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We all know what a stain is, right? … Or do we … Let’s start by saying that a stain is a discoloration. So far, so good.
The fact is, however, that not all discolorations are stains. To illustrate the point, let’s take, for example, a piece of common fabric. Fabric is typically absorbent. Therefore, if we spill some liquid onto it, the material will absorb it. If it is only water, it will leave a temporary stain. In fact, once it dries, the fabric will go back to its original color (plus, maybe, some mineral deposit can we can just brush away), but if coffee, or cooking oil is spilled on it a stain will occur because the fabric will absorb the staining agent and change its color in a permanent way, unless we do something to remove the agent from the fabric. On the other hand, if bleach is spilled on that same fabric, a discoloration will occur, but it can hardly be defined as a stain because it is actually a permanent damage to the dye that originally made the color of the fabric.
A true stain is always darker than the stained material.
If it appears as being of a lighter color it is not a stain but either a mark of corrosion (etching) made by an acid, or a caustic mark (bleaching) made by a strong base (a.k.a., alkali). In other words, a lighter color “stain” is in fact always a surface damage and has no relation whatsoever with the absorbency rate of the damaged material – stone or whatever.
There is not a single exception to this rule.
As with the fabric example, when it comes to natural stone there are stains that are in fact stains, and there are “stains” that are actually discolorations that are due to something else.
All stones are, more or less, absorbent. One may say that diamonds or gemstones are not absorbent. That’s right, but a gemstone is not actually a stone. It is in fact made of one crystal of one single mineral. All other (less noble) stones are the composition of many crystals, 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. That said, what is next is the fact that the porosity of stones varies greatly, and so does, of course, their absorbency. Some of them are extremely dense, therefore their porosity is minimal. What this translates into is the fact that the absorbency of such types of stone is so marginal that—by all practical intents and purposes—can be considered irrelevant. 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 (absorbency) 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 (mostly—but not necessarily limited to—acids) 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 seconds). Many a slate, too, will etch, and so will a few “granites” (those that instead of being a 100% silicate rock are mixed with a certain percentage of calcite.) Please call if you are considering removing a stain by yourself.
Etching, a.k.a. “Water Stains” Or “Rings”
olished marble, travertine, onyx, limestone, etc. are all calcite-based stones, and as such are affected by pH active liquids, mostly acidic in nature. In layman’s language, when an acidic liquid gets on a polished marble, travertine, many a slate, etc. surface, it etches it on contact. That is, it leaves a mark of corrosion that looks like a water-stain or ring. Such surface damage has nothing to do with the absorbency rate of the stone (typically quite low, anyway), but exclusively with its chemical makeup, which, as mentioned before, is mostly calcite (Calcium Carbonate, CaCo3). Trying to remove the “stain” by poulticing it would be useless exercise, since it is not a stain, no matter what it looks like. So, how do you remove a chemical etch-mark, which, as seen, is not a stain but a surface damage? You don’t! In fact an etch mark can be effectively compared to, and defined as, a shallow chemical scratch.
A scratch is something missing (a groove), and nobody can remove something missing. It would be like trying to remove a hole from a doughnut! The only thing one can do is to eat the doughnut, and … the hole is gone! Same thing with a scratch: you must actually remove whatever is around the groove, down to the depth of the deepest point of the scratch.
You are actually facing a full-fledged—though small in size—stone restoration project! If it is polished marble or travertine or onyx, then there’s hope. If it is hone-finished marble or travertine, or hone-finished slate (like a chalk-board), or mixed “granite”, then you probably should hire a professional stone refinisher. If it’s a cleft-finished slate (rippled on its surface), then nobody can actually do anything about it, other than attempt to mask it by applying a good quality stone color enhancer.
While marble and other calcite based stones are vulnerable to acids, granite is much more resistant. In fact, the only acid that will etch polished granite is hydrofluoric acid, commonly found in rust removers.