STAINS IN MARBLES AND STONES. HOW TO REMOVE!
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:
- Oil and grease stains. They can be removed:
- With organic solvent (ethane, ethyl ether, triethanolamine etc) applied to the soiled surface together with a thick layer of adsorbent. Good results have given the ethane when applied to the stain in combination with its cover with chalk, talc, sepiolite and allowed to evaporate slowly. Depending on how persistent the stain is, the cleaning process must be repeated more than once, renewing the adsorbent material at the intervals (in the latter, the oil or fat that created the stain is concentrated). The cleaning process needs special attention because the adsorbent material can be strongly impregnated with grease, resulting in its redeposition in the rock when it is porous.
- With alkaline solution (eg ammonia solution, sodium carbonate solutions (soda), sodium hypochlorite together with chalk, etc.) if the stain is not eliminated by the previous method. The solution hydrolyzes the stain by separating it into its basic components, which can then be dissolved and absorbed by the adsorbent.
- With hydrogen peroxide (oxyzene) on adsorbent material (mainly chalk) in case of very old and spoiled stains. The oxidation reaction may be slow enough and several repetitions are required.
- Stains of pen ink. As this type of ink is usually dissolved in water, these stains are removed by applying aqueous solutions of surfactants or sodium hypochlorite solutions in combination with suitable adsorbents (eg chalk). In the most persistent stains, hydrogen peroxide (always with adsorbent material, e.g. gypsum) is used.
- Pencil ink stains. They are removed with acetone or chlorinated hydrocarbons on a thick layer of adsorbent material, e.g. Chalk or talc. The procedure is repeated more than once in the case of persistent stains, taking care not to leave traces of the solution on the surface. If made halo, hydrogen peroxide is used in combination with chalk as a sorbent.
- Stains of varnishes. First, remove any spilled droplets of varnish from the surface of the rock with a thin spatula, taking care not to scratch the surface. The stains themselves are removed using the appropriate varnish removing liquid (methyl chloride, chlorinated hydrocarbons, etc.), applied in small amounts onto the stain and covered by a thick layer of adsorbent material (chalk, talc, attapulgite, sepiolite etc.). The process is repeated more than once by ensuring that the adsorbent material is refreshed when it appears to be sufficiently soiled by the varnish to ensure that the adsorbent material is renewed when it appears to be sufficiently soiled by the varnish removed by the solvent. If the stain is not completely removed, its oxidative dissolution is applied by means of hydrogen peroxide (always with adsorbent material). After removing the organic part of the varnish, remnants of its mineral dyestuff can be observed. These can be removed by repeatedly applying the surface of the stone with a neutral surfactant. If the type and chemical composition of the decorative rock allows, it is also recommended to apply a more active method, ie the use of slightly acidic solutions and / or metal ion solutions.
- Bitumen and asphalt stains. Removed like varnish stains, using toluene, xylene (or other aromatic hydrocarbons) on a thick layer of adsorbent. It is recommended that the test be applied to a small stain.
- Stains of tea, coffee, wine. If stains are recent and superficial, they can be removed with a common eraser. If the result is not good, it is recommended to apply aqueous solutions of sodium hypochlorite (8-10%) together with an adsorbent. In case of persistent stains or traces of hail, hydrogen peroxide is used, which will slowly carry out the complete oxidation of the organic substance along with adsorbent material.
- Stains from food in general. Where possible, they are removed with surfactants. Sodium hypochlorite solutions (8-10%) are used more than once. In cases of more persistent stains, it is advisable to treat hydrogen peroxide in combination with suitable adsorbents.
- Stains of copper salts. They have dark green shades and are generally due to alkaline carbonates or sulphates of copper. For their removal are applied methods similar to those eliminating rust stains or more simply used in repeated applications of solutions of ammonia or ammonium carbonate (about 20%), EDTA, sulphamic acid etc. in combination with the appropriate inert adsorbents (chalk, sepiolite , Attapulgite, etc.). After application, a good rinse of the surface with distilled and deionized water is necessary.
- Rust stains. Their removal is quite complex, especially when they are old and the stone is limestone and porous. When the rock is silicate (granite, quartzite, etc.), solutions of suitable mixtures of sulfuric, phosphoric, hydrochloric, oxalic and sulphamic acid can be used.
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.