Will the Wet Spots on my recent Marble Slab Installation go Away?


My contractor has installed moon face marble slabs approx 6 ft wide and 10 ft tall in my office entry. There are numerous unsightly wet spots still visible since installation 4 days ago. I’m told it’s the plaster leaching thru and will dry. Is this reasonable?


ANSWER - If you have a number of wet/dark spots on your newly installed marble slabs, it may be that the marble slab was installed in a spot bonded method.

If the marble slab was installed on a wall, it is possible that they mechanically attached the marble to the wall with wire.  When they do that they might use a plaster or a thin-set mortar to around the mechanical attachment to stabilize it and to allow then to adjust the levelness/plumbness of the slab on the wall.

If the marble slab was installed on the floor, then the spot bonded method is not considered an appropriate installation method.  This is because it leaves a lot of voids under the marble that would have a propensity to be damaged if those unsupported spots where subjected to some impact or heavy load.

A wet a thin-set mortar or plaster could transfer moisture to a stone and cause it to turn dark at the point of contact, but you would expect it to dry and go away.  Normally moisture will not migrate through a true marble because it has such a low absorption of 0.2% or less.

I have seen some cases where a certain type of mortar/plaster or mastic was used for the spots and it actually stained the stone where it was not removable.  This is a case of incompatibility between the adhesive and stone.  The installer or specifier of the adhesive should have verified the compatibility of the products.

The only think you can do now is wait and see if the spots diminish or go away.  I would put a measuring device next to them and take photos with dates on them, so you can compare them in a few weeks to see if they have changed.


6 thoughts on “Will the Wet Spots on my recent Marble Slab Installation go Away?

  1. Angelo says:

    Thinking of using marble tile for walls in a shower stall. It’s a cottage in Muskoka Ontario. Plan would be to seal the marble. My question is, what is the effect of natural lake water on marble vs treated city water.

  2. Donato Pompo says:

    Natural Lake Water chemistry will vary from lake to lake depending on its water source and what it has been subjected to. Same with treated city water as we know from recent situations where city water can be contaminated due to various situations.

    Some water will have higher concentrations of minerals and is considered hard water that has higher concentrations of calcium and magnesium. Hard water can cause buildup in pipes and can leave residuals on surfaces that need to be cleaned more frequently.

    Best thing to do is have the water tested. You can also experiment with it on samples of the material you want to use.

  3. Margaret Dowd says:

    We recently had our shower redone with white and gray Bianca marble tile. The tile is now turning a darker gray near the drain area and also two brown spots have appeared. The contractor says we must have used some cleaner but all I used was a wet rag. He says he has never seen this happen. Help

    • Donato Pompo says:

      Chances are that the darker gray area near the drain is due to the installer either did not protect the drain weep holes to allow water under the stone to evacuate into the drain or he used a drain that did not have weep holes. Only way to fix it is remove the stones around the drain and unplug the weep holes.

      The brown spots are likely rust spots. White marbles typically contain iron sulfide minerals that can cause rusting if they are exposed to excessive moisture. Or there could be a metal attachment beneath the stone that can cause rusting. If it is surface staining it can normally be cleaned with a poultice. Best to have a professional stone restoration person do the cleaning.

  4. Cilla Zosa says:

    Hi! I have a Carrera marble bathroom countertop. I now know it’s not sealed. A dark grey ring formed I believe from water sitting on top under a soap holder. How long does it tak to dry out, and is there something I can do?

    • Donato Pompo says:

      People tend to refer to marble as a soft stone, but it is far from it. Per the ASTM C503 standard the minimum density should be 162 lb/ft3 and the minimum absorption is 0.2% which is below the minimum absorption for being impervious.

      Although marble being a a calcium carbonate material is susceptible to scratching and sensitive to acid that can etch its surface. Chances are the dark grey ring isn’t a damp spot because the marble is impervious, but it could be an etched condition. Etched meaning it chemically altered the material. So it should feel like it has some texture when you rub your fingers across the spot.

      The good news is natural stone typically can be restored to being like new. A professional stone restoration company can deep clean, grind and re-polish the marble. Sealers can help, but they won’t prevent etching or staining. What is important is that you wipe up any spills right away. The soap holder should be set on top of a cloth to protect the marble.

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