Stain Management for Stone
All stones are absorbent and all natural stones are the composition of many particles, either of the same mineral, or of different minerals bonded together. The “space” in between these molecules of minerals is mostly what determines the porosity of a stone and this varies greatly, and so does their absorbency. Some of them are extremely dense, therefore their porosity is minimal.
Some other stones present a medium porosity, and others at the very end of the spectrum are extremely porous. Because of their inherent porosity, many stones will absorb liquids, and if such liquids are staining agents, a true stain will occur.
A true stain is a discoloration of the stone produced by a staining agent that was actually absorbed by the stone.
Other ‘discolorations’ have nothing to do with the porosity of the stone, but rather are a result of damage to the stone surface. All those ‘stains’ that look like ‘water spots’ or ‘water rings’ are actually marks of corrosion (etches) created by some chemically active liquid which had a chance to come in contact with the stone. All calcite-based stones such as marble, limestone, onyx, travertine, etc. are sensitive to acids. Therefore, they will etch readily within a few minutes.
A true stain is always darker than the stained material.
If it appears as being of a lighter colour it is not a stain but either an etch mark made by an acid, or a caustic mark (bleaching) made by a strong base alkaline. If it is polished or honed finished marble, travertine, slate or granite then you probably should hire a professional stone refinisher. If the etch is light and the depth is undetectable by the naked eye, and it looks and feels smooth, then a polishing compound for polished marble will work quite well without requiring the experience of a professional and no specific tools are needed, other than a piece of terry cloth.
We may also have a combination of a stain with etching. For example, if some red wine is spilled on an absorbent polished limestone, then the acidity of the wine will etch the surface on contact, while the dark colour of the wine will stain the stone by being absorbed by it. In such a case, first the stain will have to be removed and then the etch mark repaired by a professional stone restorer.