Removing stains from marbles and decorative rocks is a process quite time consuming and complicated, especially when the stains are old. The end result depends on many factors: the type of rock, the type of surface treatment, the type of stain, the way and the techniques of laying the slabs, etc

Generally, stains in decorative rocks can be distinguished in:

Organic or inorganic, surface or deep, recent or old. In order to remove them, in almost all cases, special chemicals in liquid form or dissolved in water, in combination with suitable adsorbents, must be used. The last must be spread over the surface of the rock over the stain to absorb the liquid chemical along with the agent responsible for creating the stain in the mass at the beginning of the capillary effect while preventing further spreading in the mass of the decorative rock.

The most commonly used adsorbents for this purpose are: inorganic solids in the form of very fine powder (chalk, talc, alumina, sepiolite, attapulgite, magnesium silicate, limestone, etc.), cellulose gel, (Xantana gum), certain polymers, etc., as well as liquids with high viscosity (glycerin, polyoxydrilate compounds etc), organic solids (cellulose paste, absorbent paper, cotton, fine cotton cloth, etc.). In general, adsorbents which do not react with the cleaning solution and do not alter the rock even when penetrated therein have to be preferred. They must also be easily removable after the stain removal process has ended. In all cases, it is advisable to thoroughly rinse the surface with distilled or demineralized water at the end of the process to remove salts that are always harmful, especially for porous rocks.

Also, when the nature of the stain and / or rock are not known, testing on a small part of the surface is recommended.

Specifically, depending on the type of stain, the following are noted for its removal:

When the rock is limestone, which is very sensitive to the action of acids (eg marble) with which it reacts to carbon dioxide and salts, the process is much more complex and depends on the degree of surface treatment. Raw solutions of oxalic, hydrofluoric or even phosphoric acid can be applied on rough or ground surfaces, because these acids which form insoluble calcium salts do not severely affect the limestone surface. In some cases, citric acid solutions may also be used.

Polished surfaces require more specialized and complex methods (usually based on the appropriate metal ions in a neutral or slightly alkaline solution). In this case, a long process is required, and the results are not always satisfactory, especially when the stain has penetrated deep and is old. In any case, after applying dilute acids, the surface of the decorative rock should be carefully washed with a solution of sodium carbonate and then rinsed with plenty of water.