QUESTIONMy 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?
ANSWERANSWER - 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.