The ten-second test
Look at the mark from an angle, with light raking across the surface rather than straight onto it.
A stain is darker than the stone around it. Something has been absorbed — oil, wine, rust from a tin left on a worktop, tannin from a plant pot. The surface is still smooth and still has its polish; the colour has gone in.
An etch is lighter, duller and often slightly rough. The polish has gone. What you are looking at is not something sitting on the stone but a small area where the stone itself has been chemically dissolved.
Why etching happens so easily
Marble, limestone and travertine are calcareous — essentially calcium carbonate. Acid dissolves calcium carbonate. That is the whole mechanism, and it works at room temperature in seconds.
The acids that do it are entirely ordinary. Lemon and lime juice. Vinegar and anything containing it. Wine. Tomato. Many fizzy drinks. Almost every supermarket bathroom cleaner and every limescale remover. Some cosmetics and most perfumes.
This is why a marble vanity in a London bathroom tends to develop a constellation of small dull rings over a couple of years, and why the worst damage is usually around a basin rather than anywhere obvious.
The sealer misunderstanding
An impregnating sealer sits just below the surface and slows absorption. It buys you time to wipe up a spill before it soaks in. That is the whole of its job.
It does not make stone acid-proof. A sealed marble worktop etches from spilled lemon juice exactly as an unsealed one does, because etching happens on the surface, above where the sealer sits.
This is the most expensive misunderstanding in the category, and it is actively encouraged by people who sell sealer. Sealing is worth doing. It is simply not doing the thing most people think they are buying.
What can be done
A stain is usually recoverable. The method is a poultice — an absorbent paste chosen for whatever caused the stain, applied, covered and left for hours or days to draw the staining material back out of the pores. It is slow rather than difficult, and it sometimes needs repeating.
An etch cannot be cleaned, because there is nothing on the surface to remove. The correction is mechanical: honing the damaged layer away with progressively finer diamond abrasives until the surface is even again, then polishing back to the finish the stone originally had. It removes material permanently.
That is skilled work and it is worth saying plainly that it is not a job for a hand pad from a hardware shop. A badly honed patch is more visible than the etch was, and on a book-matched slab it is a great deal more expensive to put right.
What actually prevents both
Routine, and only routine. A pH-neutral cleaner and nothing else. Spills wiped rather than left. Coasters under anything acidic, and a board under anything that will leak. No descaler within reach of the stone — which, in a London bathroom, is the hard part, because the limescale and the marble are usually a hand's width apart.
On our retained arrangements this is simply part of the specification, and it is why most of our stone clients are retained clients rather than restoration jobs.
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