Moisture: Stone Enemy Number One

Most of the problems associated with stone tile installation can be traced to moisture or water intrusion of some kind.

Many stains are caused by the presence of water. Water is an essential ingredient for the setting cleaning and restoration of stone but it can also be its number one enemy. What problems are associated with moisture and water? The following is a brief description of the problems, their prevention and remedies:


Efflorescence appears as a white powdery residue on the surface of the stone. It is a common condition on new stone installations or when the stone is exposed to a large quantity of water, such as flooding. This powder is a mineral salt from the setting bed. To remove efflorescence do not use water, buff the stone with a clean polishing pad or #0000 steel wool pad. The stone will continue to efflorescence until it is completely dry. This drying process can take several days to as long as one year.


Sub-florescence is what happens when the mineral salts migrate and do not make it all the way to the surface. In the efflorescence condition above, the salts are deposited on the surface of the stone. In sub-florescence the salts crystallize just below the surface, causing stress within the pores of the stone. The result is a condition known as spalling which appears as pits in the surface of the stone. Sub-florescence is very common on green marbles and is very common on almost all stone surfaces where de-icing salts are used.

Iron Staining

Many light colored stones contain naturally occurring deposits of iron. Iron is a mineral found in stone and can occur randomly throughout the stone. If iron is present, it will begin to oxidize when exposed to water or other oxidizers such as acids and household bleach. Stone can remain for years without yellowing then over time may slowly turn yellow and in severe causes may turn completely brown. This oxidation process is accelerated when the stone is saturated with water. This process of oxidation is similar to the rusting of metal. If you expose a brand new nail to water and air it will turn brown and rust. The same process is occurring with the iron in the stone. If water and/ or air is eliminated the iron will not oxidize. This is the reason certain white marble suddenly turn yellow. The process is difficult to reverse and replacement of the stone may be necessary. The following stain removal technique has proved successful in several cases. Before testing this procedure it is important to first determine if iron is the cause.

Testing for Iron:

  1. Before assuming the marble is yellowed due to iron be sure to attempt cleaning and stripping with a good alkaline based stripper. If these procedures fail then testing for iron will be necessary.
  2. If a flood has occurred or excessive water was used first check the water for iron. There are several inexpensive test kits available that can be used to check the iron content in water. Check with your local plumbing supply store or store carrying water softening supplies. If any amount of iron is detected then it is possible iron has entered the stone through the water supply. To eliminate the iron there are chelating chemicals that can be added to the water to prevent the iron from staining. This is very important if the stone is cleaned with this water.
  3. If the water contains no iron and even if it does the stone should be checked for iron content. Remove a small sample of the stone and contact a testing lab and have them analyzed it for total iron. If there are spare tiles that have never been installed also have them tested for total iron. If iron is present naturally in this stone, it will probably be detected in the spare tile. If the results return with iron present then the following procedure should be tested.
  4. Check the stone for moisture. A moisture meter is a useful instrument that can be employed to check the stone for moisture. If the stone contains water, it is very possible that iron is beginning to oxidize.

Removing Iron Staining:

  1. Prepare a solution of water and the following chemical: Sodium Hydro sulfite and Sodium Metabisulfite. These chemicals are available in a product called Iron-Out® from your plumbing supply or home center. Mix a solution in water and apply to the affected area. Allow solution to soak in and keep wet for several hours. Do not allow solution to dry. After several hours pick up excess solution with a wet vacuum and rinse throughly with water and a chelating agent such as EDTA. Be prepared to repolish any marble since these chemicals can cause etching.
  2. If the above procedure fails than prepare a poultice with diatomaceous earth and the Iron Out®). Mix the poultice into a thick paste and apply to a small area. Cover the poultice with plastic and allow it to sit covered for 24 hours. After 24 hours remove the poultice paste and rinse the area with water and a chelating agent. If the stain is removed, the entire surface can be treated. If the stain still remains then replacement is the only solution.

Before the above procedure can be performed, it is important that the affected area be dry. If water or moisture are still present, oxidation of iron may continue.

The yellowing of stone is a common problem. New installations should be sealed with a good quality penetrating sealer (impregnator) which will help prevent oxidation of the iron by eliminating moisture.

The above procedure has proven successful in some cases of iron staining. However, if the following test does not produce the desired results I would recommend replacement of the affected areas.

Marble and limestone in constant wet areas such as showers and pools, etc. may be impossible to remove iron. The stone as well as adjacent materials, such as grout, will also be affected.


Several type of thin stone tiles are very susceptible to warping. Many of the green marbles and a few agglomerate marbles are notorious for this warping condition. Many installers have had the surprise to find that their tile installation has become warped overnight.

Why does this a happen and can it be prevented? Warping is caused by water. Green marble set with any water based material will have a tendency to warp. The mechanism of why the tile warps is somewhat a mystery. Some believe that the water fills the pores of the stone and when the water evaporates the orientation of the stones crystal change and cause it to warp. Whatever the reason, one thing is for sure, green marble can warp when set with water based materials.

