The marble "dresses" our house in an impeccable way, but it needs maintenance, based on good elbow grease and following some care, especially when it comes to external window sills that are continuously exposed to atmospheric agents and can get stained.
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The main problem will be making the marble (usual travertine) white again. Of course, it is better to carry out a thorough cleaning operation at least twice a year.
First of all, remove the stain from the marble using a mix of bicarbonate, lemon, and warm water, and then mix as best as possible. Take a stiff bristle brush, moisten it with the mix and scrub the dark parts. If you fail to do this, put a small amount of bicarbonate and water on the dirty areas and let the paste take effect, leaving it for fifteen minutes. Then rinse.

If you are still not satisfied, pass a cotton swab with a few drops of ammonia and rub.
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Then to restore the entire external marble sill to its original whiteness and make it flawless again, put a small amount of liquid Marseille soap in warm water and mix well, then dip the brush into it and pass it over to the marble to be cleaned. Rinse the sponge with clean running water and rinse with another soft, clean cloth. Finally, dry it to make it shiny and shiny again.
It is preferable to do this on a cool and windy day in a period when the sun does not shine.
Adapted and translated by The Cop Cart Staff
Sources: Donnad