevents moisture from seeping in underneath the tiles. Grout that has become black needs to be cleaned. A build-up of dirt, and possibly even mould, is likely the cause of the discoloured grout. Cleaning black grout will restore the original beauty of the tiled surface.
Combine ½ cup of baking soda, 1/3 cup of ammonia, ¼ cup of white vinegar and seven cups of warm water in a bucket. Stir the ingredients together with a large spoon.
Transfer some of the solution to a spray bottle. Use a funnel to pour the solution from the bucket into the spray bottle.
Spray the solution onto the black grout. Wipe it clean with a damp rag. There is no need to rinse the solution off with water.
Mix ½ cup of hydrogen peroxide in a spray bottle with one cup of water, for a less expensive cleaning method. Spray the solution onto the black grout. Scrub the solution into the grout with a toothbrush. Leave the solution on the grout without rinsing. The peroxide and the black stains will dissipate naturally. If there is residue on the grout after the liquid has evaporated, wipe it clean with a dry rag.
Make an oxygen bleach solution if the black colour is still on the grout. Oxygen bleach is safe to use, even on coloured grout. Mix two tablespoons of oxygen bleach to every two cups of water in a bucket. Use a measuring cup or something similar to pour the solution directly onto the black grout. Spread the liquid all over the black grout using a paint brush. Let it sit on the grout for at least five minutes and then scrub the grout with a toothbrush. Wipe away the excess water and dirt with a clean, dry rag. Repeat this process as necessary to remove all of the black stains from the grout.
Never mix bleach or products containing bleach with ammonia. The combination can produce fumes that are highly toxic.