Cure: Once a green marble tile warps it is difficult to repair. Attempts have been made to grind the tile flat , but this usually fails since additional water is introduced during the grinding process. The green simply warps again.

Prevention: The only way to prevent warping is to install it properly with a non-water based material such as epoxy. Some installers have also ben successful in sealing the back of the tile with epoxy and installing it in a water based system(see July Stone & tile Report). Do not attempt to seal the back of the tile with a silicone sealer. The silicone acts as a water repellant and will cause the setting material to fail resulting in a bond loss.


Erosion is a condition found when stone is exposed to constant amounts of water. This is especially true with marble that is used in water fountains. While marble is a very decorative material, it is one of the worst materials to use in or around water. Marble is composed of calcium carbonate. Calcium carbonate is a water soluble mineral. Quite simply this means it will dissolve in water. Want proof? Visit the Grand Canyon. Erosion can be recognized by a slow deterioration of the stone surface. With polished stone the polish will be worn off. In older installations, the stone may become very soft, brittle and in extreme conditions, it will powder. For all architects or designers are reading this article I beg you no to use marble for water fountains. If you do, plan on very high maintenance costs and plan on replacement in about five to ten years if not sooner.

Stabilizing erosion: If you are faced with trying to stabilize a marble fountain that is already deteriorated there are some treatments that can be applied that will extend the life of the marble. These treatments are general called consolidants and serve to replace the natural binders that are lost through erosion. Consolidants can be tricky and quite often will cause discoloration of the surface. Be sure to test the consolidant carefully before use.

Mineral Crusts or Lime Putty

Mineral crusts or lime putty can be recognized by its white crust like formation on stone surfaces. These crusts are often found on outdoor stone stair, water fountains and other areas where stone is exposed to water. The crusts are a deposit of hard mineral salts consisting of calcium and magnesium. These minerals originate from the soil, setting bed or from the water itself. These salts are similar to efflorescence in that they are a mineral. They differ in that they form a hard crust that can be difficult to remove.

Crust Removal

There are only two ways to remove these mineral crusts— by abrasion or through chemical treatments. The mineral salts should be removed with an abrasive. I have found that a stiff non-ferrous wire brush can work well. Brushes can also be purchased that attach to an electric drill. Be careful and do not get too aggressive. Avoid damaging the stone surface.
Quite often abrasion alone will not remove all salt deposits. Strong acidic chemicals will be required. These chemicals can be purchased from most chemical companies that supply stone cleaning products. Be careful when using these products around calcium based stone since the acid can also damage the stone itself.


The best prevention from mineral salts is to prevent moisture form entering the stone. On steps and fountains make sure all grout joints are caulked with a water-proof material. When installing steps outdoors make sure a water proofing barrier is used. It is also a good idea to use a good stone impregnator on all surfaces to prevent water from entering the stone. Caution: Stone impregnators will not waterproof stone. Do not use them where hydro-static pressure is a concern.


To properly test for moisture, a protimeter is necessary. A protimeter is an instrument that reads moisture. The common protimeter has been designed for use with wood, drywall and other similar substances. The protimeter contains two sharp probes that are inserted into the wood or drywall to give a direct moisture reading in percent. Unfortunately, you cannot push these probes into the stone, but the protimeter can give you important data on stone moisture. By placing the pins so that the just touch the stone a relative reading can be obtained. For example, A reading of 0-6% is considered relatively dry. A reading between 7-20% is wet. A reading of of 20% is very wet. These readings only tell you that the stone is wet, a little wet or dry. A direct percent reading can not be obtain with these instruments, but can provide useful information.

Another simple technique for determining moisture in stone is to take a piece of plastic about 12 inches square and place it on the suspected stone. Tape all four edges and allow it to stay overnight or 12 hours. After 12 hours, if there is any moisture present, you will see condensation collecting under the plastic.


A protimeter can also be used to check for the presence of soluble salts. The following procedure will only tell you that salts are present. It will not tell you how much or what type. But in many cases the simple presents of salts can indicate potential spalling and/or pitting.

For this test you will need the following materials:

  • A rubber block
  • Filter paper
  • Distilled water
  • A protimeter
  • Forceps

Any type of rubber will do as long as it is clean and does not contain any salts. A piece of hard plastic can also be used. Filter paper can be purchased from a scientific supply store and sometimes from the supplier who sells protimeters. Distilled water can be purchased from the grocery store. The forceps are used to pick up the filter paper.

To check for soluble salts pick up a filter paper with the forceps. DO NOT touch the filter paper with your fingers. The human skin contains soluble salts which could be transferred to the paper giving a false reading. Place the filter paper on the rubber block. Add a drop or two of distilled water to the filter paper. Place the probes of the protimeter to the filter paper and record the reading. Next, take a new filter paper and place it on the stone to be tested. Add several drops of distilled water and take a reading. Record the reading on a piece of paper.

If the reading obtained on the filter paper from the stone is higher, then there are salts present. If it is the same or lower, salts are absent. The protimeter works by reading ionic changes. When salts are dissolved in water, the ionic changes are higher, which gives a higher reading.

An articles written by Fred Hueston, Chief Technical Director for Stone and Tile PRO.