Cure: Remove the re-crystallized layer. It can often be done by polishing the marble with a powder marble polish containing oxalic acid. Apply the powder to the marble, add water and work into a slurry with a hog-hair pad and a standard buffing machine. Continue to work until the yellowing has disappeared. If this technique fails, then the marble will have to be re-honed. This polishing and honing procedure should be performed by trained individuals. If these techniques fail to remove the yellowing, then proceed to #4.
4. Iron Staining. Many white marble tops contain naturally occurring deposits of iron. Iron is a mineral found in stone and can occur randomly. If iron is present in the marble, it will begin to oxidize when exposed to water or other oxidizers, such as acids and household bleach. White marble can remain for years without yellowing, then, over time, may slowly turn yellow and in severe causes may turn completely brown. The process is accelerated when the marble is saturated with water.
Oxidation in marble is similar to rusting in 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 marble. If water and/or air is eliminated, the iron will not oxidize. This is why certain white marbles suddenly turn yellow. The process is difficult to reverse, and replacement of the marble may be necessary. Before attempting stain removal, it is important to first determine if iron is the cause.
Testing For Iron:
1. Attempt to clean and strip the marble as outlined in Nos. 1 and 2 above before assuming iron is present in the material. If these procedures fail, then testing for iron will be necessary.
2. Test the water for iron. There are several inexpensive test kits available that can be used to do this. Check with your local plumbing supply store or any store carrying water softening supplies. There are chelating chemicals available that can be added to the water to prevent the iron from staining. This is very important if the marble is cleaned with this water.
3. Regardless of whether or not the water contains iron, the marble itself should be checked for iron content. Remove a piece of the marble (if possible) and contact a testing lab, which can analyze it for total iron content.
4. If the results return with iron present, check the marble for moisture. A moisture meter is a useful instrument for this procedure. If the marble contains water, it is very possible that the iron is beginning to oxidize.
Removing Iron Staining:
1. Prepare a solution of water and sodium hydrosulfite and sodium metabisulfite. These chemicals are available in a product called Iron-Out from your plumbing supply or home center. Mix with water and apply to the affected area. Allow the solution to soak into the marble and keep wet for several hours. Do not allow the solution to dry. Remove the excess solution with a wet vacuum, and rinse thoroughly with water and a chelating agent, such as EDTA. Be prepared to re-polish the marble, as these chemicals can cause etching.
2. If that fails, prepare a poultice with diatomaceous earth and 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. Remove the poultice paste and rinse the area with water and a chelating agent. If the stain is removed, the entire top can be treated. If the stain still remains, replacement is the only solution.