Marble is suitable for most showers and other wet area applications. There is some maintenance required if you want to keep your stone looking its best, but it is not a dealbreaker. To learn more, we spoke with Mike Loflin, Industry Research & Information Manager, at the Natural Stone Institute.
As always, we recommend consulting with your fabrication partner as each stone is composed of its own unique characteristics and some stones may react differently to water and moisture than others (depending on their mineral composition).
What causes marble to rust?
If exposed to certain liquids, marble containing certain minerals (for example, pyrite) can begin to oxidize. Oxidation is what we call rust. Minerals like pyrite won’t oxidize unless they have been exposed to water, acid or bleach.
If a marble possesses colors or colorful veining, this essentially means the slab has trace minerals present that can potentially rust. However, a marble slab will not rust unless it is exposed to oxidizing liquids (such as water, acid or bleach), and is not treated properly afterwards.
What are pyrite inclusions and why are they prominent in marble?
Pyrite is a yellowish, metallic looking sulfide mineral most commonly found in white marbles. Pyrite can group together and form small, randomly distributed groupings of iron-bearing minerals, called inclusions, that naturally occur in most white stones.
If your slab is installed, sealed, and maintained properly, then rust is not going to be a problem. From Washington D.C. to Milan, century old buildings constructed of marble have not rusted as a result of proper care and maintenance.
How can your fabricator make sure that “rust bleeding” does not occur in your stone?
- Oil-based putty and plumbing sealants should never be used in contact with stone.
- Use a cement backer board instead of water-resistant drywall board (green board). Drywall will degrade and the paper on it will become a food source for mold and mildew when subjected to moisture. Do not use unless a waterproof membrane completely protects the surface from moisture infiltration.
- Make sure all horizontal surfaces (such as shower pan, seats, sills, curbs, etc.) slope slightly downward, ensuring positive water movement.
- Properly seal around the stone to waterproof and ensure that no water gets behind or underneath the slab.
- A proper deep-penetrating (but breathable) sealant on the surface of the stone will also ensure that the oxidation process does not take place as the water is not able to penetrate the surface.
For more in depth analysis on stone installation in wet areas, visit the Natural Stone Institute website, here.
If marble is sealed properly, will it prevent the slab from further rust?
The sealer is a stop gap. A good deep-penetrating (but breathable) sealer will help to prevent rust and oxidation. Quality stone requires a quality sealer.
Proper maintenance after shower use will ensure that the sealer will last longer, as repetitive contact with moisture/water will minimize the sealer’s lifespan.
How often do you recommend sealing for this specific application to ensure moisture is repelled from the slab?
It all depends on how you maintain and care for your stone. If your stone is properly maintained, then sealing every year could be overkill. As long as water beads up on the surface of the slab, the sealer is still doing its job.
Steps you can take to help prevent rust in marble showers after installation:
- Run the ventilation fan while showering.
- Squeegee the stone after showering.
- Run a towel over stone afterwards to ensure no additional water droplets remain.
- Open the door after showering to allow for proper ventilation.
If not properly maintained, how quickly can rust/oxidation occur in the stone?
It can happen within a couple of years if not maintained properly. We frequently hear from restoration experts — “if they had only taken care of it.”
Can marble be used in a steam shower?
Yes, but it is important to select the appropriate marble for this use. For a steam shower that will exude a lot of moisture in the air, marble that is classified as “A” or “B” can be used as they embody less minerals that are subjective to rusting. It is highly recommended not to use marbles that are classified as “C” or “D” as in some cases the veining can begin to dissolve in a steam environment.
Please note, for a regular shower application, you can use marble classifications A, B, C, and D if you maintain proper maintenance.
For additional insight and a rough breakdown for stone classifications, visit the Natural Stone Institute website here.
If left untreated, is it true that rust stains can’t be extracted?
There are products available in the marketplace that work to remove rust. However if the rust stain originates from within or below the stone, it is generally not removable since the source of the stain cannot be eliminated.
One rust removal product that we have seen good results with is Tenax. Tenax has a ready-to-use liquid rust remover that works to remove the rust in some cases. In general you simply apply the liquid rust remover to the rust spots and let it sit for 1 1/2 hours and wipe off with a clean cloth. It is important to read and follow the instructions on the label closely. If not followed precisely, the results may vary.
If this or another similar product does not work, another option would be for your fabricator to re-hone the material and re-seal it to bring it back to its former glory.
For additional insight on restoration and maintenance, please visit the Natural Stone Institute website here.